Try to re-register the last msxml library that is in your system. This libraries are in the C:\windows\sytem32 folder. I'm not sure this is your problem because unregistered dlls cause more collateral damages but you can try. Execute from command line this, REGSVR32 "C:\windows\system32\MSXML4.DLL" , the number could be different, depends on the software you have installed.

I left an attempt at the original post... Absolute faith corrupts as absolutely as absolute power Eric Hoffer All that is necessary for the triumph of evil is that good men do nothing. Edmund Burke

Strings are immutable in C#, so "ref string" might not mean much, since the target would still be unchangeable. Also strings in c# are by definition Unicode, so you will likely need to define UNICODE in your c++ project. C# strings map reasonably well to BSTRs from an interop point of view. You might try making the C++ signature "Func2(10, &BSTRmystr) and match that with the stringbuilder variant odf the C# one... Absolute faith corrupts as absolutely as absolute power Eric Hoffer All that is necessary for the triumph of evil is that good men do nothing. Edmund Burke

I was trying to do the same for some time. Eventualy I realized that the best method is to allocate memory in C# and send a referance of the memory array to the C++ mthod\function. You might have a probelm determining the size of the array needed but that is a different story. Even if you get it to work, it would be very careless doing so. Gilad.