Throw Statement in VB.NET

In this article I will explain you about Throw Statement in VB.NET.
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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

In the above example we add a throw statement in the catch block basically rethrowing System.Exception causing the same exception to occur in the catch clause. So, if we have one more try block outside the inner try block, the outer try-catch block catches the exception.
 

One last thing about the throw statement: you will never need to throw system exceptions such as IndexOutOfRange or NullReferenceException, which are thrown normally by the runtime. The .NET framework developer specification requires that you don't throw these exceptions programmatically. However, if you really wanted to do it, you could write a piece of code that throws these exceptions programmatically and compiles and executes successfully.

Conclusion


Hope this article would have helped you in understanding Throw Statement in VB.NET

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