Question

I’m translating some C# code to Delphi, I understand that(msg = TMsg):

(int)msg.LParam

is just a cast(please correct me if I’m wrong), however the following boggles my mind:

Marshal.ReadInt32( (IntPtr)msg.WParam, 4 )

can someone please clarify this?

Answer 1

That just reads a 4 byte integer from a pointer. In managed .net code you don’t have pointers (unless you use unsafe code) so the framework provides tools to help interop between the native and managed world. The documentation on MSDN for the .net libraries is comprehensive and, of course, describes Marshal.ReadInt32.

The extra complication here is that there is an additional offset of 4 bytes. In reality the pointer is probably pointing to a struct and this code is picking out the integer value at offset 4 of the struct. That’s overwhelmingly the most likely explanation for the code being the way it is.

Now, a literal translation would be:

PInteger(msg.WParam+4)^

but you might alternatively write it in Delphi like this:

type
  TMyRecord = record
    i1: Integer;
    i2: Integer;
  end;
  PMyrecord = ^TMyRecord;
....
value := PMyRecord(msg.WParam)^.i2;

If you know which message this WParam relates to then you will also know what the true record is to use here. And so you would not need to define a dedicated record since Windows.pas would already do so.