using System;

​

public class MyObject

{

    public string Name { get; }

    public MyObject(string name) => Name = name;

}

​

public class MyFormatter : IFormatProvider, ICustomFormatter {

​

    public object GetFormat(Type formatType)

    {

        return this;

    }

    public string Format(string fmt, object obj, IFormatProvider formatProvider)

    {

        if(obj is MyObject myObj)

        {

            return myObj.Name;

        }

        else

        {

            return "NULL!";

        }

    }

}

public class Program

{

    public static void Main()

    {

        MyObject obj = null;

        Console.WriteLine(obj?.ToString() ?? "");

        Console.WriteLine(Convert.ToString(obj, new MyFormatter()));

        Console.WriteLine(string.Format(new MyFormatter(), "{0}", obj));

    }

}