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VB.NET Strings
VB.NET StringsString Basics in Visual BasicThe String data type represents a series of characters (each representing in turn an instance of the Char data type). This topic introduces the basic concepts of strings in Visual Basic. String VariablesAn instance of a string can be assigned a literal value that represents a series of characters. For example: VBDim MyString As String MyString = "This is an example of the String data type" A String variable can also accept any expression that evaluates to a string. Examples are shown below: VBDim OneString As String Dim TwoString As String OneString = "one, two, three, four, five" ' Evaluates to "two". TwoString = OneString.Substring(5, 3) OneString = "1" ' Evaluates to "11". TwoString = OneString & "1" Any literal that is assigned to a String variable must be enclosed in quotation marks (""). This means that a quotation mark within a string cannot be represented by a quotation mark. For example, the following code causes a compiler error: VBDim myString As String ' This line would cause an error. ' myString = "He said, "Look at this example!"" This code causes an error because the compiler terminates the string after the second quotation mark, and the remainder of the string is interpreted as code. To solve this problem, Visual Basic interprets two quotation marks in a string literal as one quotation mark in the string. The following example demonstrates the correct way to include a quotation mark in a string: VB' The value of myString is: He said, "Look at this example!" myString = "He said, ""Look at this example!""" In the preceding example, the two quotation marks preceding the word Look become one quotation mark in the string. The three quotation marks at the end of the line represent one quotation mark in the string and the string termination character. String literals can contain multiple lines: VBDim x = "hello world" The resulting string contains newline sequences that you used in your string literal (vbcr, vbcrlf, etc.). You no longer need to use the old workaround: VBDim x = <xml><![CDATA[Hello World]]></xml>.Value Characters in StringsA string can be thought of as a series of Char values, and the String type has built-in functions that allow you to perform many manipulations on a string that resemble the manipulations allowed by arrays. Like all array in .NET Framework, these are zero-based arrays. You may refer to a specific character in a string through the Chars property, which provides a way to access a character by the position in which it appears in the string. For example: VBDim myString As String = "ABCDE" Dim myChar As Char ' The value of myChar is "D". myChar = myString.Chars(3) In the above example, the Chars property of the string returns the fourth character in the string, which is D, and assigns it to myChar. You can also get the length of a particular string through the Length property. If you need to perform multiple array-type manipulations on a string, you can convert it to an array of Char instances using the ToCharArray function of the string. For example: VBDim myString As String = "abcdefghijklmnop" Dim myArray As Char() = myString.ToCharArray The variable myArray now contains an array of Char values, each representing a character from myString. The Immutability of StringsA string is immutable, which means its value cannot be changed once it has been created. However, this does not prevent you from assigning more than one value to a string variable. Consider the following example: VBDim myString As String = "This string is immutable" myString = "Or is it?" Here, a string variable is created, given a value, and then its value is changed. More specifically, in the first line, an instance of type String is created and given the value This string is immutable. In the second line of the example, a new instance is created and given the value Or is it?, and the string variable discards its reference to the first instance and stores a reference to the new instance. Unlike other intrinsic data types, String is a reference type. When a variable of reference type is passed as an argument to a function or subroutine, a reference to the memory address where the data is stored is passed instead of the actual value of the string. So in the previous example, the name of the variable remains the same, but it points to a new and different instance of the String class, which holds the new value. See also
Types of String Manipulation Methods in Visual BasicThere are several different ways to analyze and manipulate your strings. Some of the methods are a part of the Visual Basic language, and others are inherent in the String class. Visual Basic Language and the .NET FrameworkVisual Basic methods are used as inherent functions of the language. They may be used without qualification in your code. The following example shows typical use of a Visual Basic string-manipulation command: VBDim aString As String = "SomeString" Dim bString As String ' Assign "meS" to bString. bString = Mid(aString, 3, 3) In this example, the Mid function performs a direct operation on aString and assigns the value to bString. For a list of Visual Basic string manipulation methods, see String Manipulation Summary. Shared Methods and Instance MethodsYou can also manipulate strings with the methods of the String class. There are two types of methods in String: shared methods and instance methods. Shared MethodsA shared method is a method that stems from the String class itself and does not require an instance of that class to work. These methods can be qualified with the name of the class (String) rather than with an instance of the String class. For example: VBDim aString As String = String.Copy("A literal string")
In the preceding example, the String.Copy method is a static method, which acts upon an expression it is given and assigns the resulting value to bString. Instance MethodsInstance methods, by contrast, stem from a particular instance of String and must be qualified with the instance name. For example: VBDim aString As String = "A String" Dim bString As String ' Assign "String" to bString. bString = aString.Substring(2, 6) In this example, the String.Substring method is a method of the instance of String (that is, aString). It performs an operation on aString and assigns that value to bString. For more information, see the documentation for the String class. See alsoNothing and Strings in Visual BasicThe Visual Basic runtime and the .NET Framework evaluate Nothing differently when it comes to strings. Visual Basic Runtime and the .NET FrameworkConsider the following example: VBDim MyString As String = "This is my string" Dim stringLength As Integer ' Explicitly set the string to Nothing. MyString = Nothing ' stringLength = 0 stringLength = Len(MyString) ' This line, however, causes an exception to be thrown. stringLength = MyString.Length The Visual Basic runtime usually evaluates Nothing as an empty string (""). The .NET Framework does not, however, and throws an exception whenever an attempt is made to perform a string operation on Nothing. See alsoHow Culture Affects Strings in Visual BasicThis Help page discusses how Visual Basic uses culture information to perform string conversions and comparisons. When to Use Culture-Specific StringsTypically, you should use culture-specific strings for all data presented to and read from users, and use culture-invariant strings for your application's internal data. For example, if your application asks users to enter a date as a string, it should expect users to format the strings according to their culture, and the application should convert the string appropriately. If your application then presents that date in its user interface, it should present it in the user's culture. However, if the application uploads the date to a central server, it should format the string according to one specific culture, to prevent confusion between potentially different date formats. Culture-Sensitive FunctionsAll of the Visual Basic string-conversion functions (except for the Str and Val functions) use the application's culture information to make sure that the conversions and comparisons are appropriate for the culture of the application's user. The key to successfully using string-conversion functions in applications that run on computers with different culture settings is to understand which functions use a specific culture setting, and which use the current culture setting. Notice that the application's culture settings are, by default, inherited from the culture settings of the operating system. For more information, see Asc, AscW, Chr, ChrW, Format, Hex, Oct, and Type Conversion Functions. The Str (converts numbers to strings) and Val (converts strings to numbers) functions do not use the application's culture information when converting between strings and numbers. Instead, they recognize only the period (.) as a valid decimal separator. The culturally-aware analogues of these functions are:
For more information, see Str and Val. Using a Specific CultureImagine that you are developing an application that sends a date (formatted as a string) to a Web service. In this case, your application must use a specific culture for the string conversion. To illustrate why, consider the result of using the date's ToString() method: If your application uses that method to format the date July 4, 2005, it returns "7/4/2005 12:00:00 AM" when run with the United States English (en-US) culture, but it returns "04.07.2005 00:00:00" when run with the German (de-DE) culture. When you need to perform a string conversion in a specific culture format, you should use the CultureInfo class that is built into the .NET Framework. You can create a new CultureInfo object for a specific culture by passing the culture's name to the CultureInfo constructor. The supported culture names are listed in the CultureInfo class Help page. Alternatively, you can get an instance of the invariant culture from the CultureInfo.InvariantCulture property. The invariant culture is based on the English culture, but there are some differences. For example, the invariant culture specifies a 24-hour clock instead of a 12-hour clock. To convert a date to the culture's string, pass the CultureInfo object to the date object's ToString(IFormatProvider) method. For example, the following code displays "07/04/2005 00:00:00", regardless of the application's culture settings. VBDim d As Date = #7/4/2005# MsgBox(d.ToString(System.Globalization.CultureInfo.InvariantCulture)) Note Date literals are always interpreted according to the English culture. Comparing StringsThere are two important situations where string comparisons are needed:
You can perform both types of comparisons with the Visual Basic StrComp function. Specify the optional Compare argument to control the type of comparison: Text for most input and output Binary for determining exact matches. The StrComp function returns an integer that indicates the relationship between the two compared strings based on the sorting order. A positive value for the result indicates that the first string is greater than the second string. A negative result indicates the first string is smaller, and zero indicates equality between the strings. VB' Defines variables. Dim testStr1 As String = "ABCD" Dim testStr2 As String = "abcd" Dim testComp As Integer ' The two strings sort equally. Returns 0. testComp = StrComp(testStr1, testStr2, CompareMethod.Text) ' testStr1 sorts before testStr2. Returns -1. testComp = StrComp(testStr1, testStr2, CompareMethod.Binary) ' testStr2 sorts after testStr1. Returns 1. testComp = StrComp(testStr2, testStr1, CompareMethod.Binary) You can also use the .NET Framework partner of the StrComp function, the String.Compare method. This is a static, overloaded method of the base string class. The following example illustrates how this method is used: VBDim myString As String = "Alphabetical" Dim secondString As String = "Order" Dim result As Integer result = String.Compare(myString, secondString) For finer control over how the comparisons are performed, you can use additional overloads of the Compare method. With the String.Compare method, you can use the comparisonType argument to specify which type of comparison to use.
Security ConsiderationsIf your application makes security decisions based on the result of a comparison or case-change operation, then the operation should use the String.Compare method, and pass Ordinal or OrdinalIgnoreCase for the comparisonType argument. See alsoInterpolated StringsUsed to construct strings. An interpolated string looks like a template string that contains interpolated expressions. An interpolated string returns a string that replaces the interpolated expressions that it contains with their string representations. This feature is available in Visual Basic 14 and later versions. The arguments of an interpolated string are easier to understand than a composite format string. For example, the interpolated string VBConsole.WriteLine($"Name = {name}, hours = {hours:hh}")
contains two interpolated expressions, '{name}' and '{hours:hh}'. The equivalent composite format string is: VBConsole.WriteLine("Name = {0}, hours = {1:hh}", name, hours);
The structure of an interpolated string is: VB$"<text> {<interpolated-expression> [,<field-width>] [:<format-string>] } <text> ..."
where:
Important You cannot have any white space between the $ and the " that starts the string. Doing so causes a compiler error. You can use an interpolated string anywhere you can use a string literal. The interpolated string is evaluated each time the code with the interpolated string executes. This allows you to separate the definition and evaluation of an interpolated string. To include a curly brace ("{" or "}") in an interpolated string, use two curly braces, "{{" or "}}". See the Implicit Conversions section for more details. If the interpolated string contains other characters with special meaning in an interpolated string, such as the quotation mark ("), colon (:), or comma (,), they should be escaped if they occur in literal text, or they should be included in an expression delimited by parentheses if they are language elements included in an interpolated expression. The following example escapes quotation marks to include them in the result string, and it uses parentheses to delimit the expression (age == 1 ? "" : "s") so that the colon is not interpreted as beginning a format string. VBPublic Module Example
Public Sub Main()
Dim name = "Horace"
Dim age = 34
Dim s1 = $"He asked, ""Is your name {name}?"", but didn't wait for a reply."
Console.WriteLine(s1)
Dim s2 = $"{name} is {age:D3} year{(If(age = 1, "", "s"))} old."
Console.WriteLine(s2)
End Sub
End Module
' The example displays the following output:
' He asked, "Is your name Horace?", but didn't wait for a reply.
' Horace is 034 years old.
' </Snippet1>
Implicit ConversionsThere are three implicit type conversions from an interpolated string:
See alsoZero-based vs. One-based String Access in Visual BasicThis topic compares how Visual Basic and the .NET Framework provide access to the characters in a string. The .NET Framework always provides zero-based access to the characters in a string, whereas Visual Basic provides zero-based and one-based access, depending on the function. One-BasedFor an example of a one-based Visual Basic function, consider the Mid function. It takes an argument that indicates the character position at which the substring will start, starting with position 1. The .NET Framework String.Substring method takes an index of the character in the string at which the substring is to start, starting with position 0. Thus, if you have a string "ABCDE", the individual characters are numbered 1,2,3,4,5 for use with the Mid function, but 0,1,2,3,4 for use with the String.Substring method. Zero-BasedFor an example of a zero-based Visual Basic function, consider the Split function. It splits a string and returns an array containing the substrings. The .NET Framework String.Split method also splits a string and returns an array containing the substrings. Because the Split function and Split method return .NET Framework arrays, they must be zero-based. See alsoHow to: Create Strings Using a StringBuilder in Visual BasicThis example constructs a long string from many smaller strings using the StringBuilder class. The StringBuilder class is more efficient than the &= operator for concatenating many strings. ExampleThe following example creates an instance of the StringBuilder class, appends 1,000 strings to that instance, and then returns its string representation. VBPrivate Function StringBuilderTest() As String
Dim builder As New System.Text.StringBuilder
For i As Integer = 1 To 1000
builder.Append("Step " & i & vbCrLf)
Next
Return builder.ToString
End Function
See alsoHow to: Search Within a StringThis example calls the IndexOf method on a String object to report the index of the first occurrence of a substring. ExampleVBDim SearchWithinThis As String = "ABCDEFGHIJKLMNOP" Dim SearchForThis As String = "DEF" Dim FirstCharacter As Integer = SearchWithinThis.IndexOf(SearchForThis) Compiling the CodeThis example requires:
Robust ProgrammingThe IndexOf method reports the location of the first character of the first occurrence of the substring. The index is 0-based, which means the first character of a string has an index of 0. If IndexOf does not find the substring, it returns -1. The IndexOf method is case-sensitive and uses the current culture. For optimal error control, you might want to enclose the string search in the Try block of a Try...Catch...Finally Statement construction. See alsoConverting Between Strings and Other Data Types in Visual BasicThis section describes how to convert strings into other data types. How to: Convert an Array of Bytes into a String in Visual BasicThis topic shows how to convert the bytes from a byte array into a string. ExampleThis example uses the GetString method of the Encoding.Unicode encoding class to convert all the bytes from a byte array into a string. VBPrivate Function UnicodeBytesToString(
ByVal bytes() As Byte) As String
Return System.Text.Encoding.Unicode.GetString(bytes)
End Function
You can choose from several encoding options to convert a byte array into a string:
See alsoHow to: Convert Strings into an Array of Bytes in Visual BasicThis topic shows how to convert a string into an array of bytes. ExampleThis example uses the GetBytes method of the Encoding.Unicode encoding class to convert a string into an array of bytes. VBPrivate Function UnicodeStringToBytes(
ByVal str As String) As Byte()
Return System.Text.Encoding.Unicode.GetBytes(str)
End Function
You can choose from several encoding options to convert a string into a byte array:
See alsoHow to: Create a String from An Array of Char ValuesThis example creates the string "abcd" from individual characters. ExampleVBPrivate Sub MakeStringFromCharacters()
Dim characters() As Char = {"a"c, "b"c, "c"c, "d"c}
Dim alphabet As New String(characters)
End Sub
Compiling the CodeThis method has no special requirements. The syntax "a"c, where a single c follows a single character in quotation marks, is used to create a character literal. Robust ProgrammingNull characters (equivalent to Chr(0)) in the string lead to unexpected results when using the string. The null character will be included with the string, but characters following the null character will not be displayed in some situations. See alsoHow to: Convert Hexadecimal Strings to NumbersThis example converts a hexadecimal string to an integer using the Convert.ToInt32 method. To convert a hexadecimal string to a number
See alsoHow to: Convert a String to an Array of Characters in Visual BasicSometimes it is useful to have data about the characters in your string and the positions of those characters within your string, such as when you are parsing a string. This example shows how you can get an array of the characters in a string by calling the string's ToCharArray method. ExampleThis example demonstrates how to split a string into a Char array, and how to split a string into a String array of its Unicode text characters. The reason for this distinction is that Unicode text characters can be composed of two or more Char characters (such as a surrogate pair or a combining character sequence). For more information, see TextElementEnumerator and The Unicode Standard. VBDim testString1 As String = "ABC" ' Create an array containing "A", "B", and "C". Dim charArray() As Char = testString1.ToCharArray ExampleIt is more difficult to split a string into its Unicode text characters, but this is necessary if you need information about the visual representation of a string. This example uses the SubstringByTextElements method to get information about the Unicode text characters that make up a string. VB' This string is made up of a surrogate pair (high surrogate
' U+D800 and low surrogate U+DC00) and a combining character
' sequence (the letter "a" with the combining grave accent).
Dim testString2 As String = ChrW(&HD800) & ChrW(&HDC00) & "a" & ChrW(&H300)
' Create and initialize a StringInfo object for the string.
Dim si As New System.Globalization.StringInfo(testString2)
' Create and populate the array.
Dim unicodeTestArray(si.LengthInTextElements) As String
For i As Integer = 0 To si.LengthInTextElements - 1
unicodeTestArray(i) = si.SubstringByTextElements(i, 1)
Next
See also
How to: Access Characters in Strings in Visual BasicThis example demonstrates how to use the Chars[Index] property to access the character at the specified location in a string. ExampleSometimes it is useful to have data about the characters in your string and the positions of those characters within your string. You can think of a string as an array of characters (Char instances); you can retrieve a particular character by referencing the index of that character through the Chars[Index] property. VBDim myString As String = "ABCDE" Dim myChar As Char ' Assign "D" to myChar. myChar = myString.Chars(3) The index parameter of the Chars[Index] property is zero-based. Robust ProgrammingThe Chars[Index] property returns the character at the specified position. However, some Unicode characters can be represented by more than one character. For more information on how to work with Unicode characters, see How to: Convert a String to an Array of Characters. The Chars[Index] property throws an IndexOutOfRangeException exception if the index parameter is greater than or equal to the length of the string, or if it is less than zero See also
Validating Strings in Visual BasicThis section discusses how to validate strings in Visual Basic. How to: Validate File Names and Paths in Visual BasicThis example returns a Boolean value that indicates whether a string represents a file name or path. The validation checks if the name contains characters that are not allowed by the file system. ExampleVBFunction IsValidFileNameOrPath(ByVal name As String) As Boolean
' Determines if the name is Nothing.
If name Is Nothing Then
Return False
End If
' Determines if there are bad characters in the name.
For Each badChar As Char In System.IO.Path.GetInvalidPathChars
If InStr(name, badChar) > 0 Then
Return False
End If
Next
' The name passes basic validation.
Return True
End Function
This example does not check if the name has incorrectly placed colons, or directories with no name, or if the length of the name exceeds the system-defined maximum length. It also does not check if the application has permission to access the file-system resource with the specified name. See alsoHow to: Validate Strings That Represent Dates or TimesThe following code example sets a Boolean value that indicates whether a string represents a valid date or time. ExampleVBDim isValidDate As Boolean = IsDate("01/01/03")
Dim isValidTime As Boolean = IsDate("9:30 PM")
Compiling the CodeReplace ("01/01/03") and "9:30 PM" with the date and time you want to validate. You can replace the string with another hard-coded string, with a String variable, or with a method that returns a string, such as InputBox. Robust ProgrammingUse this method to validate the string before trying to convert the String to a DateTime variable. By checking the date or time first, you can avoid generating an exception at run time. See alsoUsing Regular Expressions with the MaskedTextBox Control in Visual BasicThis example demonstrates how to convert simple regular expressions to work with the MaskedTextBox control. Description of the Masking LanguageThe standard MaskedTextBox masking language is based on the one used by the Masked Edit control in Visual Basic 6.0 and should be familiar to users migrating from that platform. The Mask property of the MaskedTextBox control specifies what input mask to use. The mask must be a string composed of one or more of the masking elements from the following table.
The decimal (.), thousandths (,), time (:), date (/), and currency ($) symbols default to displaying those symbols as defined by the application's culture. You can force them to display symbols for another culture by using the FormatProvider property. Regular Expressions and MasksAlthough you can use regular expressions and masks to validate user input, they are not completely equivalent. Regular expressions can express more complex patterns than masks, but masks can express the same information more succinctly and in a culturally relevant format. The following table compares four regular expressions and the equivalent mask for each.
See alsoWalkthrough: Validating That Passwords Are ComplexThis method checks for some strong-password characteristics and updates a string parameter with information about which checks the password fails. Passwords can be used in a secure system to authorize a user. However, the passwords must be difficult for unauthorized users to guess. Attackers can use a dictionary attack program, which iterates through all of the words in a dictionary (or multiple dictionaries in different languages) and tests whether any of the words work as a user's password. Weak passwords such as "Yankees" or "Mustang" can be guessed quickly. Stronger passwords, such as "?You'L1N3vaFiNdMeyeP@sSWerd!", are much less likely to be guessed. A password-protected system should ensure that users choose strong passwords. A strong password is complex (containing a mixture of uppercase, lowercase, numeric, and special characters) and is not a word. This example demonstrates how to verify complexity. ExampleCodeVB''' <summary>Determines if a password is sufficiently complex.</summary>
''' <param name="pwd">Password to validate</param>
''' <param name="minLength">Minimum number of password characters.</param>
''' <param name="numUpper">Minimum number of uppercase characters.</param>
''' <param name="numLower">Minimum number of lowercase characters.</param>
''' <param name="numNumbers">Minimum number of numeric characters.</param>
''' <param name="numSpecial">Minimum number of special characters.</param>
''' <returns>True if the password is sufficiently complex.</returns>
Function ValidatePassword(ByVal pwd As String,
Optional ByVal minLength As Integer = 8,
Optional ByVal numUpper As Integer = 2,
Optional ByVal numLower As Integer = 2,
Optional ByVal numNumbers As Integer = 2,
Optional ByVal numSpecial As Integer = 2) As Boolean
' Replace [A-Z] with \p{Lu}, to allow for Unicode uppercase letters.
Dim upper As New System.Text.RegularExpressions.Regex("[A-Z]")
Dim lower As New System.Text.RegularExpressions.Regex("[a-z]")
Dim number As New System.Text.RegularExpressions.Regex("[0-9]")
' Special is "none of the above".
Dim special As New System.Text.RegularExpressions.Regex("[^a-zA-Z0-9]")
' Check the length.
If Len(pwd) < minLength Then Return False
' Check for minimum number of occurrences.
If upper.Matches(pwd).Count < numUpper Then Return False
If lower.Matches(pwd).Count < numLower Then Return False
If number.Matches(pwd).Count < numNumbers Then Return False
If special.Matches(pwd).Count < numSpecial Then Return False
' Passed all checks.
Return True
End Function
Sub TestValidatePassword()
Dim password As String = "Password"
' Demonstrate that "Password" is not complex.
MsgBox(password & " is complex: " & ValidatePassword(password))
password = "Z9f%a>2kQ"
' Demonstrate that "Z9f%a>2kQ" is not complex.
MsgBox(password & " is complex: " & ValidatePassword(password))
End Sub
Compiling the CodeCall this method by passing the string that contains that password. This example requires:
SecurityIf you're moving the password across a network, you need to use a secure method for transferring data. For more information, see ASP.NET Web Application Security. You can improve the accuracy of the ValidatePassword function by adding additional complexity checks:
See alsoWalkthrough: Encrypting and Decrypting Strings in Visual BasicThis walkthrough shows you how to use the DESCryptoServiceProvider class to encrypt and decrypt strings using the cryptographic service provider (CSP) version of the Triple Data Encryption Standard (TripleDES) algorithm. The first step is to create a simple wrapper class that encapsulates the 3DES algorithm and stores the encrypted data as a base-64 encoded string. Then, that wrapper is used to securely store private user data in a publicly accessible text file. You can use encryption to protect user secrets (for example, passwords) and to make credentials unreadable by unauthorized users. This can protect an authorized user's identity from being stolen, which protects the user's assets and provides non-repudiation. Encryption can also protect a user's data from being accessed by unauthorized users. For more information, see Cryptographic Services. Important The Rijndael (now referred to as Advanced Encryption Standard [AES]) and Triple Data Encryption Standard (3DES) algorithms provide greater security than DES because they are more computationally intensive. For more information, see DES and Rijndael. To create the encryption wrapper
To test the encryption wrapper
See also
Source/Reference
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