在事件中将项目添加到组合框时发生InvalidOperationException

Question

I'm making a fairly simple SQL Server Database/Table selector in WPF, and, similar to SSMS' "Connection Properties" tab (when clicking the Options button), want an item in a combo box that will connect to the server, find a list of databases or tables (depending on which combobox is active), and populate the combobox below a separator line with these databases/tables. When I run my project to do this, I get an InvalidOperationException stating 'Collection was modified; enumeration operation may not execute.' on the ShowDialog line of the form, not within the form itself.

I've wrapped my event code in a Try/Catch block to try and better understand where the exception is being thrown from, but it never catches in my own code. The exception is only thrown after my event exits, somewhere between the event and the WPF backend, with the following Stack Trace (entries before the application actually starts up have been removed):

   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at System.Collections.Generic.List`1.Enumerator.MoveNext()
   at System.Windows.Controls.Primitives.Selector.SelectionChanger.CreateDeltaSelectionChange(List`1 unselectedItems, List`1 selectedItems)
   at System.Windows.Controls.Primitives.Selector.SelectionChanger.End()
   at System.Windows.Controls.Primitives.Selector.SelectionChanger.SelectJustThisItem(ItemInfo info, Boolean assumeInItemsCollection)
   at System.Windows.Controls.ComboBox.NotifyComboBoxItemMouseUp(ComboBoxItem comboBoxItem)
   at System.Windows.Controls.ComboBoxItem.OnMouseLeftButtonUp(MouseButtonEventArgs e)
   at System.Windows.UIElement.OnMouseLeftButtonUpThunk(Object sender, MouseButtonEventArgs e)
   at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
   at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
   at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
   at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
   at System.Windows.UIElement.ReRaiseEventAs(DependencyObject sender, RoutedEventArgs args, RoutedEvent newEvent)
   at System.Windows.UIElement.OnMouseUpThunk(Object sender, MouseButtonEventArgs e)
   at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
   at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
   at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
   at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
   at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
   at System.Windows.UIElement.RaiseTrustedEvent(RoutedEventArgs args)
   at System.Windows.UIElement.RaiseEvent(RoutedEventArgs args, Boolean trusted)
   at System.Windows.Input.InputManager.ProcessStagingArea()
   at System.Windows.Input.InputManager.ProcessInput(InputEventArgs input)
   at System.Windows.Input.InputProviderSite.ReportInput(InputReport inputReport)
   at System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawMouseActions actions, Int32 x, Int32 y, Int32 wheel)
   at System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr hwnd, WindowMessage msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at System.Windows.Interop.HwndSource.InputFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
   at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
   at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
   at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
   at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
   at System.Windows.Window.ShowHelper(Object booleanBox)
   at System.Windows.Window.Show()
   at System.Windows.Window.ShowDialog()
   at VDBFImport.App.Application_Startup(Object sender, StartupEventArgs e) 

The exception says that the source is simply "mscorlib"

The event in question looks like this:

        private void CbiBrowseDB_Selected(object sender, RoutedEventArgs e) {
            try {
                using(SqlConnection conn = new SqlConnection(GetConnectionString())) {
                    try {
                        conn.Open();
                    } catch(Exception ex) {
                        MessageBox.Show("Failed to connect to the SQL Server: " + ex.Message);
                        return;
                    }

                    DataTable dbs = new DataTable();
                    SqlDataAdapter adapter = new SqlDataAdapter("SELECT [name] FROM sys.databases WHERE [name] NOT IN ('master', 'tempdb', 'model', 'msdb')", conn);
                    try {
                        adapter.Fill(dbs);
                    } catch(InvalidOperationException ex) {
                        MessageBox.Show("Failed to retrieve table list from the SQL Server: " + ex.Message);
                    }

                    try {
                        while(cbxDatabase.Items.Count > 2) {
                            cbxDatabase.Items.RemoveAt(cbxDatabase.Items.Count - 1);
                        }
                        for(int i = 0; i < dbs.Rows.Count; ++i) {
                            ComboBoxItem item = new ComboBoxItem();
                            item.Content = dbs.Rows[i].Field<string>("name");
                            cbxDatabase.Items.Add(item);
                        }
                        cbxDatabase.Items.Refresh();
                    } catch(InvalidOperationException ex) {
                        MessageBox.Show("Failed to update the list of databases: " + ex.Message + "\r\n" + ex.StackTrace);
                    }
                }
            } catch (Exception ex) {
                MessageBox.Show("Failed.");
            }
        }

I expected this to just work, but instead I get the exception thrown.The broad "catch (Exception ex)" never executes, which indicates to me that this is an issue caused by something WPF is doing in the background. I suspect it may be possible that it's due to the fact that this event is triggered by selecting an option in the expanded combobox, but this type of behavior works in SSMS, so I'm not sure what I'm doing wrong. I've seen that this issue often occurs when modifying an enumeration in a foreach loop, but I have not used any of those in this event, or anywhere else in this project.

Answer 1

You seem to handle the Selected event for the ComboBoxItem . Try to handle the SelectionChanged event for the ComboBox instead. Then you should get no InvalidOperationException .

Answer 2

You are doing

while(cbxDatabase.Items.Count > 2) {
    cbxDatabase.Items.RemoveAt(cbxDatabase.Items.Count - 1);
}

Here you are modifying the collection object in the while loop. And I do not see cbxDatabase being initialized anywhere in your code pasted above. Maybe this is the cause of the exception.