checking field values for null

Question

I've been working through this for a while and I'm missing the obvious. I'm trying to make sure that if a textbox value is left blank an error isn't thrown and instead a simple message is displayed in the textbox. However, what I've got and several other methods I've tried similar to this haven't worked. I can't figure out why this isn't working and what I need to do differently.

Basics:

This is a simple calculator that allows one to enter their waist, neck, and height measurements to use in a formula that calculates their estimated body fat percentage. The calculation works properly unless a field is left blank.

Thanks for any help!

My code:

        if (TBWaist.Text == null || TBNeck.Text == null || TBHeight.Text == null)
        {
            TBBodyFat.Text = "Value missing";
        }
        else
            if (TBWaist.Text != null & TBNeck.Text != null & TBHeight.Text != null)
            {
                double waist;
                double neck;
                double height;

                waist = Convert.ToDouble(TBWaist.Text);
                neck = Convert.ToDouble(TBNeck.Text);
                height = Convert.ToDouble(TBHeight.Text);

                TBBodyFat.Text = Convert.ToString(String.Format("{0:p2}", ((501.5 / (1.0324 - .19077 * (Math.Log10(waist - neck)) + .15456 * (Math.Log10(height))) - 450) / 100)));

Error Message

This is the error message I get if I leave the waist textbox blank. I get the same error if I leave any of the others blank as well.

Input string was not in a correct format on line 45.

waist = Convert.ToDouble(TBWaist.Text);

Answer 1

I would recommend using the TryParse methods. In this way, a blank, an empty string or something that is not convertible to a string are all treated the same. Second, I would recommend adding RequiredFieldValidators and/or RegularExpressionValidators to each of the text boxes to ensure that the user enters a value and that value is numeric. In this way, the check in your event procedure acts as a last ditch check instead of requiring a PostBack for validation

protected void Button1_Click(object sender, EventArgs e)
{
    //Navy Men's Body Fat Formula:  %Fat=495/(1.0324-.19077(log(abdomen-neck))+.15456(log(height)))-450 
    //string outputString = String.Format("At loop position {0}.\n", i);

    double waist;
    double neck;
    double height;

    if ( !double.TryParse( TBWaist.Text, out waist )
        || !double.TryParse( TBNeck.Text, out neck )
        || !double.TryParse( TBHeight.Text, out height ) )
    {
        ErrorMessageLabel.Text = "Please ensure that each value entered is numeric.";
        return;
    }   

    var bodyFat = (501.5 
        / (1.0324 - .19077 * (Math.Log10(waist - neck)) 
            + .15456 * (Math.Log10(height))
            ) - 450 ) / 100;

    TBBodyFat.Text = bodyFat.ToString("P2");
}

Answer 2

You should test for more than just null values you have to test for empty as well

use String.IsNullOrEmpty(TBWaist.Text);