Question

I’m currently successfully using Win32 API’s SendMessage function to send text between two threads using the WM_SETTEXT parameter.

What I would like to do is send a custom data type instead of primitive data types.

So let’s say I have

Type myType
    a as Integer
    b(5) as Boolean
    d(15) as Double
End Type
Dim tmp as myType

I would like to be able to:

Call SendMessage(dstHWnd, WM_SETTEXT, 0, tmp)

I am guessing I would have to use WM_COPYDATA or similar, but the other issue is this produces a error because my data type cannot be casted into Any, per function definition:

Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hWnd As Long, ByVal wMsg As Long, ByVal wParam As Integer, ByVal lParam As Any) As Long

Is it possible to coax this conversion? Or is there an alternative best-practices method (fast and optimal)?

Answer 1

Declare the last parameter of SendMessage as byref lParam as myType.

You are, however, abusing the messaging system. It’s only fine while you know what you’re doing and you’re sure no system-default procesing logic will ever be applied to that message.

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To clarify, on the receiving end, you’re doing the following to get the data.
First, you declare your message processing routine with the last parameter being ByVal lParam As Long. Also have a function:

Public Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, Source As Any, ByVal Length As Long)

Then, when you receive a message:

if uMsg = WM_SETTEXT then
  dim t as myType
  copymemory t, byval lParam, len(t)

  'Using t here
end if

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To clarify a bit further.

Because all threads are inside one process, you can simply share pointers and do that by the mean of WM_COPYDATA. You’ll only need the first member of the COPYDATASTRUCT structure.

On the sending end, you set COPYDATASTRUCT.dwData = VarPrt(your_struct).

On the receiving end, you do that same CopyMemory thing shown above.

Note that if your message processing routine is only going to receive that single message (and no other messages), then you can simply declare its last parameter as ByRef lParam As myType and use it directly, avoiding the copying.