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Exception Handling
This page was last reviewed on Oct 6, 2023.
Dot Net Perls
Exception. In VB.NET programs, many things can go wrong. Sometimes these errors are our fault. Other times, they are unavoidable, inescapable yet potentially harmful.
With Exceptions, we isolate and handle these errors. We use the Try and Catch keywords in the VB.NET language. Exception handling is powerful and convenient.
First example. To begin, we write a small program that tries to divide by zero. We use Integer.Parse here to avoid a compile-time-error.
Info The runtime itself throws the DivideByZeroException. In the Catch block, we display the exception Message.
Note The Try block is required to use a Catch block. If we use no try nor catch, the exception is still thrown.
But If we do not use a Catch statement, and none is found in calling methods, the exception will cause the program to terminate.
Module Module1 Sub Main() Try ' Try to divide by zero. Dim value As Integer = 1 / Integer.Parse("0") ' This statement is sadly not reached. Console.WriteLine("Hi") Catch ex As Exception ' Display the message. Console.WriteLine(ex.Message) End Try End Sub End Module
Arithmetic operation resulted in an overflow.
Throw. An Exception can be triggered directly by the .NET Framework. But often code will use an explicit Throw-statement. We can specify a message in the Exception constructor.
Module Module1 Sub Main() Try ' Throw a serious exception. Throw New Exception("Mega-error") Catch ex As Exception ' Display the exception's message. Console.WriteLine(ex.Message) End Try End Sub End Module
Mega-error
NullReferenceException. Strings can be Nothing (null). If you call a method on a Nothing string, you will get a NullReferenceException. You could catch this, with the Catch statement.
However It is more efficient to simply check for Nothing, with the "Not Nothing" construct. More information on Nothing is available.
Nothing
If
Dim value As String = Nothing Console.WriteLine(value.Length)
Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.
Dim value As String = Nothing If Not value Is Nothing Then Console.WriteLine(value.Length) End If
Finally. Try try-catch construct optionally has a third part. The finally-statement is always run, except when the program terminates for external reasons.
And Finally statements are run after catch blocks are reached. It can be used to execute cleanup code.
Tip The Using-statement, which ensures cleanup of resources, is implemented with the Finally statement. It is a form of syntactic sugar.
Here In this example, all 5 Console.WriteLine calls are reached. An exception is triggered in the Try-block.
Then The Catch-block's statements are executed. And finally, the Finally statement is reached.
Module Module1 Sub Main() Console.WriteLine(0) Try ' Reached. Console.WriteLine(1) ' An exception is thrown. Dim s As String = Nothing s = s.ToUpper() Catch ex As Exception ' Reached. Console.WriteLine(2) Finally ' Reached. Console.WriteLine(3) End Try Console.WriteLine(4) End Sub End Module
0 1 [in Try] 2 [in Catch] 3 [in Finally] 4
Performance. Speed is a critical consideration when using exceptions in VB.NET programs. The only fast exception is one that never occurs.
Info An exception that is thrown is slower than most other CPU-based operations.
So One strategy is to reserve exceptions for "exceptional" conditions, not normal ones. If a value may be zero, check it before dividing.
However If an important file is missing from your program's setup, an exception is warranted. This is an exceptional condition.
A summary. With computers, many things can go wrong. Exception handling isolates error checks in our code. This leads to programs that are still fast, but also simpler to read and maintain.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
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