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    <p>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:</p> <ol> <li>Make sure default .NET assembly loading mechanism ("Fusion" engine) can't find either x86 or x64 version of the platform-specific assembly</li> <li>Before the main application attempts loading the platform-specific assembly, install a custom assembly resolver in the current AppDomain</li> <li>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.</li> </ol> <p>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).</p> <p><strong>Note 1</strong>: "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")</p> <p><strong>Note 2</strong>: 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.</p> <pre><code>(cmd.exe) C: mkdir \TEMP\CrossPlatformTest cd \TEMP\CrossPlatformTest </code></pre> <p><strong>Step 1</strong>: The platform-specific assembly is represented by a simple C# class library:</p> <pre><code>// 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(); } } } </code></pre> <p><strong>Step 2</strong>: We compile platform-specific assemblies using simple command-line commands:</p> <pre><code>(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 </code></pre> <p><strong>Step 3</strong>: 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:</p> <pre><code>// 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; } } } </code></pre> <p>"Program" is the "real" implementation of the application (note that App.Run was invoked at the end of Bootstrapper.Main):</p> <pre><code>// file 'program.cs' in C:\TEMP\CrossPlatformTest namespace Cross.Platform.Program { public static class App { public static void Run() { Cross.Platform.Library.Worker.Run(); } } } </code></pre> <p><strong>Step 4</strong>: Compile the main application on command line:</p> <pre><code>(cmd.exe from Note 2) csc /reference:platform\x86\library.dll /out:program.exe program.cs bootstrapper.cs </code></pre> <p><strong>Step 5</strong>: We're now finished. The structure of the directory we created should be as follows:</p> <pre><code>(C:\TEMP\CrossPlatformTest, root dir) platform (dir) amd64 (dir) library.dll x86 (dir) library.dll program.exe *.cs (source files) </code></pre> <p>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.</p> <p>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.</p>
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    1. COWe use similar approach but the event is attached in the static constructor - this way in some cases the attachment happens before .NET attempts to load another assembly.
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    2. COPlease update to use Environment.Is64BitProcess -- as it might be different from the CPU on the machine. Otherwise - very well answered - we are using something similar.
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    3. CO`PROCESSOR_ARCHITECTURE` is correct - It actually reflects the process not the machine.
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