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VB.NET Objects and Classes
VB.NET Objects and ClassesObjects and classes in Visual BasicAn object is a combination of code and data that can be treated as a unit. An object can be a piece of an application, like a control or a form. An entire application can also be an object. When you create an application in Visual Basic, you constantly work with objects. You can use objects provided by Visual Basic, such as controls, forms, and data access objects. You can also use objects from other applications within your Visual Basic application. You can even create your own objects and define additional properties and methods for them. Objects act like prefabricated building blocks for programs — they let you write a piece of code once and reuse it over and over. This topic discusses objects in detail. Objects and classesEach object in Visual Basic is defined by a class. A class describes the variables, properties, procedures, and events of an object. Objects are instances of classes; you can create as many objects you need once you have defined a class. To understand the relationship between an object and its class, think of cookie cutters and cookies. The cookie cutter is the class. It defines the characteristics of each cookie, for example size and shape. The class is used to create objects. The objects are the cookies. You must create an object before you can access its members. To create an object from a class
Note Whenever possible, you should declare the variable to be of the class type you intend to assign to it. This is called early binding. If you don't know the class type at compile time, you can invoke late binding by declaring the variable to be of the Object Data Type. However, late binding can make performance slower and limit access to the run-time object's members. For more information, see Object Variable Declaration. Multiple instancesObjects newly created from a class are often identical to each other. Once they exist as individual objects, however, their variables and properties can be changed independently of the other instances. For example, if you add three check boxes to a form, each check box object is an instance of the CheckBox class. The individual CheckBox objects share a common set of characteristics and capabilities (properties, variables, procedures, and events) defined by the class. However, each has its own name, can be separately enabled and disabled, and can be placed in a different location on the form. Object membersAn object is an element of an application, representing an instance of a class. Fields, properties, methods, and events are the building blocks of objects and constitute their members. Member AccessYou access a member of an object by specifying, in order, the name of the object variable, a period (.), and the name of the member. The following example sets the Text property of a Label object. VBwarningLabel.Text = "Data not saved" IntelliSense listing of membersIntelliSense lists members of a class when you invoke its List Members option, for example when you type a period (.) as a member-access operator. If you type the period following the name of a variable declared as an instance of that class, IntelliSense lists all the instance members and none of the shared members. If you type the period following the class name itself, IntelliSense lists all the shared members and none of the instance members. For more information, see Using IntelliSense. Fields and propertiesFields and properties represent information stored in an object. You retrieve and set their values with assignment statements the same way you retrieve and set local variables in a procedure. The following example retrieves the Width property and sets the ForeColor property of a Label object. VBDim warningWidth As Integer = warningLabel.Width warningLabel.ForeColor = System.Drawing.Color.Red Note that a field is also called a member variable. Use property procedures when:
Use fields when:
MethodsA method is an action that an object can perform. For example, Add is a method of the ComboBox object that adds a new entry to a combo box. The following example demonstrates the Start method of a Timer object. VBDim safetyTimer As New System.Windows.Forms.Timer safetyTimer.Start() Note that a method is simply a procedure that is exposed by an object. For more information, see Procedures. EventsAn event is an action recognized by an object, such as clicking the mouse or pressing a key, and for which you can write code to respond. Events can occur as a result of a user action or program code, or they can be caused by the system. Code that signals an event is said to raise the event, and code that responds to it is said to handle it. You can also develop your own custom events to be raised by your objects and handled by other objects. For more information, see Events. Instance members and shared membersWhen you create an object from a class, the result is an instance of that class. Members that are not declared with the Shared keyword are instance members, which belong strictly to that particular instance. An instance member in one instance is independent of the same member in another instance of the same class. An instance member variable, for example, can have different values in different instances. Members declared with the Shared keyword are shared members, which belong to the class as a whole and not to any particular instance. A shared member exists only once, no matter how many instances of its class you create, or even if you create no instances. A shared member variable, for example, has only one value, which is available to all code that can access the class. Accessing nonshared membersTo access a nonshared member of an object
Accessing shared membersTo access a shared member of an object
Differences between classes and modulesThe main difference between classes and modules is that classes can be instantiated as objects while standard modules cannot. Because there is only one copy of a standard module's data, when one part of your program changes a public variable in a standard module, any other part of the program gets the same value if it then reads that variable. In contrast, object data exists separately for each instantiated object. Another difference is that unlike standard modules, classes can implement interfaces. Note When the Shared modifier is applied to a class member, it is associated with the class itself instead of a particular instance of the class. The member is accessed directly by using the class name, the same way module members are accessed. Classes and modules also use different scopes for their members. Members defined within a class are scoped within a specific instance of the class and exist only for the lifetime of the object. To access class members from outside a class, you must use fully qualified names in the format of Object.Member. On the other hand, members declared within a module are publicly accessible by default, and can be accessed by any code that can access the module. This means that variables in a standard module are effectively global variables because they are visible from anywhere in your project, and they exist for the life of the program. Reusing classes and objectsObjects let you declare variables and procedures once and then reuse them whenever needed. For example, if you want to add a spelling checker to an application you could define all the variables and support functions to provide spell-checking functionality. If you create your spelling checker as a class, you can then reuse it in other applications by adding a reference to the compiled assembly. Better yet, you may be able to save yourself some work by using a spelling checker class that someone else has already developed. The .NET Framework provides many examples of components that are available for use. The following example uses the TimeZone class in the System namespace. TimeZone provides members that allow you to retrieve information about the time zone of the current computer system. VBPublic Sub examineTimeZone()
Dim tz As System.TimeZone = System.TimeZone.CurrentTimeZone
Dim s As String = "Current time zone is "
s &= CStr(tz.GetUtcOffset(Now).Hours) & " hours and "
s &= CStr(tz.GetUtcOffset(Now).Minutes) & " minutes "
s &= "different from UTC (coordinated universal time)"
s &= vbCrLf & "and is currently "
If tz.IsDaylightSavingTime(Now) = False Then s &= "not "
s &= "on ""summer time""."
MsgBox(s)
End Sub
In the preceding example, the first Dim Statement declares an object variable of type TimeZone and assigns to it a TimeZone object returned by the CurrentTimeZone property. Relationships among objectsObjects can be related to each other in several ways. The principal kinds of relationship are hierarchical and containment. Hierarchical relationshipWhen classes are derived from more fundamental classes, they are said to have a hierarchical relationship. Class hierarchies are useful when describing items that are a subtype of a more general class. In the following example, suppose you want to define a special kind of Button that acts like a normal Button but also exposes a method that reverses the foreground and background colors. To define a class is derived from an already existing class
Derived classes inherit members from the class they are based on, allowing you to add complexity as you progress in a class hierarchy. For more information, see Inheritance Basics. Compiling the codeBe sure the compiler can access the class from which you intend to derive your new class. This might mean fully qualifying its name, as in the preceding example, or identifying its namespace in an Imports Statement (.NET Namespace and Type). If the class is in a different project, you might need to add a reference to that project. For more information, see Managing references in a project. Containment relationshipAnother way that objects can be related is a containment relationship. Container objects logically encapsulate other objects. For example, the OperatingSystem object logically contains a Version object, which it returns through its Version property. Note that the container object does not physically contain any other object. CollectionsOne particular type of object containment is represented by collections. Collections are groups of similar objects that can be enumerated. Visual Basic supports a specific syntax in the For Each...Next Statement that allows you to iterate through the items of a collection. Additionally, collections often allow you to use an Item[String] to retrieve elements by their index or by associating them with a unique string. Collections can be easier to use than arrays because they allow you to add or remove items without using indexes. Because of their ease of use, collections are often used to store forms and controls. Walkthrough: Defining ClassesThis walkthrough demonstrates how to define classes, which you can then use to create objects. It also shows you how to add properties and methods to the new class, and demonstrates how to initialize an object. Note Your computer might show different names or locations for some of the Visual Studio user interface elements in the following instructions. The Visual Studio edition that you have and the settings that you use determine these elements. For more information, see Personalizing the IDE. To define a class
To create a button to test the class
To run your application
See alsoOverloaded properties and methodsOverloading is the creation of more than one procedure, instance constructor, or property in a class with the same name but different argument types. Overloading usageOverloading is especially useful when your object model dictates that you employ identical names for procedures that operate on different data types. For example, a class that can display several different data types could have Display procedures that look like this: VBOverloads Sub Display(ByVal theChar As Char)
' Add code that displays Char data.
End Sub
Overloads Sub Display(ByVal theInteger As Integer)
' Add code that displays Integer data.
End Sub
Overloads Sub Display(ByVal theDouble As Double)
' Add code that displays Double data.
End Sub
Without overloading, you would need to create distinct names for each procedure, even though they do the same thing, as shown next: VBSub DisplayChar(ByVal theChar As Char)
' Add code that displays Char data.
End Sub
Sub DisplayInt(ByVal theInteger As Integer)
' Add code that displays Integer data.
End Sub
Sub DisplayDouble(ByVal theDouble As Double)
' Add code that displays Double data.
End Sub
Overloading makes it easier to use properties or methods because it provides a choice of data types that can be used. For example, the overloaded Display method discussed previously can be called with any of the following lines of code: VB' Call Display with a literal of type Char.
Display("9"c)
' Call Display with a literal of type Integer.
Display(9)
' Call Display with a literal of type Double.
Display(9.9R)
At run time, Visual Basic calls the correct procedure based on the data types of the parameters you specify. Overloading rulesYou create an overloaded member for a class by adding two or more properties or methods with the same name. Except for overloaded derived members, each overloaded member must have different parameter lists, and the following items cannot be used as a differentiating feature when overloading a property or procedure:
The Overloads keyword is optional when overloading, but if any overloaded member uses the Overloads keyword, then all other overloaded members with the same name must also specify this keyword. Derived classes can overload inherited members with members that have identical parameters and parameter types, a process known as shadowing by name and signature. If the Overloads keyword is used when shadowing by name and signature, the derived class's implementation of the member will be used instead of the implementation in the base class, and all other overloads for that member will be available to instances of the derived class. If the Overloads keyword is omitted when overloading an inherited member with a member that has identical parameters and parameter types, then the overloading is called shadowing by name. Shadowing by name replaces the inherited implementation of a member, and it makes all other overloads unavailable to instances of the derived class and its decedents. The Overloads and Shadows modifiers cannot both be used with the same property or method. ExampleThe following example creates overloaded methods that accept either a String or Decimal representation of a dollar amount and return a string containing the sales tax. To use this example to create an overloaded method
At run time, the compiler chooses the appropriate overloaded function that matches the parameters being used. When you click the button, the overloaded method is called first with a Price parameter that is a string and the message, "Price is a String. Tax is $5.12" is displayed. TaxAmount is called with a Decimal value the second time and the message, "Price is a Decimal. Tax is $5.12" is displayed. See also
Inheritance BasicsThe Inherits statement is used to declare a new class, called a derived class, based on an existing class, known as a base class. Derived classes inherit, and can extend, the properties, methods, events, fields, and constants defined in the base class. The following section describes some of the rules for inheritance, and the modifiers you can use to change the way classes inherit or are inherited:
Inheritance ModifiersVisual Basic introduces the following class-level statements and modifiers to support inheritance:
Overriding Properties and Methods in Derived ClassesBy default, a derived class inherits properties and methods from its base class. If an inherited property or method has to behave differently in the derived class it can be overridden. That is, you can define a new implementation of the method in the derived class. The following modifiers are used to control how properties and methods are overridden:
Suppose you want to define classes to handle payroll. You could define a generic Payroll class that contains a RunPayroll method that calculates payroll for a typical week. You could then use Payroll as a base class for a more specialized BonusPayroll class, which could be used when distributing employee bonuses. The BonusPayroll class can inherit, and override, the PayEmployee method defined in the base Payroll class. The following example defines a base class, Payroll, and a derived class, BonusPayroll, which overrides an inherited method, PayEmployee. A procedure, RunPayroll, creates and then passes a Payroll object and a BonusPayroll object to a function, Pay, that executes the PayEmployee method of both objects. VBConst BonusRate As Decimal = 1.45D
Const PayRate As Decimal = 14.75D
Class Payroll
Overridable Function PayEmployee(
ByVal HoursWorked As Decimal,
ByVal PayRate As Decimal) As Decimal
PayEmployee = HoursWorked * PayRate
End Function
End Class
Class BonusPayroll
Inherits Payroll
Overrides Function PayEmployee(
ByVal HoursWorked As Decimal,
ByVal PayRate As Decimal) As Decimal
' The following code calls the original method in the base
' class, and then modifies the returned value.
PayEmployee = MyBase.PayEmployee(HoursWorked, PayRate) * BonusRate
End Function
End Class
Sub RunPayroll()
Dim PayrollItem As Payroll = New Payroll
Dim BonusPayrollItem As New BonusPayroll
Dim HoursWorked As Decimal = 40
MsgBox("Normal pay is: " &
PayrollItem.PayEmployee(HoursWorked, PayRate))
MsgBox("Pay with bonus is: " &
BonusPayrollItem.PayEmployee(HoursWorked, PayRate))
End Sub
The MyBase KeywordThe MyBase keyword behaves like an object variable that refers to the base class of the current instance of a class. MyBase is frequently used to access base class members that are overridden or shadowed in a derived class. In particular, MyBase.New is used to explicitly call a base class constructor from a derived class constructor. For example, suppose you are designing a derived class that overrides a method inherited from the base class. The overridden method can call the method in the base class and modify the return value as shown in the following code fragment: VBClass DerivedClass
Inherits BaseClass
Public Overrides Function CalculateShipping(
ByVal Dist As Double,
ByVal Rate As Double) As Double
' Call the method in the base class and modify the return value.
Return MyBase.CalculateShipping(Dist, Rate) * 2
End Function
End Class
The following list describes restrictions on using MyBase:
For more information and another example, see How to: Access a Variable Hidden by a Derived Class. The MyClass KeywordThe MyClass keyword behaves like an object variable that refers to the current instance of a class as originally implemented. MyClass resembles Me, but every method and property call on MyClass is treated as if the method or property were NotOverridable. Therefore, the method or property is not affected by overriding in a derived class.
The following example compares Me and MyClass. VBClass baseClass
Public Overridable Sub testMethod()
MsgBox("Base class string")
End Sub
Public Sub useMe()
' The following call uses the calling class's method, even if
' that method is an override.
Me.testMethod()
End Sub
Public Sub useMyClass()
' The following call uses this instance's method and not any
' override.
MyClass.testMethod()
End Sub
End Class
Class derivedClass : Inherits baseClass
Public Overrides Sub testMethod()
MsgBox("Derived class string")
End Sub
End Class
Class testClasses
Sub startHere()
Dim testObj As derivedClass = New derivedClass()
' The following call displays "Derived class string".
testObj.useMe()
' The following call displays "Base class string".
testObj.useMyClass()
End Sub
End Class
Even though derivedClass overrides testMethod, the MyClass keyword in useMyClass nullifies the effects of overriding, and the compiler resolves the call to the base class version of testMethod. See alsoObject Lifetime: How Objects Are Created and DestroyedAn instance of a class, an object, is created by using the New keyword. Initialization tasks often must be performed on new objects before they are used. Common initialization tasks include opening files, connecting to databases, and reading values of registry keys. Visual Basic controls the initialization of new objects using procedures called constructors (special methods that allow control over initialization). After an object leaves scope, it is released by the common language runtime (CLR). Visual Basic controls the release of system resources using procedures called destructors. Together, constructors and destructors support the creation of robust and predictable class libraries. Using Constructors and DestructorsConstructors and destructors control the creation and destruction of objects. The Sub New and Sub Finalize procedures in Visual Basic initialize and destroy objects; they replace the Class_Initialize and Class_Terminate methods used in Visual Basic 6.0 and earlier versions. Sub NewThe Sub New constructor can run only once when a class is created. It cannot be called explicitly anywhere other than in the first line of code of another constructor from either the same class or from a derived class. Furthermore, the code in the Sub New method always runs before any other code in a class. Visual Basic implicitly creates a Sub New constructor at run time if you do not explicitly define a Sub New procedure for a class. To create a constructor for a class, create a procedure named Sub New anywhere in the class definition. To create a parameterized constructor, specify the names and data types of arguments to Sub New just as you would specify arguments for any other procedure, as in the following code: VBSub New(ByVal s As String) Constructors are frequently overloaded, as in the following code: VBSub New(ByVal s As String, i As Integer) When you define a class derived from another class, the first line of a constructor must be a call to the constructor of the base class, unless the base class has an accessible constructor that takes no parameters. A call to the base class that contains the above constructor, for example, would be MyBase.New(s). Otherwise, MyBase.New is optional, and the Visual Basic runtime calls it implicitly. After you write the code to call the parent object's constructor, you can add any additional initialization code to the Sub New procedure. Sub New can accept arguments when called as a parameterized constructor. These parameters are passed from the procedure calling the constructor, for example, Dim AnObject As New ThisClass(X). Sub FinalizeBefore releasing objects, the CLR automatically calls the Finalize method for objects that define a Sub Finalize procedure. The Finalize method can contain code that needs to execute just before an object is destroyed, such as code for closing files and saving state information. There is a slight performance penalty for executing Sub Finalize, so you should define a Sub Finalize method only when you need to release objects explicitly. Note The garbage collector in the CLR does not (and cannot) dispose of unmanaged objects, objects that the operating system executes directly, outside the CLR environment. This is because different unmanaged objects must be disposed of in different ways. That information is not directly associated with the unmanaged object; it must be found in the documentation for the object. A class that uses unmanaged objects must dispose of them in its Finalize method. The Finalize destructor is a protected method that can be called only from the class it belongs to, or from derived classes. The system calls Finalize automatically when an object is destroyed, so you should not explicitly call Finalize from outside of a derived class's Finalize implementation. Unlike Class_Terminate, which executes as soon as an object is set to nothing, there is usually a delay between when an object loses scope and when Visual Basic calls the Finalize destructor. Visual Basic .NET allows for a second kind of destructor, IDisposable.Dispose, which can be explicitly called at any time to immediately release resources. Note A Finalize destructor should not throw exceptions, because they cannot be handled by the application and can cause the application to terminate. How New and Finalize Methods Work in a Class HierarchyWhenever an instance of a class is created, the common language runtime (CLR) attempts to execute a procedure named New, if it exists in that object. New is a type of procedure called a constructor that is used to initialize new objects before any other code in an object executes. A New constructor can be used to open files, connect to databases, initialize variables, and take care of any other tasks that need to be done before an object can be used. When an instance of a derived class is created, the Sub New constructor of the base class executes first, followed by constructors in derived classes. This happens because the first line of code in a Sub New constructor uses the syntax MyBase.New()to call the constructor of the class immediately above itself in the class hierarchy. The Sub New constructor is then called for each class in the class hierarchy until the constructor for the base class is reached. At that point, the code in the constructor for the base class executes, followed by the code in each constructor in all derived classes and the code in the most derived classes is executed last.
When an object is no longer needed, the CLR calls the Finalize method for that object before freeing its memory. The Finalize method is called a destructor because it performs cleanup tasks, such as saving state information, closing files and connections to databases, and other tasks that must be done before releasing the object.
IDisposable InterfaceClass instances often control resources not managed by the CLR, such as Windows handles and database connections. These resources must be disposed of in the Finalize method of the class, so that they will be released when the object is destroyed by the garbage collector. However, the garbage collector destroys objects only when the CLR requires more free memory. This means that the resources may not be released until long after the object goes out of scope. To supplement garbage collection, your classes can provide a mechanism to actively manage system resources if they implement the IDisposable interface. IDisposable has one method, Dispose, which clients should call when they finish using an object. You can use the Dispose method to immediately release resources and perform tasks such as closing files and database connections. Unlike the Finalize destructor, the Dispose method is not called automatically. Clients of a class must explicitly call Dispose when you want to immediately release resources. Implementing IDisposableA class that implements the IDisposable interface should include these sections of code:
Deriving from a Class that Implements IDisposableA class that derives from a base class that implements the IDisposable interface does not need to override any of the base methods unless it uses additional resources that need to be disposed. In that situation, the derived class should override the base class's Dispose(disposing) method to dispose of the derived class's resources. This override must call the base class's Dispose(disposing) method. VBProtected Overrides Sub Dispose(ByVal disposing As Boolean)
If Not Me.disposed Then
If disposing Then
' Insert code to free managed resources.
End If
' Insert code to free unmanaged resources.
End If
MyBase.Dispose(disposing)
End Sub
A derived class should not override the base class's Dispose and Finalize methods. When those methods are called from an instance of the derived class, the base class's implementation of those methods call the derived class's override of the Dispose(disposing) method. Garbage Collection and the Finalize DestructorThe .NET Framework uses the reference-tracing garbage collection system to periodically release unused resources. Visual Basic 6.0 and earlier versions used a different system called reference counting to manage resources. Although both systems perform the same function automatically, there are a few important differences. The CLR periodically destroys objects when the system determines that such objects are no longer needed. Objects are released more quickly when system resources are in short supply, and less frequently otherwise. The delay between when an object loses scope and when the CLR releases it means that, unlike with objects in Visual Basic 6.0 and earlier versions, you cannot determine exactly when the object will be destroyed. In such a situation, objects are said to have non-deterministic lifetime. In most cases, non-deterministic lifetime does not change how you write applications, as long as you remember that the Finalize destructor may not immediately execute when an object loses scope. Another difference between the garbage-collection systems involves the use of Nothing. To take advantage of reference counting in Visual Basic 6.0 and earlier versions, programmers sometimes assigned Nothing to object variables to release the references those variables held. If the variable held the last reference to the object, the object's resources were released immediately. In later versions of Visual Basic, while there may be cases in which this procedure is still valuable, performing it never causes the referenced object to release its resources immediately. To release resources immediately, use the object's Dispose method, if available. The only time you should set a variable to Nothing is when its lifetime is long relative to the time the garbage collector takes to detect orphaned objects. See also
Object Initializers: Named and Anonymous TypesObject initializers enable you to specify properties for a complex object by using a single expression. They can be used to create instances of named types and of anonymous types. DeclarationsDeclarations of instances of named and anonymous types can look almost identical, but their effects are not the same. Each category has abilities and restrictions of its own. The following example shows a convenient way to declare and initialize an instance of a named class, Customer, by using an object initializer list. Notice that the name of the class is specified after the keyword New. VBDim namedCust = New Customer With {.Name = "Terry Adams"}
An anonymous type has no usable name. Therefore an instantiation of an anonymous type cannot include a class name. VBDim anonymousCust = New With {.Name = "Hugo Garcia"}
The requirements and results of the two declarations are not the same. For namedCust, a Customer class that has a Name property must already exist, and the declaration creates an instance of that class. For anonymousCust, the compiler defines a new class that has one property, a string called Name, and creates a new instance of that class. Named TypesObject initializers provide a simple way to call the constructor of a type and then set the values of some or all properties in a single statement. The compiler invokes the appropriate constructor for the statement: the parameterless constructor if no arguments are presented, or a parameterized constructor if one or more arguments are sent. After that, the specified properties are initialized in the order in which they are presented in the initializer list. Each initialization in the initializer list consists of the assignment of an initial value to a member of the class. The names and data types of the members are determined when the class is defined. In the following examples, the Customer class must exist, and must have members named Name and City that can accept string values. VBDim cust0 As Customer = New Customer With {.Name = "Toni Poe",
.City = "Louisville"}
Alternatively, you can obtain the same result by using the following code: VBDim cust1 As New Customer With {.Name = "Toni Poe",
.City = "Louisville"}
Each of these declarations is equivalent to the following example, which creates a Customer object by using the parameterless constructor, and then specifies initial values for the Name and City properties by using a With statement. VBDim cust2 As New Customer()
With cust2
.Name = "Toni Poe"
.City = "Louisville"
End With
If the Customer class contains a parameterized constructor that enables you to send in a value for Name, for example, you can also declare and initialize a Customer object in the following ways: VBDim cust3 As Customer =
New Customer("Toni Poe") With {.City = "Louisville"}
' --or--
Dim cust4 As New Customer("Toni Poe") With {.City = "Louisville"}
You do not have to initialize all properties, as the following code shows. VBDim cust5 As Customer = New Customer With {.Name = "Toni Poe"}
However, the initialization list cannot be empty. Uninitialized properties retain their default values. Type Inference with Named TypesYou can shorten the code for the declaration of cust1 by combining object initializers and local type inference. This enables you to omit the As clause in the variable declaration. The data type of the variable is inferred from the type of the object that is created by the assignment. In the following example, the type of cust6 is Customer. VBDim cust6 = New Customer With {.Name = "Toni Poe",
.City = "Louisville"}
Remarks About Named Types
Anonymous TypesAnonymous types use object initializers to create instances of new types that you do not explicitly define and name. Instead, the compiler generates a type according to the properties you designate in the object initializer list. Because the name of the type is not specified, it is referred to as an anonymous type. For example, compare the following declaration to the earlier one for cust6. VBDim cust13 = New With {.Name = "Toni Poe",
.City = "Louisville"}
The only difference syntactically is that no name is specified after New for the data type. However, what happens is quite different. The compiler defines a new anonymous type that has two properties, Name and City, and creates an instance of it with the specified values. Type inference determines the types of Name and City in the example to be strings. Caution The name of the anonymous type is generated by the compiler, and may vary from compilation to compilation. Your code should not use or rely on the name of an anonymous type. Because the name of the type is not available, you cannot use an As clause to declare cust13. Its type must be inferred. Without using late binding, this limits the use of anonymous types to local variables. Anonymous types provide critical support for LINQ queries. For more information about the use of anonymous types in queries, see Anonymous Types and Introduction to LINQ in Visual Basic. Remarks About Anonymous Types
See also
How to: Declare an Object by Using an Object InitializerObject initializers enable you to declare and instantiate an instance of a class in a single statement. In addition, you can initialize one or more members of the instance at the same time, without invoking a parameterized constructor. When you use an object initializer to create an instance of a named type, the parameterless constructor for the class is called, followed by initialization of designated members in the order you specify. The following procedure shows how to create an instance of a Student class in three different ways. The class has first name, last name, and class year properties, among others. Each of the three declarations creates a new instance of Student, with property First set to "Michael", property Last set to "Tucker", and all other members set to their default values. The result of each declaration in the procedure is equivalent to the following example, which does not use an object initializer. VBDim student0 As New Student
With student0
.First = "Michael"
.Last = "Tucker"
End With
For an implementation of the Student class, see How to: Create a List of Items. You can copy the code from that topic to set up the class and create a list of Student objects to work with. To create an object of a named class by using an object initializer
See also
Anonymous TypesVisual Basic supports anonymous types, which enable you to create objects without writing a class definition for the data type. Instead, the compiler generates a class for you. The class has no usable name, inherits directly from Object, and contains the properties you specify in declaring the object. Because the name of the data type is not specified, it is referred to as an anonymous type. The following example declares and creates variable product as an instance of an anonymous type that has two properties, Name and Price. VB' Variable product is an instance of a simple anonymous type.
Dim product = New With {Key .Name = "paperclips", .Price = 1.29}
A query expression uses anonymous types to combine columns of data selected by a query. You cannot define the type of the result in advance, because you cannot predict the columns a particular query might select. Anonymous types enable you to write a query that selects any number of columns, in any order. The compiler creates a data type that matches the specified properties and the specified order. In the following examples, products is a list of product objects, each of which has many properties. Variable namePriceQuery holds the definition of a query that, when it is executed, returns a collection of instances of an anonymous type that has two properties, Name and Price. VBDim namePriceQuery = From prod In products
Select prod.Name, prod.Price
Variable nameQuantityQuery holds the definition of a query that, when it is executed, returns a collection of instances of an anonymous type that has two properties, Name and OnHand. VBDim nameQuantityQuery = From prod In products
Select prod.Name, prod.OnHand
For more information about the code created by the compiler for an anonymous type, see Anonymous Type Definition. Caution The name of the anonymous type is compiler generated and may vary from compilation to compilation. Your code should not use or rely on the name of an anonymous type because the name might change when a project is recompiled. Declaring an Anonymous TypeThe declaration of an instance of an anonymous type uses an initializer list to specify the properties of the type. You can specify only properties when you declare an anonymous type, not other class elements such as methods or events. In the following example, product1 is an instance of an anonymous type that has two properties: Name and Price. VB' Variable product1 is an instance of a simple anonymous type.
Dim product1 = New With {.Name = "paperclips", .Price = 1.29}
' -or-
' product2 is an instance of an anonymous type with key properties.
Dim product2 = New With {Key .Name = "paperclips", Key .Price = 1.29}
If you designate properties as key properties, you can use them to compare two anonymous type instances for equality. However, the values of key properties cannot be changed. See the Key Properties section later in this topic for more information. Notice that declaring an instance of an anonymous type is like declaring an instance of a named type by using an object initializer: VB' Variable product3 is an instance of a class named Product.
Dim product3 = New Product With {.Name = "paperclips", .Price = 1.29}
For more information about other ways to specify anonymous type properties, see How to: Infer Property Names and Types in Anonymous Type Declarations. Key PropertiesKey properties differ from non-key properties in several fundamental ways:
EqualityInstances of anonymous types can be equal only if they are instances of the same anonymous type. The compiler treats two instances as instances of the same type if they meet the following conditions:
An instance of an anonymous types that has no key properties is equal only to itself. VB' prod1 and prod2 have no key values.
Dim prod1 = New With {.Name = "paperclips", .Price = 1.29}
Dim prod2 = New With {.Name = "paperclips", .Price = 1.29}
' The following line displays False, because prod1 and prod2 have no
' key properties.
Console.WriteLine(prod1.Equals(prod2))
' The following statement displays True because prod1 is equal to itself.
Console.WriteLine(prod1.Equals(prod1))
Two instances of the same anonymous type are equal if the values of their key properties are equal. The following examples illustrate how equality is tested. VBDim prod3 = New With {Key .Name = "paperclips", Key .Price = 1.29}
Dim prod4 = New With {Key .Name = "paperclips", Key .Price = 1.29}
' The following line displays True, because prod3 and prod4 are
' instances of the same anonymous type, and the values of their
' key properties are equal.
Console.WriteLine(prod3.Equals(prod4))
Dim prod5 = New With {Key .Name = "paperclips", Key .Price = 1.29}
Dim prod6 = New With {Key .Name = "paperclips", Key .Price = 1.29,
.OnHand = 423}
' The following line displays False, because prod5 and prod6 do not
' have the same properties.
Console.WriteLine(prod5.Equals(prod6))
Dim prod7 = New With {Key .Name = "paperclips", Key .Price = 1.29,
.OnHand = 24}
Dim prod8 = New With {Key .Name = "paperclips", Key .Price = 1.29,
.OnHand = 423}
' The following line displays True, because prod7 and prod8 are
' instances of the same anonymous type, and the values of their
' key properties are equal. The equality check does not compare the
' values of the non-key field.
Console.WriteLine(prod7.Equals(prod8))
Read-Only ValuesThe values of key properties cannot be changed. For example, in prod8 in the previous example, the Name and Price fields are read-only, but OnHand can be changed. VB' The following statement will not compile, because Name is a key ' property and its value cannot be changed. ' prod8.Name = "clamps" ' OnHand is not a Key property. Its value can be changed. prod8.OnHand = 22 Anonymous Types from Query ExpressionsQuery expressions do not always require the creation of anonymous types. When possible, they use an existing type to hold the column data. This occurs when the query returns either whole records from the data source, or only one field from each record. In the following code examples, customers is a collection of objects of a Customer class. The class has many properties, and you can include one or more of them in the query result, in any order. In the first two examples, no anonymous types are required because the queries select elements of named types:
However, appropriate named types are not always available. You might want to select customer names and addresses for one purpose, customer ID numbers and locations for another, and customer names, addresses, and order histories for a third. Anonymous types enable you to select any combination of properties, in any order, without first declaring a new named type to hold the result. Instead, the compiler creates an anonymous type for each compilation of properties. The following query selects only the customer's name and ID number from each Customer object in customers. Therefore, the compiler creates an anonymous type that contains only those two properties. VBDim custs3 = From cust In customers
Select cust.Name, cust.ID
Both the names and the data types of the properties in the anonymous type are taken from the arguments to Select, cust.Name and cust.ID. The properties in an anonymous type that is created by a query are always key properties. When custs3 is executed in the following For Each loop, the result is a collection of instances of an anonymous type with two key properties, Name and ID. VBFor Each selectedCust In custs3
Console.WriteLine(selectedCust.ID & ": " & selectedCust.Name)
Next
The elements in the collection represented by custs3 are strongly typed, and you can use IntelliSense to navigate through the available properties and to verify their types. For more information, see Introduction to LINQ in Visual Basic. Deciding Whether to Use Anonymous TypesBefore you create an object as an instance of an anonymous class, consider whether that is the best option. For example, if you want to create a temporary object to contain related data, and you have no need for other fields and methods that a complete class might contain, an anonymous type is a good solution. Anonymous types are also convenient if you want a different selection of properties for each declaration, or if you want to change the order of the properties. However, if your project includes several objects that have the same properties, in a fixed order, you can declare them more easily by using a named type with a class constructor. For example, with an appropriate constructor, it is easier to declare several instances of a Product class than it is to declare several instances of an anonymous type. VB' Declaring instances of a named type.
Dim firstProd1 As New Product("paperclips", 1.29)
Dim secondProd1 As New Product("desklamp", 28.99)
Dim thirdProd1 As New Product("stapler", 5.09)
' Declaring instances of an anonymous type.
Dim firstProd2 = New With {Key .Name = "paperclips", Key .Price = 1.29}
Dim secondProd2 = New With {Key .Name = "desklamp", Key .Price = 28.99}
Dim thirdProd2 = New With {Key .Name = "stapler", Key .Price = 5.09}
Another advantage of named types is that the compiler can catch an accidental mistyping of a property name. In the previous examples, firstProd2, secondProd2, and thirdProd2 are intended to be instances of the same anonymous type. However, if you were to accidentally declare thirdProd2 in one of the following ways, its type would be different from that of firstProd2 and secondProd2. VB' Dim thirdProd2 = New With {Key .Name = "stapler", Key .Price = 5.09}
' Dim thirdProd2 = New With {Key .Name = "stapler", Key .Price = "5.09"}
' Dim thirdProd2 = New With {Key .Name = "stapler", .Price = 5.09}
More importantly, there are limitations on the use of anonymous types that do not apply to instances of named types. firstProd2, secondProd2, and thirdProd2 are instances of the same anonymous type. However, the name for the shared anonymous type is not available and cannot appear where a type name is expected in your code. For example, an anonymous type cannot be used to define a method signature, to declare another variable or field, or in any type declaration. As a result, anonymous types are not appropriate when you have to share information across methods. An Anonymous Type DefinitionIn response to the declaration of an instance of an anonymous type, the compiler creates a new class definition that contains the specified properties. If the anonymous type contains at least one key property, the definition overrides three members inherited from Object: Equals, GetHashCode, and ToString. The code produced for testing equality and determining the hash code value considers only the key properties. If the anonymous type contains no key properties, only ToString is overridden. Explicitly named properties of an anonymous type cannot conflict with these generated methods. That is, you cannot use .Equals, .GetHashCode, or .ToString to name a property. Anonymous type definitions that have at least one key property also implement the System.IEquatable<T> interface, where T is the type of the anonymous type. For more information about the code created by the compiler and the functionality of the overridden methods, see Anonymous Type Definition. See also
How to: Infer Property Names and Types in Anonymous Type DeclarationsAnonymous types provide no mechanism for directly specifying the data types of properties. Types of all properties are inferred. In the following example, the types of Name and Price are inferred directly from the values that are used to initialize them. VB' Variable product is an instance of a simple anonymous type.
Dim product = New With {Key .Name = "paperclips", .Price = 1.29}
Anonymous types can also infer property names and types from other sources. The sections that follow provide a list of the circumstances where inference is possible, and examples of situations where it is not. Successful InferenceAnonymous types can infer property names and types from the following sources:
Inference FailuresName inference will fail in many circumstances, including the following:
See alsoAnonymous Type DefinitionIn response to the declaration of an instance of an anonymous type, the compiler creates a new class definition that contains the specified properties for the type. Compiler-Generated CodeFor the following definition of product, the compiler creates a new class definition that contains properties Name, Price, and OnHand. VB' Variable product is an instance of an anonymous type.
Dim product = New With {Key .Name = "paperclips", Key .Price = 1.29, .OnHand = 24}
The class definition contains property definitions similar to the following. Notice that there is no Set method for the key properties. The values of key properties are read-only. VBPublic Class $Anonymous1
Private _name As String
Private _price As Double
Private _onHand As Integer
Public ReadOnly Property Name() As String
Get
Return _name
End Get
End Property
Public ReadOnly Property Price() As Double
Get
Return _price
End Get
End Property
Public Property OnHand() As Integer
Get
Return _onHand
End Get
Set(ByVal Value As Integer)
_onHand = Value
End Set
End Property
End Class
In addition, anonymous type definitions contain a parameterless constructor. Constructors that require parameters are not permitted. If an anonymous type declaration contains at least one key property, the type definition overrides three members inherited from Object: Equals, GetHashCode, and ToString. If no key properties are declared, only ToString is overridden. The overrides provide the following functionality:
Explicitly named properties of an anonymous type cannot conflict with these generated methods. That is, you cannot use .Equals, .GetHashCode, or .ToString to name a property. Anonymous type definitions that include at least one key property also implement the System.IEquatable<T> interface, where T is the type of the anonymous type. Note Anonymous type declarations create the same anonymous type only if they occur in the same assembly, their properties have the same names and the same inferred types, the properties are declared in the same order, and the same properties are marked as key properties. See alsoSource/Reference
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