String issues with COM when used on .NET

[phinoppix]

Hi,

I'm having a problem using a COM component on C#; the component is developed in Delphi 5. I have this function:

[COLOR=green]// Delphi COM[/color]
function  TestString(const strOMA: WideString): WideString; stdcall;

This is the callable wrapper:

[PreserveSig]
string TestString([In, MarshalAs(UnmanagedType.BStr)]string StrOma);

The error thrown from .NET is:

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

I am expecting a debug string generated when TestString runs, but never happened (so the error might be during marshaling of parameters?). I also noticed that I only get the error when I have WideString as a parameter or as return type.

I changed WideString to PChar, and added a Marshal attribute on C#, and it worked. Still, I'm really hoping to fix this problem only on the .NET side. Can anyone shed a light on the probable cause of the error?