DLLImport -> how to handle a HANDLE in C#

Question

in my C# code I want to import a C++ DLL. I use the dllimport and it works fine with a some of the functions. But in one function I get a HANDLE which I need later to call another function.

   [DllImport("SiUSBXp.dll")]
   public static extern int SI_Open(UInt32 deviceNum,ref IntPtr devHandle );   // this function gets the HANDLE
   [DllImport("SiUSBXp.dll")]
   public static extern int SI_Write([In]IntPtr devHandle, [In, Out] byte[] inputByte, UInt32 size,ref UInt32 bytesWritten); // this function needs the HANDLE

In my code these functions are called like this:

   IntPtr devHandle = new IntPtr();
   UInt32 bytesWritten = new UInt32();
   byte[] byteArr = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
   SI_Open(0, ref devHandle);
   SI_Write(devHandle, byteArr, 10, ref bytesWritten);

If I do it like this I get an "System.AccessViolationException". I searched here and in the internet but didnt find a specific answer. How do I use the IntPtr correctly, so it works? Best Regards

Toby

Answer 1

try this:

   [DllImport("SiUSBXp.dll")]
   public static extern int SI_Open(UInt32 deviceNum, ref IntPtr devHandle);   // this function gets the HANDLE
   [DllImport("SiUSBXp.dll")]
   public static extern int SI_Write(IntPtr devHandle, ref byte[] inputByte, UInt32 size, ref UInt32 bytesWritten); // this function needs the HANDLE

EDIT:

@Hans Passant is right. This is the correct way to pass a byte[] into a LPVOID parameter. ref used to coerce an object into LPVOID, but isn't needed for an array. What happens when you try this?

   [DllImport("SiUSBXp.dll")]
   public static extern int SI_Write(IntPtr devHandle, byte[] inputByte, UInt32 size, ref UInt32 bytesWritten); // this function needs the HANDLE

Did you try the answer @Simon Mourier gave? He was first to provide this declaration and his answer deserves to be accepted.