checking field values for null - asp.net

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);

Answer 3

How about using a RequiredFieldValidator?
Alternatively, you can check that the value is an empty string and/or null
if (
String.IsNullOrEmpty(TBWaist.Text) ||
String.IsNullOrEmpty(TBNeck.Text) ||
String.IsNullOrEmpty(TBHeight.Text)
)

Answer 4

Try a compare like
if( String.IsNullOrEmpty( TBWaist.Text ) == true )
{
// error case
}
else
{
// do stuff
}

Answer 5

You should use String.IsNullOrEmpty() instead. Your input field values are not null, simply empty, hence the conversion fails.
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);
if (String.IsNullOrEmpty(TBWaist.Text) || String.IsNullOrEmpty(TBNeck.Text) || String.IsNullOrEmpty(TBHeight.Text))
{
TBBodyFat.Text = "Value missing";
}
else
{
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)));
}
}