Why is the SelectedItem property of my DataGrid subclass set to Nothing?

Question

I have subclassed the WPF DataGrid in my VB.NET application because I will need to use this component frequently but also need to have some extra features, in this case adding a new row when the tab key is pressed on the bottom right cell.

I have Overidden the OnKeyDown event of the base class. This is being triggered when the tab keys is pressed while the grid is focused, exactly as I want. However, when the event is triggered I need to be able to determine whether I'm on the bottom-right cell or not. To do this I'd like to get the SelectedItem property of my DataGrid and use that to determine which cell is selected.

I am doing all of this programmatically because I don't want users to have to write any more XAML than they would have to for a regular datagrid. It should function in exactly the same way except if you tab on the bottom right cell a new row is added and the first cell of that row is selected. this should apply no matter how many rows/columns the user has in the datagrid.

The code below shows what I want to do but the SelectedItem is not set.

    Protected Overrides Sub OnKeyDown(e As KeyEventArgs)
        MyBase.OnKeyDown(e)
        If e.Key = Key.Tab Then

            Dim colIndex As Integer = Me.Columns.IndexOf(Me.CurrentColumn)
            Dim colCount As Integer = Me.Columns.Count - 1

            If -1 = colIndex Then
                'the next line throws a System.NullReferenceException because SelectedItem is not set
                If SelectedItem.Equals(Items(Items.Count - 1)) Then
                    Focus()
                    Dim dgrCI = New DataGridCellInfo(Items(Items.Count - 1), Columns(colIndex))
                    ScrollIntoView(Items(Items.Count - 1))
                    BeginEdit()

                End If
            End If
        End If
    End Sub

The SelectedItem property should be set to the last item in the table but is instead set to Nothing. Why is this?

Edit: The answer from Ppp is correct. it seems that by the time the OnKeydown event is triggered the DataGrid has already lost focus. I resolved this issue by using the OnPreviewKeydown event instead.

Answer 1

When your tab key is pressed in the last cell you basically tab out of the datagrid which is why I am guessing your SelectedItem is null.