防止在文本字段为空时用户按退格按钮时出现错误消息

Question

Whenever the user presses the backspace button on an empty text field, the following error message appears:

An unhandled exception of type 'System.ArgumentOutOfRangeException' occurred in mscorlib.dll. Additional information: StartIndex cannot be less than zero.

How can I get it to ignore backspace button presses if the textfield is empty.

   private void btnback_Click(object sender, EventArgs e)
    {
        if (remainTxt.BackColor == Color.FromArgb(245, 244, 162))
        {
            remainTxt.Text = remainTxt.Text.Remove(remainTxt.Text.Length - 1, 1);
        }
        else if (totalTxt.BackColor == Color.FromArgb(245, 244, 162))
        {
            totalTxt.Text = totalTxt.Text.Remove(totalTxt.Text.Length - 1, 1);
        }
        else if (paidTxt.BackColor == Color.FromArgb(245, 244, 162))
        {
            paidTxt.Text = paidTxt.Text.Remove(paidTxt.Text.Length - 1, 1);
        }
    }

Answer 1

You have to check if Text length is not 0. The Text.Remove function's first argument in your handler gets -1 as first parameter. This causes the exception to be thrown, because this is not a valid index. You should wrap the entire method body in a if ( remainTxt.Text.Length > 0 ) block

Answer 2

Sounds like you're trying to solve the problem in the wrong way. The problem you're having is that your code is not checking to see if the text box is empty before you run some logic on it.

Change your code to check if !string.IsNullOrEmpty(remainTxt.Text) (and the others) before attempting to subtract 1 from Length property. Because the textbox is empty, Length - 1 is -1, and is indeed, out of range.

Answer 3

I would abstract out the remove last character operation into a function that guards against an empty text field

private void RemoveLast(TextBox tb) {
  if (tb.Text.Length > 0) { 
    tb.Text = tb.Text.Remove(tb.Text.Length - 1, 1);
  }
}

Then switch the event handler to use that function

private void btnback_Click(object sender, EventArgs e)
{
    if (remainTxt.BackColor == Color.FromArgb(245, 244, 162))
    {
        RemoveLast(remainTxt);
    }
    else if (totalTxt.BackColor == Color.FromArgb(245, 244, 162))
    {
        RemoveLast(totalTxt);
    }
    else if (paidTxt.BackColor == Color.FromArgb(245, 244, 162))
    {
        RemoveLast(paidTxt);
    }
}