How can I pass MemoryStream data to unmanaged C++ DLL using P/Invoke

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

You won't be able to pass the MemoryStream directly (as the C++ method won't know about MemoryStream), but I'm presuming you will be able to pass the raw byte buffer that a MemoryStream uses.

Please post the signature of the unmanaged method you are calling.

Answer 2

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);
   IntPtr address = handle.AddrOfPinnedObject ();

   doSomething(address, rawData.Length);
 }