Label throws error if text boxes are blank

Question

I have the code below which works fine, but if I leave the text boxes blank it throws an error. I would like to first check if any are blank then throw an error message in lblSalary.Text. How could I re-write this? C#

Thanks

    protected void Button1_Click(object sender, EventArgs e)
    {
        Double AnnualHours;
        AnnualHours = Convert.ToDouble(txtAnnualHours.Text);
        Double Rate;
        Rate = Convert.ToDouble(txtRate.Text);
        Double Salary;
        Salary = (AnnualHours * Rate);
        lblSalary.Text = "$" + Salary.ToString();
    }
}

Is this a homework question? If so, you should add the "homework" tag. — ethan, Nov 23 '11 at 16:50

score 4 · Accepted Answer · edited May 23 '17 at 12:03

Instead of convert.ToDouble use double.TryParse and handle the case where it returns false.

double AnnualHours;
if (!double.TryParse(txtAnnualHours.Text, out AnnualHours))
{
    // Set label text here, as we couldn't convert to a double
}
else
{
    // AnnualHours contains the double value.
}

NOTE: Because you're dealing with money, you should use the decimal type, not the double type. Decimal can correctly represent base 10 numbers (123, 157.2, 0.5) exactly, whereas double cannot.

See here: decimal vs double! - Which one should I use and when?

score 1 · Answer 2 · answered Nov 23 '11 at 16:48

Use Double.TryParse() to parse string in safe manner

protected void Button1_Click(object sender, EventArgs e)
{
    double annualHours = 0;
    double.TryParse(txtAnnualHours.Text, out annualHours);
    double rate = 0;
    double.TryParse(txtRate.Text, out rate);
    double salary = (annualHours * rate);
    lblSalary.Text = "$" + salary.ToString();
}

If you need to do some conditional calculations depends on whether value was provided or not (user entered a valid number in textbox):

bool isAnnualHoursValid = double.TryParse(txtAnnualHours.Text, out annualHours);
if (isAnnualHoursValid)
{
     // ask user to enter valid value or use some default
}

Label throws error if text boxes are blank

2 Answers2