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VB.NET Errors
  Error Types
 Syntax Errors
 Run-Time Errors
 Logic Errors
 See also
  Error Messages
 Run Time Errors
 Compile Time Errors
  Source/Reference

VB.NET Errors

Error Types

In Visual Basic, errors fall into one of three categories: syntax errors, run-time errors, and logic errors.

Syntax Errors

Syntax errors are those that appear while you write code. If you're using Visual Studio, Visual Basic checks your code as you type it in the Code Editor window and alerts you if you make a mistake, such as misspelling a word or using a language element improperly. If you compile from the command line, Visual Basic displays a compiler error with information about the syntax error. Syntax errors are the most common type of errors. You can fix them easily in the coding environment as soon as they occur.

Note

The Option Explicit statement is one means of avoiding syntax errors. It forces you to declare, in advance, all the variables to be used in the application. Therefore, when those variables are used in the code, any typographic errors are caught immediately and can be fixed.

Run-Time Errors

Run-time errors are those that appear only after you compile and run your code. These involve code that may appear to be correct in that it has no syntax errors, but that will not execute. For example, you might correctly write a line of code to open a file. But if the file does not exist, the application cannot open the file, and it throws an exception. You can fix most run-time errors by rewriting the faulty code or by using exception handling, and then recompiling and rerunning it.

Logic Errors

Logic errors are those that appear once the application is in use. They are most often faulty assumptions made by the developer, or unwanted or unexpected results in response to user actions. For example, a mistyped key might provide incorrect information to a method, or you may assume that a valid value is always supplied to a method when that is not the case. Although logic errors can be handled by using exception handling (for example, by testing whether an argument is Nothing and throwing an ArgumentNullException), most commonly they should be addressed by correcting the error in logic and recompiling the application.

See also

 

Error Messages

When you write, compile, or run a Visual Basic application, the following types of errors can occur:

  1. Design-time errors, which occur when you write an application in Visual Studio.

  2. Compile-time errors, which occur when you compile an application in Visual Studio or at a command prompt.

  3. Run-time errors, which occur when you run an application in Visual Studio or as a stand-alone executable file.

For information about how to troubleshoot a specific error, see Additional Resources for Visual Basic Programmers.

Run Time Errors

If a Visual Basic application tries to perform an action that the system can't execute, a run-time error occurs, and Visual Basic throws an Exception object. Visual Basic can generate custom errors of any data type, including Exception objects, by using the Throw statement. An application can identify the error by displaying the error number and message of a caught exception. If an error isn't caught, the application ends.

The code can trap and examine run-time errors. If you enclose the code that produces the error in a Try block, you can catch any thrown error within a matching Catch block. For information about how to trap errors at run time and respond to them in your code, see Try...Catch...Finally Statement.

Compile Time Errors

If the Visual Basic compiler encounters a problem in the code, a compile-time error occurs. In the Code Editor, you can easily identify which line of code caused the error because a wavy line appears under that line of code. The error message appears if you either point to the wavy underline or open the Error List, which also shows other messages.

If an identifier has a wavy underline and a short underline appears under the rightmost character, you can generate a stub for the class, constructor, method, property, field or enum. For more information, see Generate From Usage.

By resolving warnings from the Visual Basic compiler, you might be able to write code that runs faster and has fewer bugs. These warnings identify code that may cause errors when the application is run. For example, the compiler warns you if you try to invoke a member of an unassigned object variable, return from a function without setting the return value, or execute a Try block with errors in the logic to catch exceptions. For more information about warnings, including how to turn them on and off, see Configuring Warnings in Visual Basic.

Source/Reference


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References

  1. Active Server Pages,  , http://msdn.microsoft.com/en-us/library/aa286483.aspx
  2. ASP Overview,  , http://msdn.microsoft.com/en-us/library/ms524929%28v=vs.90%29.aspx
  3. ASP Best Practices,  , http://technet.microsoft.com/en-us/library/cc939157.aspx
  4. ASP Built-in Objects,  , http://msdn.microsoft.com/en-us/library/ie/ms524716(v=vs.90).aspx
  5. Response Object,  , http://msdn.microsoft.com/en-us/library/ms525405(v=vs.90).aspx
  6. Request Object,  , http://msdn.microsoft.com/en-us/library/ms524948(v=vs.90).aspx
  7. Server Object (IIS),  , http://msdn.microsoft.com/en-us/library/ms525541(v=vs.90).aspx
  8. Application Object (IIS),  , http://msdn.microsoft.com/en-us/library/ms525360(v=vs.90).aspx
  9. Session Object (IIS),  , http://msdn.microsoft.com/en-us/library/ms524319(8v=vs.90).aspx
  10. ASPError Object,  , http://msdn.microsoft.com/en-us/library/ms524942(v=vs.90).aspx
  11. ObjectContext Object (IIS),  , http://msdn.microsoft.com/en-us/library/ms525667(v=vs.90).aspx
  12. Debugging Global.asa Files,  , http://msdn.microsoft.com/en-us/library/aa291249(v=vs.71).aspx
  13. How to: Debug Global.asa files,  , http://msdn.microsoft.com/en-us/library/ms241868(v=vs.80).aspx
  14. Calling COM Components from ASP Pages,  , http://msdn.microsoft.com/en-us/library/ms524620(v=VS.90).aspx
  15. IIS ASP Scripting Reference,  , http://msdn.microsoft.com/en-us/library/ms524664(v=vs.90).aspx
  16. ASP Keywords,  , http://msdn.microsoft.com/en-us/library/ms524672(v=vs.90).aspx
  17. Creating Simple ASP Pages,  , http://msdn.microsoft.com/en-us/library/ms524741(v=vs.90).aspx
  18. Including Files in ASP Applications,  , http://msdn.microsoft.com/en-us/library/ms524876(v=vs.90).aspx
  19. ASP Overview,  , http://msdn.microsoft.com/en-us/library/ms524929(v=vs.90).aspx
  20. FileSystemObject Object,  , http://msdn.microsoft.com/en-us/library/z9ty6h50(v=vs.84).aspx
  21. http://msdn.microsoft.com/en-us/library/windows/desktop/ms675944(v=vs.85).aspx,  , ADO Object Model
  22. ADO Fundamentals,  , http://msdn.microsoft.com/en-us/library/windows/desktop/ms680928(v=vs.85).aspx
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