c# – 我们不能在PreviewKeyDown事件中获取文本框的当前值？

这是我的代码,

private void txtDoscountValue_PreviewKeyDown(object sender,KeyEventArgs e)
    {
        try
        {
          if  (txtDoscountValue.Text == string.Empty)
              discountPercentage = 0;
          else
              discountPercentage = float.Parse(txtDoscountValue.Text);
              CalculateCart(cartItems,discountPercentage,0);
        }
        catch
        { }
    }