I'm trying to use P/Invoke to call functions in an unmanaged C++ DLL from C#. The C++ DLL uses CString's as function parameters and returns, such as
CString AFX_EXT_API GetUserName(CString& userID)
Unfortunately, the C++ is legacy code that I cannot change to use the more universal LPSTR (or even char *).
Is there any way to marshal the CString into a .NET compatible object? Or somehow decorate a .NET char[] to be marshaled to a CString?
You can't create a CString in managed code. So clearly you need an extra layer between the managed code and the native code.
This gives you an opportunity to make a C++/CLI DLL which sits between. You can call this code from your managed assembly without needing P/invoke. And from the C++/CLI middle layer you can create a CString.
However, there is one caveat. You must be using the same C++ runtime as the native DLL. This may be possible but it is quite likely that it will be a stumbling block. For example if the DLL was written with MSVC6 then you will need to build your intermediate layer with MSVC6 too and that rules out C++/CLI. Back to P/invoke and char* in that case.
I would stress that it is terrible practice to export a DLL interface based on CString and I'd be looking for alternatives to the DLL.