Validation in code behind

Use this information to override the ValidateForm method, which prevents a form from being submitted by the user.

The following code snippet shows an example of how to achieve this. 

public override bool ValidateForm(out string errorMessage)
{
bool isValid = base.ValidateForm(out errorMessage);
		if (isValid)
		{
			// Do additional check here
			if (ValidatingTextBox1.Text == "55")
			{
				isValid = false;
				errorMessage = "Oh no! Not 55!";	
			}
		}
		return isValid;
}