Someone asked me the following today (paraphrased)

"If I have versions 1.0.0.0 and 2.2.2.2 of TestAsssebly both in the gac, and a program TestApp.exe that needs TestAssembly, can I instantiate classes from each assembly inside the same app domain.  Can I use both versions of the Foo class in my program?"

So I sat down to do a simple experiment.  Two versions of Test Assembly are created in the GAC and a console program is run.  Foo.ToString() returns the full name of the assembly that the Foo instance comes from.

Program:

using System;

using System.Collections.Generic;

using System.Text;

using System.Reflection;

using TestAssembly;

namespace TestApp

{

    class Program

    {

        static void Main(string[] args)

        {

            try

            {

                Console.WriteLine("Loading Foo...");

                Assembly foo1 =

                AppDomain.CurrentDomain.Load("TestAssembly, Version=1.0.0.0, Culture=neutral, PublicKeyToken=612615ef384d0cd0, processorArchitecture=MSIL");

                Foo f = new TestAssembly.Foo();

                Console.WriteLine("Typeof Foo: " + f.ToString());

                object f2 = foo1.CreateInstance("TestAssembly.Foo");

                Console.WriteLine("Typeof f2: " + f2.ToString());

                Foo f3 = (Foo)foo1.CreateInstance("TestAssembly.Foo");

                Console.WriteLine("Typeof f3: " + f3.ToString());

            }

            catch (Exception ex)

            {

                Console.WriteLine(ex);

            }

        }

    }

}

Output:

C:\Projects\StrongNameTest\TestApp\bin\Debug>TestApp.exe
Loading Foo...
Typeof Foo: TestAssembly.Foo TestAssembly, Version=2.2.2.2, Culture=neutral, Pub
licKeyToken=612615ef384d0cd0
Typeof f2: TestAssembly.Foo TestAssembly, Version=1.0.0.0, Culture=neutral, Publ
icKeyToken=612615ef384d0cd0
System.InvalidCastException: Unable to cast object of type 'TestAssembly.Foo' to
 type 'TestAssembly.Foo'.
   at TestApp.Program.Main(String[] args) in c:\Projects\StrongNameTest\TestApp\
Program.cs:line 25

The part in bold is entertaining, at least.  I have a strong suspician that if I interact with f3 as an Object only, using Reflection to Invoke properties and methods then I could in fact use all the version 1.0.0.0 Foo functionality (which is vast and important) within the same app domain.

I also hit a bit of a snag when testing this application.  I was previously unaware that Visual Studio does not let you add references to assemblies added to the GAC via gacutil on the local machine.  See: http://msdn2.microsoft.com/en-us/library/wkze6zky(VS.80).aspx