Focus grid after up/down button pressed

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C#

WPF

Gridview

Hello!

I have problems with focus gridView item after up/down button (from keyboard) pressed.
I have textbox with textchanged event, so when i change text, it's looking for items in datagrid which contains textbox.text - that work's great. But now, when i press up/down arrow i'd like to focus first element of dataGrid.
So in code it should be something like:

private void Window_PreviewKeyDown_1(object sender, KeyEventArgs e)
        {
            if (e.Key == Key.Down || e.Key == Key.Up)
            {
//raise only when nothing is selected, otherway just move selection item in gridview up/down
                if(myDG.SelectedItem == null)
                    myDG.SelectedIndex = 0;
            }
        {

And actually I can see that it's getting selected, but item background is gray, not blue.

Thanks for any help!