Pass string from C# to C DLL

Question

I am trying to pass a string from C# to a C DLL. From what I have read .NET should do the conversion from string to char* for me, however I get "error CS1503: Argument '1': cannot convert from 'string' to 'char*'" Can someone advise me of where I have went wrong? Thanks.

C# code

[DllImport("Source.dll", ExactSpelling = true, CallingConvention = CallingConvention.Cdecl), SuppressUnmanagedCodeSecurity]
public static unsafe extern bool StreamReceiveInitialise(char* filepath);

const string test = "test";
// This method that will be called when the thread is started
public void Stream()
{
    if (StreamReceiveInitialise(test))
    {


    }
}

C DLL

extern "C"
{
    __declspec(dllexport) bool __cdecl StreamReceiveInitialise(char* filepath);
}

Answer 1

Declare your external method as:

public static extern bool StreamReceiveInitialise(string filepath);

Answer 2

Use a StringBuilder in place of char*. See this

[DllImport("Source.dll")]
public static extern bool StreamReceiveInitialise(StringBuilder filepath);

Answer 3

Do it like this:

[DllImport("Source.dll", CallingConvention = CallingConvention.Cdecl, CharSet=CharSet.ANSI)]
static extern bool StreamReceiveInitialise([MarshalAs(UnmanagedType.LPStr)] string filepath);

(Marshalling as UnmanagedType.LPStr is the default, but I like being explicit).