See the question and my original answer on StackOverflow

You need to instruct the C# compiler (that CSharpCodeProvider uses indirectly) that you want to link to another mscorlib.dll, using the /nostdlib option. Here is a sample that should do it:

static void Main(string[] args)
    // defines references
    List<string> references = new List<string>();

    // get a reference to the mscorlib you want
    var mscorlib_2_x86 = Path.Combine(

    // ... add other references (System.dll, etc.)

    var provider = new CSharpCodeProvider(
                   new Dictionary<string, string> { { "CompilerVersion", "v4.0" } });
    var parameters = new CompilerParameters(references.ToArray(), "program.exe");
    parameters.GenerateExecutable = true;

    // instruct the compiler not to use the default mscorlib
    parameters.CompilerOptions = "/nostdlib";              

    var results = provider.CompileAssemblyFromSource(parameters,
        @"using System;

        class Program
            static void Main(string[] args)
                Console.WriteLine(""Hello world from CLR version: "" + Environment.Version);

If you run this, it should compile a program.exe file. If you run that file, it should display something like this:

Hello world from CLR version: 2.0.50727.7905