Gabe wondered when the WM_COPY­DATA message was introduced.

The WM_COPY­DATA message was introduced by Win32. It did not exist in 16-bit Windows.

But it was there all along.

The The WM_COPY­DATA message was carefully designed so that it worked in 16-bit Windows automatically. In other words, you retained your source code compatibility between 16-bit and 32-bit Windows without having to do a single thing. Phew, one fewer breaking change between 16-bit and 32-bit Windows.

As Neil noted, there's nothing stopping you from sending message 0x004A in 16-bit Windows with a window handle in the wParam and a pointer to a COPY­DATA­STRUCT in the lParam. Since all 16-bit applications ran in the same address space, the null marshaller successfully marshals the data between the two processes.

In a sense, support for the WM_COPY­DATA message was ported downlevel even before the message existed!