How to set validations in the web methods under Web Service to limit text entry length?

[WebMethod]
public string HelloWorld(string input)
{

   // you can throw new exception
   if (string.IsNullOrEmpty(input) || input.Length > 14)               
       throw new ArgumentException("Invalid Input");

   //Or you can limit the input without throwing exception  
   if (string.IsNullOrEmpty(input) &&  input.Length > 14)               
       input = input.Substring(0,14)

   // do something with input 
}