DllImport and char*

Question

I have a method I want to import from a DLL and it has a signature of:

BOOL GetDriveLetter(OUT char* DriveLetter)

I\'ve tried

Answer 1

The StringBuilder is probably the way to go, but you have to set the capacity of the string builder before calling the function. Since C# has no idea how much memory that GetDriveLeter will use, you must make sure the StringBuilder has enough space. The marshaller will then pass a char* allocated to that length to the function and marhsall it back to the StringBuilder.

[return:MarshalAsAttribute(UnmanagedType.I4)]
private static extern bool GetDriveLetter(StringBuilder DriveLetter);

public static bool GetDriveLetter(out string driverLetter) {
  StringBuilder buffer = new StringBuilder(10);
  bool ret = GetDriveLetter(buffer);
  driveLetter = buffer.ToString();
  return ret;
}

See the p/invoke sample for GetWindowText(), for an example.