May 9th, 2014
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When was the WM_COPYDATA message introduced, and was it ported downlevel?

Raymond Chen

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!

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Raymond Chen

Raymond has been involved in the evolution of Windows for more than 30 years. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie-jeebies. The Web site spawned a book, coincidentally also titled The Old New Thing (Addison Wesley 2007). He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information.

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