Return contents of a std::wstring from C++ into C#

Question

I have an unmanaged C++ DLL that I have wrapped with a simple C interface so I can call PInvoke on it from C#. Here is an example method in the C wrapper:

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Answer 1

You need to specify MarshalAs attribute for the return value:

    [DllImport( "my.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
    [return : MarshalAs(UnmanagedType.LPWStr)]
    private static extern string GetMyString();

Make sure the function is indeed cdecl and that the wstring object is not destroyed when the function returns.

Answer 2

You are going to have to rewrite your getMyString function for the reasons mentioned in Hans Passant's answer.

You need to have the C# code pass a buffer in to your C++ code. That way the your code (ok, the CLR Marshaller) controls the lifetime of the buffer and you don't get into any undefined behavior.

Below is an implementation:

C++

void getMyString(wchar_t *str, int len)
{
    wcscpy_s(str, len, someWideString.c_str());
}

C#

[DllImport( "my.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Unicode )]
private static extern void GetMyString(StringBuffer str, int len);
public string GetMyStringMarshal()
{
    StringBuffer buffer = new StringBuffer(255);
    GetMyString(buffer, buffer.Capacity);
    return buffer.ToString();
}