P/Invoke with strings in Compact Framework 2.0 SP2

If I call the native DLL like this:

    string dfhgfdgh = "jhdgjhdjghjdtgyhn";

[DllImport("StringTestNativeDLL", CallingConvention = CallingConvention.Winapi)]

public static extern byte GetVehicleIdentification(ref string vehicleIdentification);

private void button1_Click(object sender, EventArgs e)

{

  GetVehicleIdentification(ref dfhgfdgh);

  textBox1.Text = dfhgfdgh;

}

and the native DLL looks like this:

char m_VehicleIdentification[200];

extern "C" __declspec(dllexport) BYTE __stdcall GetVehicleIdentification(wchar_t** vehicleIdentification)

{

  strcpy(m_VehicleIdentification, "1234567890");

  strcpy (*vehicleIdentification, m_VehicleIdentification);

  return 0;

}

The text in my textbox is a sequence of squares equal to half the number of characters in the string above i.o.w. 5 squares (unrecognized characters)

It seems like it’s trying to interpret the ascii-string as a unicode string (2 bytes per character). If I try to

public static extern byte GetVehicleIdentification([MarshalAs(UnmanagedType.LPStr)]ref string vehicleIdentification);

I get a “Not supported”-exception. I get the same result for LPTStr LPWStr BStr BTStr and all other string types which I have tried.

Is it not possible in the Compact Framework to call a native function which has a char** in-parameter and get a valid string back in any other way than to work with wchar_t** in the native function? I don’t want to use unsafe C#-code. (If cut the ref and use char* instead in the native function I get the same result, □□□□□□□□□□□□□.)

You should declare your constant string L"1234567890", not "1234567890", to make it Unicode, not ASCII, since you declared your function with Unicode types.  You should use wcscpy_s (or wcscpy) instead of strcpy, too.

My P/Invoke is rusty, but I thought you were supposed to use StringBuilders to pass string references to native code, not Strings.

Is there a particular reason you prefer ASCII over Unicode?

You should be able to use either strings or StringBuilders.

The important thing, above anything else is that since you're dealing with an unmanaged code that is expecting to use a reference to a unicode string that you should specify to the Marshaler how to marshal your string.

You'll need to use the attributing:

[MarshalAs(UnmanagedType.LPWStr)] // I'm guessing LPWStr, test this.

Read up on UnmanagedType, and marshalling strings

Edit, you may have to pass it as IntPtr instead, as you're needing to deal with levels of indirection.