Use c++ class library in C#, x86 vs x64, Framework 3.5 vs 4.0

Hi,

I'm developing a simple c++ class  library to use in C# console application. All works fine if I develop with Framework 4.0 for x86 and for x64. The same code used with Framework 3.5 for x86 is ok, but it doesn't work for x64. In compilation I see 2 warnings :

Assembly generation -- Referenced assembly 'mscorlib.dll' targets a different processor

Assembly generation -- Referenced assembly 'System.Data.dll' targets a different processor

When I run the program, it stops with an FileLoadException! What I have to set correctly to work in x64 with Framework 3.5?!