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This is the code what makes the textBox, and adds to the grid:

public static TextBox TextEditor(this FrameworkElement obj, Grid parent)
        {
            Label text = obj as Label;
            TextBox edit = new TextBox();
            edit.Text = text.Content.ToString();
            edit.VerticalContentAlignment = VerticalAlignment.Center;
            edit.Margin = obj.Margin;
            edit.SetSize(obj);
            edit.HorizontalAlignment = HorizontalAlignment.Left;
            edit.VerticalAlignment = VerticalAlignment.Stretch;
            edit.FontSize = 15;
            parent.Children.Add(edit);
            edit.KeyDown += (s, e) =>
            {
                if(e.Key == Key.Enter)
                {
                    text.Content = edit.Text;
                    parent.Children.Remove(edit);
                }
                if(e.Key == Key.Escape)
                {
                    parent.Children.Remove(edit);
                }
            };
            parent.SizeChanged += (s, e) =>
            {
                edit.SetSize(obj);
            };
           edit.Focusable = true;
           return edit;
        }

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private void Type_MouseDoubleClick(object sender, MouseButtonEventArgs e)
        {
            Type.TextEditor(PreviewItems).Focus();
        }

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I tried every method in this post: WPF: Can t get my control to take focus