If you declare a string variable and don't assign any value in C#. Then by default it take a blank or null value itself . in this case if u use .ToString() method then our program should throw null reference exception. and in Convert.ToString() our program doesn't throw any exception. bcoz by default it takes a blank value instead of null.

see the code example . u will get a clear view.

using System;

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

namespace csharpExamples

{
class Program
{
static void Main(string[] args)
{
string abc = "" ;
abc = null;
Console.Write(abc.ToString());
Console.ReadKey();
}
}

And if u use Convert.ToString() . then u need to change this line of code ..

Console.Write(abc.ToString()); to Console.Write(Convert.ToString(abc));

to see the output of above program..

This unhandled exception thrown by NullReference.
I hope this article will be helpful in programming ..
Thanks.