String.ToString() method on null String Object

Question

I have the following code:

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //Your code goes here
            var obj=(person)Activator.CreateInstance(typeof(person));
            Console.WriteLine(obj);
        }
    }
     public class person
        {
            public int id { get; set; }
            public string name { get; set; }
            public DateTime dob { get; set; }

            public override string ToString()
            {
                return id.ToString() + " " + name + " " + dob.ToString();
            }
        }
}

which yields the following output:

0  1/1/0001 12:00:00 AM

However, if change the person.ToString() to the following:

public override string ToString()
{
        return id.ToString() + " " + name.ToString() + " " + dob.ToString();
}

I get the following error:

System.NullReferenceException: Object reference not set to an instance of an object.
   at Rextester.person.ToString()

Can someone shed some light on it.

Edited

1) So...how is that line different than your code above? 2) Why are you using `Activator` and not just the constructor? — Servy, Aug 23 '13 at 14:55
possible duplicate of [What is a NullReferenceException in .NET and how do I fix it?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-in-net-and-how-do-i-fix-it) — User, Aug 23 '13 at 14:55
Why not just use the default constructor for `person`? Also I can't tell what you changed. — BrianM, Aug 23 '13 at 14:58
Please edit your answer to tell use what change you really made to Person.ToString. Without that, it's hard to answer your question. — hatchet - done with SOverflow, Aug 23 '13 at 15:07

score 4 · Accepted Answer · edited May 23 '17 at 12:05

4

I'm guessing your code samples aren't correct as it stands in your question and you're actually seeing this behavior:

return id.ToString() + " " + name + " " + dob.ToString();

works

return id.ToString() + " " + name.ToString() + " " + dob.ToString();

doesn't work

This is because adding a null value to a string is legal but calling a method on a null instance is not.

See this question: Why is adding null to a string legal?

edited May 23 '17 at 12:05

Community

1
1

answered Aug 23 '13 at 14:58

nvuono

3,193
23
26

2

@Servy - it's not the code he's posted, but clearly the code he's posted is not right since it's identical to the first version. nvuono's guess at what was intended makes a lot of sense. – hatchet - done with SOverflow Aug 23 '13 at 15:14
2

@hatchet There are any number of things that could be done instead as well. Rather than guessing, it's more productive to simply wait for the OP to edit his *real* code in. – Servy Aug 23 '13 at 15:17

String.ToString() method on null String Object

1 Answers1