Checking for numeric value entered in text box in Visual Basic

Question

I am working on a program for my Visual Basic class and have a quick question. One of the things we were encouraged to do was to check to make sure the quantity entered in a

Answer 1

A more correct way to do that is to use the TryParse method available in the Int32 or Double class

If Double.TryParse(txtQuantity.Text, Quantity) Then
     If intCount < Quantity Then
         lstRecipe.Items.Add(Quantity & " " & lstIngredients.Text)
          intCount += 1
     End If
 Else
     MessageBox.Show("The quantity entered is not numeric. Please add a numeric    quantity.")
 End If

And you could also remove the code that test for the empty textbox.

The TryParse method wants two parameters, the first one is the string that could be converted, the second parameter is the variable that receives the result of the conversion if it is possible. If the conversion cannot be executed the function returns false.

There are numerous reasons to prefer Double.TryParse instead of IsNumeric.

The first reason is that with TryParse you also get the result of the conversion while with IsNumeric you would have to do the conversion after the check.

The second reason is that you could give to IsNumeric whatever object you want (also a Button for example) and it accepts it. You would never discover this kind of errors at compile time. Instead, with TryParse, you could only pass a string as its first parameter.