ASP.NET 3.5 For Dummies®

19.4. Checking and Comparing Values

The CompareValidator control offers three validators in one: compare values in two ASP.NET controls, compare a value in a control against a constant value, and test whether a user has entered a valid data type.

The available operators are Equal, NotEqual, GreaterThan, GreaterThanEqual, LessThan, LessThanEqual, and DataTypeCheck.

The operator names are self-explanatory. For example, GreaterThanEqual validates when the input value is greater than or equal to a second control's value (or a constant value).

19.4.1. Comparing values in two controls

The CompareValidator can determine whether the value in one text box is greater than the value in a second text box. Follow these steps to display an error message if the validation test fails:

1. Add two ASP.NET TextBox controls to a Web form.

2. Add a Button control to the page.

3. From the Validation category of the Toolbox, add a CompareValidator to the page.

4. In the CompareValidator control's Properties window, set the following properties and corresponding values:

   Property	Value
   ControlToCompare	TextBox2 (or whatever your second text box is named)
   ControlToValidate	TextBox1 (or whatever yours is called)
   Display	Dynamic
   ErrorMessage	The top ^ must be greater!
   Operator	GreaterThan
   SetFocusOnError	True
   Type	Double

As shown in Figure 19-1, the CompareValidator complains at runtime if the value in the first text box isn't greater than the value in the bottom text box.

Figure 19-1. The Compare Validator judging text boxes.