Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object

Question

I am trying to print the contents of a Stack.

When I make the attempt I get the following error.

Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.

Which is happening on the foreach line of my code. I'm unsure why this is happening since I thought I was using the example given on the page I linked. Example would be...

        foreach( string number in numbers )
        {
            Console.WriteLine(number);
        }

Following is my code. Everything appears to be functional except this part which throws the error.

            foreach(var s in stack)
            {
                Console.WriteLine(s);
            }

...and this is my code.

using System;

namespace Exercise
{
    class Program
    {
        static void Main()
        {
            var stack = new Stack();
            stack.Push(1);
            stack.Push(2);
            stack.Push(3);

            foreach(var s in stack)
            {
                Console.WriteLine(s);
            }
        }
    }
}

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace Exercise
{
    internal class Stack : IEnumerable
    {
        private object _object;
        private List<object> list = new List<object>();
        private IEnumerator Enumerator;
        public IEnumerator GetEnumerator() => Enumerator;

        internal object Pop()
        {
            if (list.Count == 0)
                throw new InvalidOperationException("Cannot use .Pop() if list count equals 0.");

            _object = list.FirstOrDefault();

            list.RemoveAt(0);

            return _object;
        }

        internal void Push(object obj)
        {
            _object = obj;

            if (_object == null)
                throw new InvalidOperationException("Cannot use .Push() if object is null.");

            list.Insert(0, _object);
        }

        internal void Clear()
        {
            if (list.Count == 0)
                throw new InvalidOperationException("Cannot use .Clear() if list is empty.");

            list.Clear();
        }
    }
}

What am I doing wrong and how can I fix this to print the contents of the stack?

Gur Galler · Accepted Answer · 2020-04-01 10:59:35Z

3

Your GetEnumerator method returns null because the field Enumerator was never explicitly initialized, so it got the default value null.

Then, the foreach loop calls .GetEnumerator(), receives a null and tries to access the .Current property of null, so you get a NullReferenceExcpetion.

To fix this, you can use the following implementation:

public IEnumerator GetEnumerator()
{
    while (list.Any())
        yield return Pop();
}

edited Apr 1, 2020 at 10:59

answered Mar 31, 2020 at 17:44

Gur Galler

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2 Comments

rfmodulator Over a year ago

I like your solution, but it doesn't Enumerate the entire collection because you're increasing i as list.Count decreases. In this example, when list.Count is 1, i is 2, and the for loop ends. Instead, use while (list.Any()).

Milliorn Over a year ago

Thank you Gur Galler for that. That was what I was missing. Also, thank you @rfmodulator for mentioning the fix to Gur Galler code. Everything works now.

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