`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.

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 Module0
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.

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.