Control Flow and Error Handling in VS .NET

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Control Flow and Error Handling
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The Try Catch Finally Statement
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So far, you ve seen how the exception-throwing mechanism works and what informa tion an exception object carries. Now you re ready to fully appreciate the power and flexibility of the new Try Catch Finally block.
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The Catch Keyword
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Whenever you execute code that might throw an exception, you should enclose it in a Try End Try block. The portion of code between the Try keyword and the first Catch keyword is guarded against exceptions, and if an exception is thrown, Visual Basic passes the control to the first Catch block, which is also called an exception filter. In the Catch code block, you can examine the properties of the exception object and decide how to react to the error. Here s a simple example:
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Dim x, y As Integer Try x = x \ y If y is 0, the following statement is never executed. y = CInt(10 ^ x) Catch ex As Exception If ex.Message = Attempted to divide by zero. Then Deal with division by zero errors here. Else Deal here with other types of exceptions. End If End Try
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As soon as an error occurs or an exception is thrown, to comply with the new termi nology the program jumps to the Catch block, executes that code block, and then jumps to the first statement after the End Try keyword. Testing the Message string isn t the correct way to deal with exceptions, however. Instead, you should have multiple Catch blocks, each one testing a different exception object:
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Try x = x \ y y = CInt(10 ^ x) Catch ex As DivideByZeroException Deal here with divide by zero exceptions. Catch ex As OverflowException Deal here with overflow exceptions. Catch ex As Exception Deal here with all other exceptions. End Try
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Visual Basic compares the type of the exception object being thrown with the expres sions in Catch clauses in the order in which they appear, and it executes the first one
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Part I:
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The Basics
that matches. It s usually a good idea to have a final Catch expression that matches the System.Exception object because this code is guaranteed to execute if no previous Catch expression matches the exception. A Catch clause for the System.Exception object always matches any exception because all exception objects inherit from Sys tem.Exception. (This last Catch clause is conceptually similar to the Else clause in a Select Case block.) Because All Catch expressions are evaluated in the order in which they appear, you should test for the most specific exceptions first, followed by less specific ones. The test for the System.Exception object, if present, should be in the last Catch block because it matches any exception; consequently, no Catch block after it can ever execute. When sorting Catch blocks, though, you should have a look at the exception hierarchy depicted in Figure 3-2, and check that you never catch an exception object after catching its parent exception. For example, the Catch block for a DivideByZeroException object should never follow the Catch block for a less specific ArithmeticException object. The As expression in the Catch block is optional. You can omit it if you don t need to examine the exception object s properties to make a decision.
Try Catch ex As DivideByZeroException Catch ex As OverflowException Catch ex As ArithmeticException Catch less specific arithmetic exceptions here. Catch Console.WriteLine( An error has occurred. ) End Try
You can exit from a Try End Try structure at any time by calling the Exit Try state ment, which can appear inside the Try block or any Catch block. A closer look at the exception hierarchy reveals that the common language runtime can throw a few exceptions that inherit from ApplicationException or directly from Excep tion. For example, the PathTooLongException and IsolatedStorageException classes derive directly from Exception; a few exceptions in the System.IO namespace namely DirectoryNotFoundException, FileNotFoundException, and EndOfStreamException inherit from IOException, which in turn derives directly from Exception (and not from SystemException). Even worse, some exception classes in the System.Reflection namespace inherit from ApplicationException instead of SystemException. This means that you can t assume that you can trap all system-related exceptions using a Catch fil ter on SystemException. You also can t assume that a Catch clause on ApplicationEx ception will filter only application-related exceptions. The .NET runtime defines a few exceptions that occur in really exceptional (and cata strophic) circumstances namely, StackOverflowException, OutOfMemoryException,
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