C# Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object

Question

Hello, I have a problem understanding why this piece of code:

using System;

class Person
{
    public Person()
    {
    }
}


class NameApp
{
    public static void Main()
    {
        Person me = new Person();
        Object you = new Object();

        me = you as Person;
        //me = (Person) you;

        System.Console.WriteLine("Type: {0}", me.GetType()); //This line throws exception
    }
}

Throws this exception:

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object. at NameApp.Main() in C:\Users\Nenad\documents\visual studio 2010\Projects\Exercise 11.3\Exercise 11.3\Program.cs:line 21

Plenty of people pointing out the obvious - but what did you expect you as Person to do? — Damien_The_Unbeliever
– Damien_The_Unbeliever, Commented Mar 5, 2013 at 7:42
I was just following tutorial from a book - most likely author misplaced "you" and "me", but there is not info about it in ERRATA, on companion website, so I just wanted to check it here to be sure, just in case that I didn't miss something. — Nenad Bulatović
– Nenad Bulatović, Commented Mar 5, 2013 at 12:45

Habib · Accepted Answer · 2013-03-05 07:36:25Z

2

Your line

me = you as Person;

is failing and assigning null to me because you can't cast a base class type object to child class.

as (C# Reference)

The as operator is like a cast operation. However, if the conversion isn't possible, as returns null instead of raising an exception.

You probably want to cast person as object, since me is an Object, but you is not a person.

answered Mar 5, 2013 at 7:36

Habib

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1 Comment

Nenad Bulatović Over a year ago

I was just following tutorial from a book - most likely author misplaced "you" and "me", but there is not info about it in ERRATA, on companion website, so I just wanted to check it here to be sure, just in case that I didn't miss something.

Jens Kloster · Accepted Answer · 2013-03-05 07:36:31Z

2

This code will always set me as null

   Object you = new Object();
   me = you as Person;

because Obejct is not person

but person is object :

object you = new Person();
me = you as Person;

answered Mar 5, 2013 at 7:36

Jens Kloster

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Comments

mgttlinger · Accepted Answer · 2013-03-05 07:43:55Z

1

If you cast with the as keyword and the cast is not possible it returns null. Then in your case you call me.GetType() with me being null at the moment so an exception is thrown.

If you cast like (Person) objectOfTypeThatDoesNotExtendPerson an exception is thrown instantly at the cast.

answered Mar 5, 2013 at 7:43

mgttlinger

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Comments

Darin Dimitrov · Accepted Answer · 2013-03-05 07:36:41Z

1

Object you = new Object();
me = you as Person;

you is an object, not a Person so you as Person will simply return null.

answered Mar 5, 2013 at 7:36

Darin Dimitrov

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MarcinJuraszek · Accepted Answer · 2013-03-05 07:36:55Z

1

me = you as Person;

me is null if you cannot be casted to Person (and that's what happening in your situation, because new Object() cannot be casted to Person.

answered Mar 5, 2013 at 7:36

MarcinJuraszek

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nvoigt · Accepted Answer · 2013-03-05 07:37:28Z

1

The as operator returns null if the object was not of the type you requested.

me = you as Person;

you is an Object, not a Person, so (you as Person) is null and therefore me is null. When you call GetType() on me afterwards, you get a NullReferenceException.

answered Mar 5, 2013 at 7:37

nvoigt

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Comments

purplesoft · Accepted Answer · 2013-03-05 07:37:56Z

1

public static void Main()
{
    Person me = new Person();
    Object you = new Object();

    // you as person = null
    me = you as Person;
    //me = (Person) you;

    System.Console.WriteLine("Type: {0}", me.GetType()); //This line throws exception
}

answered Mar 5, 2013 at 7:37

purplesoft

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Comments

Dejo · Accepted Answer · 2013-03-05 07:38:22Z

1

Object can not be cast to a Person. It is a princip of object oriented programming.

Object is a parent class of a Person. Every class inherits it,

you can cast Person to Object, but cannot cast Object to Person

answered Mar 5, 2013 at 7:38

Dejo

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