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copied!<p>Actually... I think you're making a mistake. The value at <code>GWLP_USERDATA</code> and the space allocated according to <code>cbWndExtra</code> are two separate things...</p> <p>The <code>GWLP_USERDATA</code> value is part of the space Windows allocates for <em>every</em> window. It's so common for window classes to need a pointer-sized bit of storage that Windows just includes it in the base cost of a window, along with all of the other pre-defined "window words". Look at the documentation for the <code>nIndex</code> parameter to <a href="http://msdn.microsoft.com/en-us/library/ms633585.aspx" rel="nofollow noreferrer"><code>GetWindowLongPtr()</code></a>:</p> <blockquote> <p>Specifies the zero-based offset to the value to be retrieved. Valid values are in the range zero through the number of bytes of extra window memory, minus the size of an integer.</p> </blockquote> <p>So, <code>GetWindowLongPtr(hWnd, 0)</code> retrieves the first <code>sizeof(LONG_PTR)</code> bytes allocated in response to <code>cbWndExtra</code>, <code>GetWindowLongPtr(hWnd, sizeof(LONG_PTR))</code> gives you the next pointer-sized data, and so on. As its name implies, <code>nIndex</code> acts like an index into an array of bytes (though it always returns a pointer-sized value). Now take a look at the index values assigned to the pre-defined window data constants: they're all negative, including <code>GWLP_USERDATA</code>! In effect, <code>GetWindowLongPtr()</code> starts indexing into the middle of the window data, with the data common to all windows residing "before" the 0 index, and any window class specific data residing after it.</p> <p>Dialog windows are built on top of the baseline support provided for normal windows. Since they require more data than normal windows, you're required to specify at least <code>DLGWINDOWEXTRA</code> bytes in <code>cbWndExtra</code>. Like any other such data, it's accessed via a positive value passed to <code>GetWindowLongPtr()</code>.</p> <p>Therefore, when you ask for </p> <pre><code>GetWindowLongPtr(hWnd, GWLP_USERDATA + DLGWINDOWEXTRA + 0); </code></pre> <p>...you're actually getting the data at index <code>-21 + 30 + 0 = 9</code>: a value somewhere in the dialog manager's own data. Not what you want!</p> <p>By now you should realize, that when accessing data allocated via <code>cbWndExtra = DLGWINDOWEXTRA + extra</code> you only need to offset your request by <code>DLGWINDOWEXTRA</code>. So:</p> <pre><code>GetWindowLongPtr(hWnd, DLGWINDOWEXTRA + 0); </code></pre> <p>...will get you the first element of extra data. <code>GWLP_USERDATA</code> should be used <em>only</em> when you wish to get or set the one pointer of always-allocated user data associated with every window.</p>

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