How do I marshall a pointer to a pointer of an array of structures?

Question

My C declarations are as follows:

int myData(uint myHandle, tchar *dataName, long *Time, uint *maxData, DATASTRUCT **data);

typedef struct {
  byte Rel;
  _         


        

Answer 1

A pointer to a pointer could be represented in your dllimport declaration as ref IntPtr data, so your declaration would become:

[DllImportAttribute("myData.dll", EntryPoint = "myData")]
public static extern int myData(uint myHandle, [MarshalAs(UnmanagedType.LPTStr)] string dataName, out long Time, out uint maxData, ref IntPtr data);

(As an aside, I think a long in C is just equivalent to an int in C#. Long in C# is an Int64, which would be a long long in C)

Marshalling your DATASTRUCT[] to an IntPtr can be done using the GCHandle class

DATASTRUCT [] dataInformation = new DATASTRUCT[3];
GCHandle gch = GCHandle.Alloc(dataInformation , GCHandleType.Pinned);
IntPtr ptr = gch.AddrOfPinnedObject();
myData(myHandle, dataToShow, out Time, out maxData, ref ptr);
//It's absolutely essential you do this next bit so the object can be garbage collected again, 
//but it should only be done once the unmanaged code is definitely done with the reference.    
gch.Free(); 

Using the Marshal class and the StructureToPtr or Copy methods of it would also be an option but for the purposes of proving the concept at least the GCHandle should do the trick, it's just not ideal for scenarios where the unmanaged code does long running operations because you've pinned this object in place and the GC can't move it until you free it.