Passing a C# string to an unmanaged C DLL in Windows Mobile

Question

I've got an unmanaged c++ DLL that I need to call from a Windows Mobile C# app.

I've got the C# wrapper and it works nicely in desktop. I can call the DLL functions from a C# desktop program and pass strings around with no problem.

However, when I compile the lib and the wrapper for the mobile platform, I get an error in the DllImport lines saying that the CharSet.ANSI is not recognized. The only options I'm allowed to write are CharSet.Auto and CharSet.Unicode.

The problem is that, regardless of this setting, the strings that are received in the c++ functions are wide char strings, and not plain char* strings that is what they expect.

We can use wcstombs() to translate all strings at the beginning of each c++ function, but I'd rather not modify the lib to such an extent...

Is there a way to fix the marshalling between C# and C that works with the .NET Compact Framework?

Answer 1

No, there isn't.

Microsoft documentation specifies that:

[...] the .NET Compact Framework only supports Unicode, and consequently only includes the CharSet.Unicode (and CharSet.Auto which equals Unicode) value, and does not support any of the clauses of the Declare statement. This means that the ExactSpelling property is also not supported.

As a result, if your DLL function expects an ANSI string, you'll need to perform the conversion in the DLL, or convert the string to a byte array using the overloaded GetBytes method of the ASCIIEncoding class, before calling the function, since the .NET Compact Framework will always pass a pointer to the Unicode string. [...]

The solution is:

Functions in the DLL

int MARSHALMOBILEDLL_API testString(const char* value);
const char* MARSHALMOBILEDLL_API testReturnString(const char* value);

Wrapper

[DllImport("marshalMobileDll.dll")]
public static extern int testString(byte[] value);

[DllImport("marshalMobileDll.dll")]
public static extern System.IntPtr testReturnString(byte[] value);

Calling Code

string s1 = "1234567";
int v = Wrapper.testString( Encoding.ASCII.GetBytes(s1));

string s2 = "abcdef";
IntPtr ps3 = Wrapper.testReturnString(Encoding.ASCII.GetBytes(s2));
string s3 = IntPtrToString(ps3);


private string IntPtrToString(IntPtr intPtr)
{
  string retVal = "";

  byte b = 0;
  int i = 0;
  while ((b = Marshal.ReadByte(intPtr, i++)) != 0)
  {
    retVal += Convert.ToChar(b);
  }
  return retVal;
}