Passing a object to a cpp function

[solita]

I have a C# application.I need to pass a object say a of class A to a function in cpp.The cpp code is compiled to a dll file.
I need to dllimport and pass the object from c# to cpp where cpp changes the data members of this object and these changes need to be reflected in C#.
The class I need to pass to cpp function also contain objects of some other classes.

C#

[DllImport("My.d ll", EntryPoint = "TestFn", ExactSpelling = false)]
public static extern void TestFn(ref A a);

calling in c#

TestFn(ref a);

Rightnow I am getting this exception :

Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

[Frinavale]

What does your TestFn method do?

[solita]

TestFn will populate some of the members of object a.