Using a 32bit or 64bit dll in C# DllImport

Question

Here is the situation, I'm using a C based dll in my dot.net application. There are 2 dlls, one is 32bit called MyDll32.dll and the other is a 64bit version called MyDll64.dll.

There is a static variable holding the DLL file name: string DLL_FILE_NAME.

and it is used in the following way:

[DllImport(DLL_FILE_NAME, CallingConvention=CallingConvention.Cdecl, EntryPoint=Func1")]
private static extern int is_Func1(int var1, int var2);

Simple so far.

As you can imagine, the software is compiled with "Any CPU" turned on.

I also have the following code to determine if the system should use the 64bit file or the 32bit file.

#if WIN64
        public const string DLL_FILE_NAME = "MyDll64.dll";
#else
        public const string DLL_FILE_NAME = "MyDll32.dll";        
#endif

By now you should see the problem.. DLL_FILE_NAME is defined in compilation time and not in execution time so the right dll isn't loaded according to the execution context.

What would be the correct way to deal with this issue? I do not want two execution files (one for 32bit and the other for 64bit)? How can I set DLL_FILE_NAME before it is used in the DllImport statement?

Answer 1

I've found the simplest way to do this is to import the two methods with different names, and calling the right one. The DLL won't be loaded until the call is made so it's fine:

[DllImport("MyDll32.dll", EntryPoint = "Func1", CallingConvention = CallingConvention.Cdecl)]
private static extern int Func1_32(int var1, int var2);

[DllImport("MyDll64.dll", EntryPoint = "Func1", CallingConvention = CallingConvention.Cdecl)]
private static extern int Func1_64(int var1, int var2);

public static int Func1(int var1, int var2) {
    return IntPtr.Size == 8 /* 64bit */ ? Func1_64(var1, var2) : Func1_32(var1, var2);
}

Of course, if you have many imports, this can be become quite cumbersome to maintain manually.