[.net] How do I fix the Visual Studio compile error, "mismatch between processor architecture"?

For C# projects, the target of x86 does what it sounds like. It says that this assembly only supports x86 architectures. Likewise for x64. Any CPU on the other hand says that I don't care which architecture, I support both. So, the next 2 questions are (1) what is the configuration of the executable that uses these dlls? and (2) what is the bitness of your OS/Computer? The reason I ask is because if your executable is compiled to run in 64-bit, then it NEEDS all dependencies to be able to run in 64-bit mode as well. Your Any CPU assembly should be able to be loaded, but perhaps it is referencing some other dependency that is only capable of running in x86 configuration. Check all dependencies and dependencies-of-dependencies to make sure everything is either "Any CPU" or "x64" if you plan to run the executable in 64-bit mode. Otherwise, you'll have issues.

In many ways, Visual Studio does not make compiling a mixture of Any CPU and various architecture dependent assemblies easy. It is doable, but it often requires that an assembly that would otherwise be "Any CPU" to have to be compiled separately for x86 and x64 because some dependency-of-a-dependency somewhere has two versions.