Throw Statement in VB.NET

All along we have been talking about catching exceptions, but you may have been wondering who is throwing them or how you can throw one yourself. If an exception occurs during the evaluation of an expression, the language runtime automatically throws the appropriate exception. If an exception must be thrown programmatically, you would use the throw statement. Below example of using the throw statement.

Throw Example

    Public Class ThrowTest
        Public Shared Sub fn(ByVal age As Int32)
            If age < 0 Then
            ' throw an argument out of range exception if the age is
            ' less than zero.
                Throw New ArgumentOutOfRangeException("Age Cannot Be Negative ")
            End If
        End Sub

        Shared Sub Main()
            Try
                fn(-10)
            Catch e As Exception
                Console.WriteLine([String].Concat(e.StackTrace, e.Message))
                Console.ReadLine()
            End Try
        End Sub
    End Class

In this example we have a function called fn, which takes age as an argument. In this function we check if the age is a negative value, and if so, throw ArgumentOutOfRangeException. As you can see, throwing an exception is a fairly simple task.

Let's modify this example, as shown in next given below example.

Example of Exception

    Imports System.Text
        Public Class ThrowTest
            Public Shared Sub fn(ByVal age As Int32)
                If age < 0 Then
                    Throw New ArgumentOutOfRangeException("Age Can Not Be Negative ")
                End If
            End Sub

            Shared Sub Main()
                Try
                    Try
                        fn(-10)
                    Catch e As Exception
                        Console.WriteLine([String].Concat(e.StackTrace, e.Message))
                    Throw
                        ' or we could also have called throw e ;
                    End Try
                Catch e As Exception
                    Console.WriteLine("In the outer catch")
                    ' Executing this statement would cause a
                    ' NullreferenceException
                    'Console.WriteLine(e.InnerException.Message);
                    Console.WriteLine([String].Concat(e.StackTrace, e.Message))
                End Try
                Console.ReadLine()
            End Sub
        End Class