Using Side-by-Side assemblies to load the x64 or x32 version of a DLL

I created a simple solution that is able to load platform-specific assembly from an executable compiled as AnyCPU. The technique used can be summarized as follows:

1. Make sure default .NET assembly loading mechanism (“Fusion” engine) can’t find either x86 or x64 version of the platform-specific assembly
2. Before the main application attempts loading the platform-specific assembly, install a custom assembly resolver in the current AppDomain
3. Now when the main application needs the platform-specific assembly, Fusion engine will give up (because of step 1) and call our custom resolver (because of step 2); in the custom resolver we determine current platform and use directory-based lookup to load appropriate DLL.

To demonstrate this technique, I am attaching a short, command-line based tutorial. I tested the resulting binaries on Windows XP x86 and then Vista SP1 x64 (by copying the binaries over, just like your deployment).

Note 1: “csc.exe” is a C-sharp compiler. This tutorial assumes it is in your path (my tests were using “C:\WINDOWS\Microsoft.NET\Framework\v3.5\csc.exe”)

Note 2: I recommend you create a temporary folder for the tests and run command line (or powershell) whose current working directory is set to this location, e.g.

(cmd.exe)
C:
mkdir \TEMP\CrossPlatformTest
cd \TEMP\CrossPlatformTest

Step 1: The platform-specific assembly is represented by a simple C# class library:

// file 'library.cs' in C:\TEMP\CrossPlatformTest
namespace Cross.Platform.Library
{
    public static class Worker
    {
        public static void Run()
        {
            System.Console.WriteLine("Worker is running");
            System.Console.WriteLine("(Enter to continue)");
            System.Console.ReadLine();
        }
    }
}

Step 2: We compile platform-specific assemblies using simple command-line commands:

(cmd.exe from Note 2)
mkdir platform\x86
csc /out:platform\x86\library.dll /target:library /platform:x86 library.cs
mkdir platform\amd64
csc /out:platform\amd64\library.dll /target:library /platform:x64 library.cs

Step 3: Main program is split into two parts. “Bootstrapper” contains main entry point for the executable and it registers a custom assembly resolver in current appdomain:

// file 'bootstrapper.cs' in C:\TEMP\CrossPlatformTest
namespace Cross.Platform.Program
{
    public static class Bootstrapper
    {
        public static void Main()
        {
            System.AppDomain.CurrentDomain.AssemblyResolve += CustomResolve;
            App.Run();
        }

        private static System.Reflection.Assembly CustomResolve(
            object sender,
            System.ResolveEventArgs args)
        {
            if (args.Name.StartsWith("library"))
            {
                string fileName = System.IO.Path.GetFullPath(
                    "platform\\"
                    + System.Environment.GetEnvironmentVariable("PROCESSOR_ARCHITECTURE")
                    + "\\library.dll");
                System.Console.WriteLine(fileName);
                if (System.IO.File.Exists(fileName))
                {
                    return System.Reflection.Assembly.LoadFile(fileName);
                }
            }
            return null;
        }
    }
}

“Program” is the “real” implementation of the application (note that App.Run was invoked at the end of Bootstrapper.Main):

// file 'program.cs' in C:\TEMP\CrossPlatformTest
namespace Cross.Platform.Program
{
    public static class App
    {
        public static void Run()
        {
            Cross.Platform.Library.Worker.Run();
        }
    }
}

Step 4: Compile the main application on command line:

(cmd.exe from Note 2)
csc /reference:platform\x86\library.dll /out:program.exe program.cs bootstrapper.cs

Step 5: We’re now finished. The structure of the directory we created should be as follows:

(C:\TEMP\CrossPlatformTest, root dir)
    platform (dir)
        amd64 (dir)
            library.dll
        x86 (dir)
            library.dll
    program.exe
    *.cs (source files)

If you now run program.exe on a 32bit platform, platform\x86\library.dll will be loaded; if you run program.exe on a 64bit platform, platform\amd64\library.dll will be loaded. Note that I added Console.ReadLine() at the end of the Worker.Run method so that you can use task manager/process explorer to investigate loaded DLLs, or you can use Visual Studio/Windows Debugger to attach to the process to see the call stack etc.

When program.exe is run, our custom assembly resolver is attached to current appdomain. As soon as .NET starts loading the Program class, it sees a dependency on ‘library’ assembly, so it tries loading it. However, no such assembly is found (because we’ve hidden it in platform/* subdirectories). Luckily, our custom resolver knows our trickery and based on the current platform it tries loading the assembly from appropriate platform/* subdirectory.