int age = 25;
string  s = age.ToString();  // age value will be converted to String
string s  = Convert.ToString(age);  // age value will be converted to string
 
// But when we have  a null value for an object
int age = 0;
string  s = age.ToString();  // age value will not be converted to String, 
//as .ToString(); 无法处理将会抛出异常
 
string s  = Convert.ToString(age);  // age value will be converted to string, 
//as Convert.ToString(); 可以正确处理，不会抛出异常;
//csharp/7535