WPF DataGrid MouseLeftButtonDown not firing

Question

I have a common task. Implement CheckBox checking in DataGrid by one-click. I deside make a DataGridExtended class, derived from DataGrid, and implement something like that:

XAML:

<DataGrid x:Class="DataGrid.DataGridExtended"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         mc:Ignorable="d" 
         d:DesignHeight="300" d:DesignWidth="300">

</DataGrid>

CODE:

public partial class DataGridExtended : System.Windows.Controls.DataGrid
{
    private int _oldRowIndex;
    private int _oldColumnIndex;

    public DataGridExtended()
    {
        MouseLeftButtonUp += DataGridExtendedMouseLeftButtonUp;
        MouseLeftButtonDown += DataGridExtendedMouseLeftButtonDown;
    }


    private void DataGridExtendedMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
        // Если сендер реально DataGridExtended
        var dataGridExt = sender as DataGridExtended;
        if (dataGridExt == null)
            return;

        // Получаем текущую ячейку
        var currentCell = dataGridExt.CurrentCell;

        _oldRowIndex = dataGridExt.SelectedIndex;
        _oldColumnIndex = dataGridExt.CurrentColumn.DisplayIndex;
    }

    private void DataGridExtendedMouseLeftButtonUp(object sender, MouseButtonEventArgs e)
    {
        // Если сендер реально DataGridExtended
        var dataGridExt = sender as DataGridExtended;
        if (dataGridExt == null)
            return;

        var rowIndex = dataGridExt.SelectedIndex;
        var columnIndex = dataGridExt.CurrentColumn.DisplayIndex;


        // Получаем текущую ячейку
        var currentCell = dataGridExt.CurrentCell;

        //if (_oldRowIndex != rowIndex || _oldColumnIndex != columnIndex)
        //    return;

        // Получаем текущую колонку
        var currentColumn = currentCell.Column;

        // Получаем контент текущей ячейки
        var cellContent = currentColumn.GetCellContent(currentCell.Item);

        // Если кликнули по чекбоксу
        var checkBox = cellContent as CheckBox;
        if (checkBox == null)
            return;

        // Ставием его в фокус
        checkBox.Focus();

        // Меняем чек на противоположный
        checkBox.IsChecked = !checkBox.IsChecked;

        // Получаем выражение привязки для чекбокса
        var bindingExpression = checkBox.GetBindingExpression(ToggleButton.IsCheckedProperty);

        // Если привязка есть - обновляем ее
        if (bindingExpression != null)
            bindingExpression.UpdateSource();
    }
}

DataGridExtendedMouseLeftButtonUp handler works fine, but DataGridExtendedMouseLeftButtonDown doesn't firing. And that is the problem.

Without DataGridExtendedMouseLeftButtonDown invoking, checking behaviour is not what I want. Namely, checking is working even I move cursor out from grid :E Trying to use PreviewMouseLeftButtonDown instead MouseLeftButtonDown give wrong effect :(

So, how can I solve my problem? Don't offer use different approaches to implement one-click checking plz :) Like using XAML-style for example...

Answer 1

In WPF, we often get situations where a particular Click handler appears not to work. The reason for this is usually because a control (or our own code) is handling that event and setting e.Handled = true;, which stops the event from being passed any further. In these situations, it is generally accepted that you should try to access the event before this happens and so we turn to the matching/related Preview event.

In your situation, I would recommend that you use the PreviewMouseLeftButtonDown event. You said that something is not initialized by then, but that doesn't make any sense to me. You said that you need to save the previous value, but you could do that just from your DataGridExtendedMouseLeftButtonUp event handler.

When the user releases the mouse button the first time, then you have their new value. Save this in a variable. When the user releases the mouse button the next and each subsequent time, then save their previous value from the variable as the old value and then read their new value into the variable.

Answer 2

Try MouseDown event and then figure out right or left