InvalidOperationException when removing items in a collection

Author: Alvin Bruney MVP

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SUMMARY

You receive an InvalidOperationException with the following error message: Collection was modified: enumeration operation may not execute.

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When you iterate a collection and remove items in the collection, you receive an InvalidOperationException with the following error message: Collection was modified: enumeration operation may not execute.

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CAUSE

Code iterating a collection cannot modify the collection because the run-time makes certain assumptions about the size of the collection.

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RESOLUTION

you can work around the limitation by creating a new collection dynamically to hold the enumerated collection. Here is the complete code.

using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication
{
    class Program
    {
        static void Main(string[] args)
        {
            System.Collections.ArrayList arr = new System.Collections.ArrayList();
            arr.Add("1");
            arr.Add("2");
            arr.Add("3");

            /*This throws an exception
            foreach (string s in arr)
            {
                arr.Remove(s);
            }
            */

            //where as this works correctly
            Console.WriteLine(arr.Count);
            foreach (string s in new System.Collections.ArrayList(arr))
            {
                arr.Remove(s);
            }
            Console.WriteLine(arr.Count);
            Console.ReadKey();
        }
    }
}

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MORE INFORMATION

This link http://msdn2.microsoft.com/en-us/library/system.collections.ienumerator(vs.71).aspx discusses the limitation in further detail. It is important to note that the code does not remove the restriction on iterating a collection. Instead, the code simply moves the restriction from one container (arr) to the other (ArrayList).

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APPLIES TO

Microsoft ASP.NET 2.0

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kbpubmvp kbpubtypecca kbhowto KB555972

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