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POWindows Form ComboBox fires SelectionChangeCommitted, SelectedValueChanged and SelectedIndexChanged event when application loses focus
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copied!<p>I have .Net 2.0 Windows form containing combobxes. I have written following code to populate combobox and then binding it to ProductType property of type Int.</p> <pre><code>// Populate Combo cmbProduct.DropDownStyle = System.Windows.Forms.ComboBoxStyle.DropDownList; cmbProduct.DataSource = new DataView(productDataSet.Tables[0]); cmbProduct.DisplayMember = displayColumnName_; cmbProduct.ValueMember = idColumnaName_; // Add Databindings cmbProduct.DataBindings.Add("SelectedValue", this, "ProductType").DataSourceUpdateMode = DataSourceUpdateMode.OnPropertyChanged; </code></pre> <p><strong>ISSUE</strong></p> <ol> <li>Run application. </li> <li>Click on ComboBox's drop-down arrow but do not select any item.</li> <li>Press any keys (such as ALT+TAB, Windows Key etc) which will shift focus from current Windows form application.</li> </ol> <p>This throws Exception and application crashes. Following are the details taken from Output window.</p> <blockquote> <p>A first chance exception of type 'System.ArgumentException' occurred in System.Windows.Forms.dll Object of type 'System.DBNull' cannot be converted to type 'System.Int32'. at System.ComponentModel.ReflectPropertyDescriptor.SetValue(Object component, Object value) at System.Windows.Forms.BindToObject.SetValue(Object value) at System.Windows.Forms.Binding.PullData(Boolean reformat, Boolean force) at System.Windows.Forms.Binding.Target_PropertyChanged(Object sender, EventArgs e) at System.EventHandler.Invoke(Object sender, EventArgs e) at System.Windows.Forms.ListControl.OnSelectedValueChanged(EventArgs e) at System.Windows.Forms.ComboBox.OnSelectedValueChanged(EventArgs e) at System.Windows.Forms.ComboBox.OnSelectedIndexChanged(EventArgs e) at System.Windows.Forms.ComboBox.WmReflectCommand(Message& m) at System.Windows.Forms.ComboBox.WndProc(Message& m) at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m) at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m) at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam) at System.Windows.Forms.UnsafeNativeMethods.SendMessage(HandleRef hWnd, Int32 msg, IntPtr wParam, IntPtr lParam) at System.Windows.Forms.Control.SendMessage(Int32 msg, IntPtr wparam, IntPtr lparam) at System.Windows.Forms.Control.ReflectMessageInternal(IntPtr hWnd, Message& m) at System.Windows.Forms.Control.WmCommand(Message& m) at System.Windows.Forms.Control.WndProc(Message& m) at System.Windows.Forms.ScrollableControl.WndProc(Message& m) at System.Windows.Forms.ContainerControl.WndProc(Message& m) at System.Windows.Forms.Form.WndProc(Message& m) at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m) at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m) at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam) at System.Windows.Forms.UnsafeNativeMethods.CallWindowProc(IntPtr wndProc, IntPtr hWnd, Int32 msg, IntPtr wParam, IntPtr lParam) at System.Windows.Forms.NativeWindow.DefWndProc(Message& m) at System.Windows.Forms.Control.DefWndProc(Message& m) at System.Windows.Forms.Control.WmCommand(Message& m) at System.Windows.Forms.Control.WndProc(Message& m) at System.Windows.Forms.ComboBox.WndProc(Message& m) at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m) at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m) at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam) at System.Windows.Forms.UnsafeNativeMethods.PeekMessage(MSG& msg, HandleRef hwnd, Int32 msgMin, Int32 msgMax, Int32 remove) at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData) at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context) at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context) at System.Windows.Forms.Application.Run(Form mainForm)</p> </blockquote> <p>I added some debug statment in event handlers to check the event sequence. The output window deatails are as shown below :</p> <pre><code>cmbProduct_SelectionChangeCommitted occured - SelectedValue is NULL cmbProduct_SelectedValueChanged occured - New SelectedValue is NULL The thread 0x1728 has exited with code 0 (0x0). cmbProduct_SelectedIndexChanged occured - New SelectedIndex is -1 The thread 0x1250 has exited with code 0 (0x0). </code></pre> <p><strong>Question</strong> </p> <p>Why .Net fires <code>SelectionChangeCommitted</code>, <code>SelectedValueChanged</code> and <code>SelectedIndexChanged</code> events when the application loses its focus with combobox state is OPEN ?</p>

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