Comparison

Version1

        protected override void OnKeyDown(KeyEventArgs e)
        {
            var modifiers = e.Device.Modifiers;
            var key = e.Key;

            if (IsNavigational(key))
            {
                var shouldSelectAlong = modifiers.HasFlag(ModifierKeys.Shift);
                var isWordJumpEnabled = modifiers.HasFlag(ModifierKeys.Control);
                this.MoveCaret(TranslateToDirection(key, modifiers), shouldSelectAlong, isWordJumpEnabled);
            }
            else if (IsTextInput(e, modifiers))
            {
                this.InsertText(e.Text);
            }
            else
            {
                this.TryPerformSpecialInput(key, modifiers);
            }

            e.Handled = true;
        }

Version2

 protected override void OnKeyDown(KeyEventArgs e)
        {
            string text = this.Text ?? string.Empty;
            int caretIndex = this.CaretIndex;
            bool movement = false;
            bool textEntered = false;
            var modifiers = e.Device.Modifiers;

            switch (e.Key)
            {
                case Key.A:
                    if (modifiers == ModifierKeys.Control)
                    {
                        this.SelectionStart = 0;
                        this.SelectionEnd = this.Text.Length;
                    }

                    break;
                case Key.Left:
                    this.MoveHorizontal(-1, modifiers);
                    movement = true;
                    break;

                case Key.Right:
                    this.MoveHorizontal(1, modifiers);
                    movement = true;
                    break;

                case Key.Up:
                    this.MoveVertical(-1, modifiers);
                    movement = true;
                    break;

                case Key.Down:
                    this.MoveVertical(1, modifiers);
                    movement = true;
                    break;

                case Key.Home:
                    this.MoveHome(modifiers);
                    movement = true;
                    break;

                case Key.End:
                    this.MoveEnd(modifiers);
                    movement = true;
                    break;

                case Key.Back:
                    if (!this.DeleteSelection() && this.CaretIndex > 0)
                    {
                        this.Text = text.Substring(0, caretIndex - 1) + text.Substring(caretIndex);
                        --this.CaretIndex;
                    }

                    break;

                case Key.Delete:
                    if (!this.DeleteSelection() && caretIndex < text.Length)
                    {
                        this.Text = text.Substring(0, caretIndex) + text.Substring(caretIndex + 1);
                    }

                    break;

                case Key.Enter:
                    if (this.AcceptsReturn)
                    {
                        goto default;
                    }

                    break;

                case Key.Tab:
                    if (this.AcceptsTab)
                    {
                        goto default;
                    }

                    break;

                default:
                    if (!string.IsNullOrEmpty(e.Text))
                    {
                        this.DeleteSelection();
                        caretIndex = this.CaretIndex;
                        text = this.Text;
                        this.Text = text.Substring(0, caretIndex) + e.Text + text.Substring(caretIndex);
                        ++this.CaretIndex;
                        textEntered = true;
                    }

                    break;
            }

            if (movement && ((modifiers & ModifierKeys.Shift) != 0))
            {
                this.SelectionEnd = this.CaretIndex;
            }
            else if (movement || textEntered)
            {
                this.SelectionStart = this.SelectionEnd = this.CaretIndex;
            }

            e.Handled = true;
        }