第一次调用时，Preview Mouse Down事件将返回引用了null的CurrentColumn的发件人

Question

I run my application and fill datagrid with data. Then I click on some row and handle event in following way:

  private void dataGridCanTabParamList_PreviewMouseDown(object sender, MouseButtonEventArgs e)
    {
        var buffer = sender as DataGrid;

        if ((buffer == null) || (buffer.CurrentColumn == null) )
            return;

        SetCanPropertyDesription(buffer.CurrentColumn.Header.ToString());

    }

When I run this event first time the CurrentColumn is null, when I run this event second time clicking in exactly the same position CurrentColumn contains data. CurrentItem is also empty when clicked first time.

Why I don't see the data at first click?

Answer 1

PreviewMouseDownEvent is tunneling event which is raised before actual MouseDown event.

And MouseDown event(handled by DataGridCell) is responsible for the selection of column in dataGrid. So, at first time your no cell is selected hence CurrentItem and CurrentColumn is null at that time.

See the propagation of events that how it works -

Answer 2

That is because CurrentColumn and CurrentItem refer to selected Column/Item. When you first click, nothing is selected as you handle the tunneled event (so your code is executed before the DataGrid actually set the current item) . When you click for the second time, CurrentColumn and CurrentItem have already been set.