Strange C# platform invoke / DLLImport behaviour

Question

I have an unmanaged DLL I am referencing in my project using [DLLImport], but am receiving odd results when I match method signatures.

Here is an example signature from the DLL:

DLLEXPORT unsigned long OpenPort(unsigned long  ulPort,
                                     unsigned long  ulBaudRate,
                                     unsigned long  ulByteSize,
                                     unsigned long  ulPartity,
                                     unsigned long  ulStopBits,
                                     unsigned long  ulFlowControl)

And here is my C# code to import the function:

[DllImport("C:/my.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern uint OpenPort(ulong ulPort, ulong ulBaudRate,
    ulong ulByteSize, ulong ulParity, ulong ulStopBits, ulong ulFlowControl);

Notice I declare this with a return type of uint, as when I attempt to use ulong I get unexpected results (long numbers usually that look a bit like memory addresses).

However, the function works and returns expected results if I use a return type of int/uint. Can anyone shed any light on this behaviour for me?

Thanks.

What is sizeof(unsigned long) on your platform? It might be 4 bytes. ulong is always 8. — ta.speot.is, Nov 04 '13 at 11:49
@ta.speot.is yep, you're right.. I didn't know the size of an unsigned long was inconsistent between C# and C++. Nothing confusing about that :) — Alfie, Nov 04 '13 at 12:03
What's the reason for not using the build in serial port class? — Gusdor, Nov 04 '13 at 12:08
@Gusdor the DLL is for third party hardware - that function is also responsible for initiating communication with the device etc. — Alfie, Nov 04 '13 at 12:13
@Alfie David's answer has an example which is almost certainly what you want and answers the question more directly than mine. Would you mark him as the correct answer? — ta.speot.is, Nov 04 '13 at 13:45
@ta.speot.is Thank you. I appreciate the dialog we had, and how you responded to my questioning. — David Heffernan, Nov 04 '13 at 14:05
@DavidHeffernan I appreciate the time you took to question and discuss. — ta.speot.is, Nov 04 '13 at 14:08

David Heffernan · Accepted Answer · 2013-11-04T13:39:09.310

I'm assuming that your target platform is Windows, based on the name of your library in the DllImport attribute. On Windows, the C++ long type (unsigned as well as signed, obviously) is 4 bytes wide, for both 32 and 64 bit. So, you need to declare your p/invoke using uint rather than ulong.

The correct declaration is:

[DllImport("C:/my.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern uint OpenPort(
    uint ulPort, 
    uint ulBaudRate,
    uint ulByteSize, 
    uint ulParity, 
    uint ulStopBits, 
    uint ulFlowControl
);

Now, if your target platform is other than Windows, then you'd need to know what unsigned long is on that platform to give specific advise.

[This answer](http://stackoverflow.com/a/1764206/242520) is an excellent starting point for more information on the latter paragraph. — ta.speot.is, Nov 04 '13 at 13:42

Strange C# platform invoke / DLLImport behaviour

1 Answers1