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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Symptoms

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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Properties

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