Hi,
Yo don't need to do any manual marshaling to call your COM object fucntion
if the object expect a struct array. The managed definition of your method
should be:

getdata(int num, [In, MarshalAs(UnmanagedType.LPArray, SizeParamIndex=0)]
ref mystruct[] structarr);
//If the code of your com object will modify array element you must apply
the Out attribute.

To call that function do the following:
mystruct[] arr = new mystruct[ArraySize];
//Fill your array with useful values.
YouComObject.getdata(arr.length, ref arr);

If you can change the definition of your COM object I advise you to use
SAFEARRAYs instead of C-Style array. You can find more examples of
marshaling structures and arrays in my article:
http://www.thecodeproject.com/useritems/ManWMF.asp (C# translation of the
Windows Media Format SDK)

The error you receive is because you pass to you COM function a pointer to
mystruct* and the function is expecting mystruct** (that's why I cluded the
ref keyword). Your code could work if you do that:

IntPtr buf=Marshalptr(myarray);
GCHandle h = GCHandle.Alloc(buff, GCHandleType.Pinned);
try
{
 comclass.getdata(3, h.AddressOfPinnedObject());
}
finally
{
 h.Free();
}

Take care of who must be free the allocated memory.
Regards,
Idael.