Why the error Collection was modified; enumeration operation may not execute occurs and how to handle it in C#?

CsharpServer Side ProgrammingProgramming

This error occurs when a looping process is being running on a collection (Ex: List) and the collection is modified (data added or removed) during the runtime.

Example

 Live Demo

using System;
using System.Collections.Generic;
namespace DemoApplication {
   public class Program {
      static void Main(string[] args) {
         try {
            var studentsList = new List<Student> {
               new Student {
                  Id = 1,
                  Name = "John"
               },
               new Student {
                  Id = 0,
                  Name = "Jack"
               },
               new Student {
                  Id = 2,
                  Name = "Jack"
               }
            };
            foreach (var student in studentsList) {
               if (student.Id <= 0) {
                  studentsList.Remove(student);
               }
               else {
                  Console.WriteLine($"Id: {student.Id}, Name: {student.Name}");
               }
            }
         }
         catch(Exception ex) {
            Console.WriteLine($"Exception: {ex.Message}");
            Console.ReadLine();
         }
      }
   }
   public class Student {
      public int Id { get; set; }
      public string Name { get; set; }
   }
}

Output

The output of the above code is

Id: 1, Name: John
Exception: Collection was modified; enumeration operation may not execute.

In the above example, the foreach loop is executed on the studentsList. When the Id of the student is 0, the item will be removed from the studentsList. Because of this change the studentsList gets modified (resized) and an exception is being thrown during the runtime.

Fix for the above issue

To overcome the above issue, perform a ToList() operation on the studentsList before the start of each iteration.

foreach (var student in studentsList.ToList())

Example

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Threading.Tasks;
namespace DemoApplication {
   public class Program {
      static void Main(string[] args) {
         var studentsList = new List<Student> {
            new Student {
               Id = 1,
               Name = "John"
            },
            new Student {
               Id = 0,
               Name = "Jack"
            },
            new Student {
               Id = 2,
               Name = "Jack"
            }
         };
         foreach (var student in studentsList.ToList()) {
            if (student.Id <= 0) {
               studentsList.Remove(student);
            }
            else {
               Console.WriteLine($"Id: {student.Id}, Name: {student.Name}");
            }
         }
         Console.ReadLine();
      }
   }
   public class Student {
      public int Id { get; set; }
      public string Name { get; set; }
   }
}

The output of the above code is

Output

Id: 1, Name: John
Id: 2, Name: Jack
raja
Updated on 08-Aug-2020 11:02:45

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