validate Textbox input for decimal values in c#

You could add a custom validator to the page, and handle the ServerValidate event

 
  protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args) {
    decimal value;
 
    // Test for decimal
    if (!decimal.TryParse(args.Value, out value)) {
      args.IsValid = false;
      return;
    }
 
    // Test for 2 decimal places
    args.IsValid = ((value * 100) % 1 == 0);
  }

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If you don't require users to type the decimals but only want to prevent them from inputting more than 2, you can use a CompareValidator, set the ControlToValidate to your textbox, set the Operator to DataTypeCheck and Type to Currency.
It will allow numbers like 123 | 123.0 | 123.00 | 123.99