A look at the difference between Convert.ToString() and .ToString() method.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ExampleOfString
{
    class Program
    {
        static void Main(string[] args)
        {
            String UserName = null ;
            try
            {
                //Below statement WILL THROW an exception as .ToString() does not handles null values
                String strUserId = UserName.ToString();
            }
            catch (NullReferenceException ex)
            {
                Console.WriteLine("Error:{0}", ex.Message);
            }

            try
            {
                //Below statement WILL NOT THROW an exception as convert.tostring handles implicitly
                String strUserId = Convert.ToString(UserName);
                Console.WriteLine("UserName:{0}", strUserId);
            }
            catch (NullReferenceException ex)
            {
                Console.WriteLine("Error:{0}", ex.Message);
            }
        }
    }
}

/*
Below is the output of the console application

Error:Object reference not set to an instance of an object.
UserName:
Press any key to continue . . .
*/