Question

I need your help with the following scenario:

I am reading some data from hardware into a MemoryStream (C#) and I need to pass this data in memory to a dll implemented in unmanaged C++ (using pointer ??).
The data read (into stream) is very large (megabytes). I understand that I can P/Invoke this dll but what I am not sure is how to pass the pointer / reference of the stream data to the C++ API ?

I must admit I am confused as I am new to C# – do I need to use unsafe / fixed since data is large or these are irrelevant as MemoryStream object is managed by GC ? Some example code / detailed description would be very helpful. Thanks

Signature of unmanaged API:

BOOL doSomething(void * rawData, int dataLength)

Answer 1

If it’s just expecting bytes you can read the MemoryStream into a byte array and then pass a pointer to that to the method.

You have to declare the external method:

[DllImport("mylibrary.dll", CharSet = CharSet.Auto)]
public static extern bool doSomething(IntPtr rawData, int dataLength);

Then, read the bytes from the MemoryStream into a byte array. Allocate a GCHandle which:

Once allocated, you can use a GCHandle
to prevent the managed object from
being collected by the garbage
collector when an unmanaged client
holds the only reference. Without such
a handle, the object can be collected
by the garbage collector before
completing its work on behalf of the
unmanaged client.

And finally, use the AddrOfPinnedObject method to get an IntPtr to pass to the C++ dll.

private void CallTheMethod(MemoryStream memStream)
{
   byte[] rawData = new byte[memStream.Length];
   memStream.Read(rawData, 0, memStream.Length);
   
   GCHandle rawDataHandle = GCHandle.Alloc(rawData, GCHandleType.Pinned);
   try
   {
     IntPtr address = rawDataHandle.AddrOfPinnedObject ();

     doSomething(address, rawData.Length);
   }
   finally
   {
     if (rawDataHandle.IsAllocated)
       rawDataHandle.Free();
   }
 }