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This article discusses the options for handling exceptions in Microsoft Visual Basic .NET or Microsoft Visual Basic 2005.
Visual Basic .NET or Visual Basic 2005 supports two different ways to keep an unexpected error from terminating an application: unstructured error handling and structured error handling. These events are run-time errors--also called exceptions-- that are responses to abnormal or exceptional conditions that are caused by the execution of a block of code.
Unstructured error handling is the name that is used in Visual Basic .NET or Visual Basic 2005 to refer to the error handling method that is used in Microsoft Visual Basic 6.0. Structured error handling is introduced for the first time to Visual Basic programmers in .NET and is performed by using the Try...Catch...Finally statement, which has long been a feature of other programming languages. Structured error handling introduces a simpler way to create and maintain programs with robust, comprehensive error handlers.
Although Visual Basic .NET or Visual Basic 2005 supports both methods, the methods cannot be implemented simultaneously in the same procedure. The only exception to this is the Error statement, which can be used in structured error handling. It is recommended that all error handling in Visual Basic .NET or Visual Basic 2005 be performed with structured error handling. Unstructured error handling can degrade the performance of the application and result in code that is difficult to debug and maintain.
In the case of a run-time error, both error handling methods look for a local error handler that is defined in a particular block of code. If no local error handler is present, the exception is propagated up the call stack, until a matching handler is found. The call stack represents all procedures that have been called prior to the current point of execution and have not yet been terminated. If no handler is found in the procedures in the call stack when an error occurs, the application is terminated.
Unstructured Error HandlingUnstructured error handling is implemented with the On Error statement, which is placed at the beginning of a code block to handle all possible exceptions that occur during the execution of the code. All Visual Basic 6.0 error handlers in .NET are objects that can be accessed by using the Microsoft.VisualBasic.Information.Err namespace. The handler is set to Nothing each time the procedure is called. You should place only one On Error statement in each procedure, because additional statements disable all previous handlers that are defined in that procedure.
On Error StatementThe On Error statement is used to enable an error-handling routine, disable an error handling routine, or specify where to branch the code in the event of an error.
Used to enable the error-handling routine, starting at the location that is specified by the line argument. The line argument can be either a line label or a line number that is located within the closing procedure. A run-time error activates the error handler and branches the control to the specified line. If the specified line is not located within the same procedure as the On Error statement, a compile error occurs.
To avoid unexpected behavior, place an Exit Sub statement, an Exit Function statement, or an Exit Property statement just before the line label or line number. This prevents the error-handling code from running when no error has occurred.
Disables the enabled error handler that is defined within the current procedure and resets it to Nothing.
Disables the enabled exception that is defined within the current procedure and resets it to Nothing.
Moves the control of execution to the statement that follows immediately after the statement that caused the run-time error to occur, and continues the execution from this point forward. This is the preferred form to use to access objects, rather than using the On Error GoTo statement.
In the following example code, the error handler is enabled on the first line of the routine with the On Error GoTo Unstructured statement. The location of the error handling routine is identified with the Unstructured line label. The error routine implements a simple Select Case statement that executes the corresponding block of code, depending on the error that occurred.
The Resume Next statement at the end of the error handling procedure returns control of the execution back to the line that follows the line that caused the error to occur.
The error handler is then disabled with the On Error GoTo 0 statement, followed by the On Error Resume Next statement, which reactivates the error handler. If a run-time error occurs, the statement causes the execution to branch to the line that immediately follows the line that caused the error to occur, the same way that the Resume Next statement does in the error handling routine. In this case, that line is the If statement that evaluates the error number and displays it to the user, as well as clearing the error object.
Structured Error HandlingWith structured error handling, Visual Basic now has an effective way to prevent unexpected errors from terminating the execution of the application. Structured error handling also provides the programmers with a simpler way to create robust applications that are easier to maintain.
Structured error handling is implemented in Visual Basic. NET or Visual Basic 2005 with a Try...Catch...Finally block of statements. The Try...Catch...Finally block provides, for the first time to Visual Basic, the capability of nested error handling.
Try...Catch...Finally StatementsThe following code demonstrates the structure of a Try...Catch...Finally statement.
The code that is expected to generate a run-time error should be placed in the Try block for monitoring by the error handler. If this code produces an error during execution, Visual Basic examines all of the Catch statements implemented within the Try...Catch...Finally block to find a condition that matches the error. If Visual Basic finds a matching condition, the control of execution is transferred to the first line of code within the Catch statement. If Visual Basic does not find a matching condition, the error is propagated to the outer Try...Catch...Finally statement. This statement can be located in the same procedure (nested statements) or in a previous procedure that called the one that produced an error. This process is repeated until a matching statement is found. If a matching statement is not found, an error is produced, and the application is terminated.
The Finally statement is executed last, regardless of whether any errors were found. In other words, if no matching Catch statement is found, the Finally statement executes prior to the propagation to the outer statements.
This hierarchy and propagation are demonstrated with the following code:
In the preceding example, an error is generated in a fnStructured() procedure. This is a Stack Overflow error, caused by division with a zero (0). This procedure implements two Try...Catch...Finally blocks of statements, but neither one has matching Catch statements, and the error is not caught. Before the control is propagated in the call stack, the code that is encapsulated in the Finally block is executed. The Try...Catch...Finally block that is implemented in the Sub Main has a general condition that catches all exceptions that are thrown in its Try block.
The following is the output of the preceding procedure:
fnStructured: Inner Finally executed ! fnStructured: Outer Finally executed ! Exception Information: System.OverflowException: Arithmetic operation resulted in an overflow. at StructuredError.StructuredError.fnStructured() in C:\Documents and Settings\heikkiri\My Documents\Visual Studio Projects\Temp\StructuredError\StructuredError.vb:line 21 at StructuredError.StructuredError.Main() in C:\Documents and Settings\heikkiri\My Documents\Visual Studio Projects\Temp\StructuredError\StructuredError.vb:line 5 Main: Finally executed !
The Err ObjectThe Err object, which is inherited from Microsoft Visual Basic 6.0, can only be used to catch errors in a procedure that implements unstructured error handling with the On Error statement.
The Err object contains information about a run-time error that occurred during the execution of the application. As the execution enters a procedure that contains error handling, the properties of the Err object are set to zero (0) or zero-length-string (""). If an error occurs during the execution, the properties are then set to provide unique information about the error that occurred, through its properties. An instance of the Err object can be obtained through a Microsoft.VisualBasic.Information.Err function; however, because the Err object is an intrinsic object with global scope, it is not necessary to create an instance of this object at run time.
In case of a run-time error, the properties of the Err object can be used to handle the error (depending on its type), as well as to display information to the user about the error that occurred. The following table shows the different properties that are associated with the Err object.
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The Err object has two methods associated with it: Raise and Clear.
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As stated earlier in this article, the preferred method of error handling in Visual Basic .NET or Visual Basic 2005 is structured error handling, which is implemented with a Try...Catch...Finally block. All exceptions that are caught in Visual Basic .NET or Visual Basic 2005 are derived from this class, for example, OverFlowException. In case of a run-time error, Visual Basic .NET or Visual Basic 2005 throws an Exception object that can be either system generated or custom generated. The method for catching these Exceptions is described in the "Structured Error Handling" section of this article.
When the exception is thrown, the global Err object is set to the corresponding values, and a new instance of an Exception object is created. This instance contains more information about the error that occurred. For example, the StackTrace property contains a list of methods that were called and led to the occurrence of the error.
The following table shows the public properties that are available for the exceptions that are caught in the structured error handling.
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The following table shows the public methods that are implemented by the Exception class.
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The Err object was retained in Visual Basic .NET or Visual Basic 2005 for compatibility reasons and to ease the migration from Visual Basic 6.0 to Visual Basic .NET or Visual Basic 2005 applications. It is recommended that all error handling in Visual Basic .NET or Visual Basic 2005 applications be performed by using structured error/exception handling.