The CLR binder today allows you to either accidentally or intentionally load two versions of a given assembly in an AppDomain. The pain point of this design is that it fails at run time when specific operations are performed with respect to types that are references in both the assemblies which will be majority of the cases.

 

Let us make a small sample to illustrate the problem that you will encounter if you had two versions of an assembly in an AppDomain:

 

Let us call this DomSample.cs

 

using System;

using System.Reflection;

class DomSample {

    public static void Main() {

        try {

            Assembly a1 = Assembly.Load("Foo, Version=2.0.0.0, Culture=neutral, PublicKeyToken=1faea1974f697f94");

            Assembly a2 = Assembly.Load("Foo, Version=1.0.0.0,  Culture=neutral, PublicKeyToken=1faea1974f697f94");

            Type ty1 = Type.GetType("Foo, Foo, Version=1.0.0.0, Culture=neutral, PublicKeyToken=1faea1974f697f94");

            Type ty2 = Type.GetType("Foo, Foo, Version=2.0.0.0, Culture=neutral, PublicKeyToken=1faea1974f697f94");

            Console.WriteLine("V1 Type = {0}", ty1.Assembly.FullName.ToString());

            Console.WriteLine("V2 Type = {0}", ty2.Assembly.FullName.ToString());

            if (!Type.Equals(ty1, ty2)){

                Console.WriteLine("The types are different");

            }

            Foo t1 = (Foo)Activator.CreateInstance(ty1);

            Foo t2 = (Foo)Activator.CreateInstance(ty2);

        }

        catch (Exception e)

        {

            Console.WriteLine(e.ToString());

        }

    }

}

 

Let us call this v1/Helper.cs

 

using System;

using System.Reflection;

[assembly: System.Reflection.AssemblyVersion("1.0.0.0")]

public class Helper {

    public void HelperMethod() {

        Console.WriteLine("Version = {0}", (Assembly.GetExecutingAssembly()).ToString());

    }

}

 

Let us call this v2/Helper.cs

 

using System;

using System.Reflection;

[assembly: System.Reflection.AssemblyVersion("2.0.0.0")]

public class Helper {

    public void HelperMethod() {

        Console.WriteLine("Version = {0}", (Assembly.GetExecutingAssembly()).ToString());

    }

}

 

Let us compile the two versions of the helper methods and place them in the GAC using gacutil /I Helper.dll accordingly. Let us compile the DomSample.cs using one of the versions of the helper assembly and execute it.

 

C:\temp\AppDomain>DomMain.exe

V1 Type = Helper, Version=1.0.0.0, Culture=neutral, PublicKeyToken=1faea1974f697f94

V2 Type = Helper, Version=2.0.0.0, Culture=neutral, PublicKeyToken=1faea1974f697f94

The types are different

System.InvalidCastException: Unable to cast object of type 'Helper' to type 'Helper'.

   at DomSample.Main() in c:\temp\AppDomain\DomMain.cs:line 30

 

The two types that you created are different and when you try to cast these types you get an InvalidCastException.

 

Have any of you encountered this? What is your thought about this problem? I would be interested to hear more about this.