C# does not support exception filter. However, VB and IL support it. To add exception filter to C#, we can build a function in VB or IL, then call it in C#.

The example below is one way we may implement the function to support exception filter. It asks the caller to provide four delegates, and call them at the right places.

Imports Microsoft.VisualBasic

 

Namespace Utility

 

    Public Delegate Sub TryDelegate()

    Public Delegate Function FilterDelegate(ByVal exception As System.Exception) As Boolean

    Public Delegate Sub CatchDelegate(ByVal exception As System.Exception)

    Public Delegate Sub FinallyDelegate()

 

    Public Class ExceptionFilter

        Inherits System.Object

 

        Public Shared Sub TryFilterCatch(ByVal tryDelegate As TryDelegate, ByVal filterDelegate As FilterDelegate, ByVal catchDelegate As CatchDelegate, ByVal finallyDelegate As FinallyDelegate)

            Try

                tryDelegate()

            Catch ex As System.Exception When catchDelegate <> Nothing And filterDelegate(ex)

                catchDelegate(ex)

            Finally

                If (finallyDelegate <> Nothing) Then

                    finallyDelegate()

                End If

            End Try

        End Sub

 

    End Class

 

End Namespace

 

In C# we can call Utility.TryFilterCatch as the following:

using System;

using Utility;

 

namespace ExceptionFilterUsage

{

    class Program

    {

        static void Main(string[] args)

        {

            TryDelegate tryDelegate = delegate()

            {

                Console.WriteLine("In try.");

                throw new ApplicationException();

            };

 

            FilterDelegate filterDelegate = delegate(Exception e)

            {

                Console.WriteLine("In exception filter.");

 

                if (e is ApplicationException)

                {

                    Console.WriteLine("ApplicationException is thrown, which should never happen.");

                    return false;

                }

                return true;

            };

 

            CatchDelegate catchDelegate = delegate(Exception e)

            {

                Console.WriteLine("In Catch: Exception: {0}", e);

            };

 

            FinallyDelegate finallyDelegate = delegate()

            {

                Console.WriteLine("In finally.");

            };

 

            ExceptionFilter.TryFilterCatch(tryDelegate, filterDelegate, catchDelegate, finallyDelegate);

        }

    }

}