Access violation exception when calling C++ function from C#

I have the following C++ function (belongs to a COM interface, derived from IUnknown), which I want to call from C# code:

C++ declaration as documented:

HRESULT Function1([in] STRUCT1 *s1, [in, out] STRUCT2 *s2, [in] SIZE_T var1);

Declaration inside a working C++ program:

    STDMETHOD(Function1)(
    THIS_
    __out STRUCT1 * s1,
    __in_ecount_opt(var1) const STRUCT2 * s2,
    SIZE_T var1
    ) PURE;

In C# territory I define the following:

[StructLayout(LayoutKind.Sequential)]
public struct STRUCT1
{
    public uint  member1;     //HRESULT member1
    public ulong member2;     //SIZE_T member2
}

[StructLayout(LayoutKind.Sequential)]
public struct STRUCT2 
{
    public IntPtr  member1;   //VOID *member1;
    public ulong   member2;   //SIZE_T  member2;
    public STRUCT3 member3;   //STRUCT3 member3;
}

[StructLayout(LayoutKind.Sequential)]
public struct STRUCT3
{
    public int member1;  //int member1
}

I implement this function in C# as follows :

[ComImport, ComVisible(false), InterfaceType(ComInterfaceType.InterfaceIsIUnknown),
Guid("…")]
public interface Iinterface1 
{
……


    uint Function1(ref STRUCT1 s1, ref STRUCT2 s2, ulong var1);
……
}

and I invoke the function like this:

            STRUCT1 temp1 = new STRUCT1();
            temp1.member1 = 0;
            temp1.member2 = 0;

            STRUCT2 temp2 = new STRUCT2();
            STRUCT3 temp3 = new STRUCT3();
            temp3.member1 = 0;
            temp2.member1 = IntPtr.Zero;
            temp2.member2 = 0;
            temp2.member3 = temp3;
            ulong var1 = 1;

            res1 = COMobject.Function1(ref temp1, ref temp2, var1);

When function is executed I get an Access Violation Exception :

“An unhandled exception of type 'System.AccessViolationException' occurred in prog1.exe Additional information: Attempted to read or write protected memory. This is often an indication that other memory is corrupt. “

I implemented most of functions of the same interface with no problems, as well as a lot of other interfaces in this application. This one really puzzles me.

I would really appreciate your help on this..

Thank you in advance.