Thottam R. Sriram

Type forwarding using TypeForwardedTo attribute in Runtime.CompilerServices

We used this feature to solve a problem internally and I was tempted to blog about this cool feature. Type forwarding allows you to move a type from one assembly to another without having to recompile the application that uses the original assembly.

Here is how this works:

Writing the original library (original.cs)

using System;

public class ClassToBeForwardedLater

{

    public void SomeMethod()

    {

        Console.WriteLine("Inside ClassToBeForwardedLater in Original.dll");

    }

}

Let us compile this to original.dll as below

csc /t:library original.cs

Building the application using the original assembly (application.exe)

using System;

class Application

{

    public static void Main()

    {

        ClassToBeForwardedLater ctbf = new ClassToBeForwardedLater();

        ctbf.SomeMethod();

    }

}

Let us compile the executable as below

csc /r:original.dll application.cs

Let us run application.exe. You will get the following output

Re-implementing the original assembly in a new assembly (newlibrary.cs)

using System;

public class ClassToBeForwardedLater

{

    public void SomeMethod()

    {

        Console.WriteLine("Inside ClassToBeForwarded in newlibrary");

    }

}

Let us compile the newlibrary.dll as below

csc /t:library newlibrary.cs

Re-publish the original assembly, but this time with a type forwarded

using System;

using System.Runtime.CompilerServices;

[assembly:TypeForwardedTo(typeof(ClassToBeForwardedLater))]

      Let us compile the original library as below

csc /t:library /r:newlibrary.dll original.cs

Let us run application.exe again and see what we get.

The method in the newlibrary is invoked though the application was built with a reference to original.dll. If you do ildasm on application.exe and view the metadata section you will see a reference to original.dll.