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POWeird behavior caused by using .Net ComboBox properties SelectionStart & SelectionLength in "DropDownList" mode
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copied!<p>We have a sample application with such handler for a combobox in "DropDownList" mode:</p> <pre><code> private void comboBox1_Leave(object sender, EventArgs e) { comboBox1.SelectionStart = 0; comboBox1.SelectionLength = 0; } </code></pre> <p>the code above behaves differently depending whether the application has CALLWNDPROC hook loaded or not. If application has a CALLWNDPROC hook in it - the code above would throw exception whenever combobox loses focus. Without the hook - that code doesn't throw.</p> <p>these are few lines from exception description:</p> <pre><code>System.ArgumentOutOfRangeException: InvalidArgument=Value of '-2136611475' is not valid for 'start'. Parameter name: start at System.Windows.Forms.ComboBox.Select(Int32 start, Int32 length) at System.Windows.Forms.ComboBox.set_SelectionLength(Int32 value) at ComboCrash.Form1.comboBox1_Leave(Object sender, EventArgs e) in T:\tmp.proj\ComboCrash\ComboCrash\Form1.cs:line 32 at System.Windows.Forms.Control.OnLeave(EventArgs e) at System.Windows.Forms.Control.NotifyLeave() at System.Windows.Forms.ContainerControl.UpdateFocusedControl() </code></pre> <p>The question is: What might be the cause of that different behavior with a hook installed?</p> <p>PS1: I am not a C# developer, but it seems to me that concept of textual selection is not applicable for DropDownList comboboxes (as they don't have a textbox), is it correct? </p> <p>PS2: Application that installs the hook and a hook DLL are written in C++. Hook function is as simple as: </p> <pre><code>return (CallNextHookEx(hook_handle, code, wParam, lParam)); </code></pre>

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