C# calling a method from inner class that has reference to outerclass

Question

I have a class called Foo, it has an inner class Inner. The Inner class has reference to outer class's field.

public class Foo
{
    int foo = 2;

    public class Inner
    {
        Foo f;
        public Inner(Foo f)
        {
            f=f;
        }

        public override String ToString()
        {
            return "Inner[foo="+f.foo+"]" ;
        }
    }
}

This is my program.

public class Program
{
   public static void Main(string[] args)
   {    
      Foo foo=new Foo();
      Foo.Inner inner=new Foo.Inner(foo);
      Console.WriteLine(inner.ToString());
   }
}

There is no problem to compile it, but it fails when I run it. It gives me an exception

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object. at Foo.Inner.ToString()

What is going on here?

Why do you want to create a nested class? Why can't it be two independent classes with a readonly property of type `Inner` in `Foo`? — Nikhil Agrawal, Jul 08 '14 at 05:32
http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it — Soner Gönül, Jul 08 '14 at 05:34

Nikhil Agrawal · Accepted Answer · 2014-07-08T05:45:40.963

6

Problem:
Your problem is in doing f = f. It is not doing anything productive. This line does not update the Inner field f and it remains unassigned or null. So when you do Console.WriteLine(inner.ToString());, it throws error at "Inner[foo=" + f.foo + "]"

Also VS shows a warning

enter image description here

Solution:
1. Change constructor variable to something else. Like this

Foo f;
public Inner(Foo f1)
{
    f = f1;
}

OR
2. Add this to distinguish between variables.

Foo f;
public Inner(Foo f)
{
    this.f = f;
}

edited Jul 08 '14 at 05:45

answered Jul 08 '14 at 05:35

Nikhil Agrawal

47,018
22
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I wonder whether a "release" compilation would cut the whole operation, avoiding the exception...a nice test to do! – Mario Vernari Jul 08 '14 at 05:36
@MarioVernari no it wont, if it cut it then the code won't work! – lahsrah Jul 08 '14 at 05:44
1

@MarioVernari: It still throws exception in Release mode even though warning is ignored but this line still remains `return "Inner[foo=" + f.foo + "]";`. – Nikhil Agrawal Jul 08 '14 at 05:57

score 3 · Answer 2 · answered Jul 08 '14 at 05:37

3

To fix the problem you can chnage the following

f=f;

to

this.f = f;

answered Jul 08 '14 at 05:37

3dd

2,520
13
20

score 0 · Answer 3 · answered Jul 08 '14 at 05:41

0

You have to assgined a Foo obejct like

        Foo f;
        public Inner(Foo _f)
        {
            f = _f;
        }

answered Jul 08 '14 at 05:41

Dhaval Patel

7,471
6
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C# calling a method from inner class that has reference to outerclass

3 Answers3