Question

Given the following code, I’m curious to know how to avoid the following exception

System.InvalidOperationException was unhandled
Message=Collection was modified; enumeration operation may not execute.
Source=mscorlib
StackTrace:
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at System.Collections.Generic.List`1.Enumerator.MoveNext()
   at PBV.Program.Main(String[] args) in C:\Documents and Settings\tmohojft\Local Settings\Application Data\Temporary Projects\PBV\Program.cs:line 39
   at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
   at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
   at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()
InnerException: 

CODE:

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

namespace PBV
{
class Program
{
    struct structItem
    {
        public int y { get; set; }
        public int z { get; set; }
    }

    struct testStruct
    {
        public int x { get; set; }
        public List<structItem> items { get; set; }
    }

    static void Main(string[] args)
    {
        testStruct a = new testStruct();
        structItem b = new structItem();

        for (byte i = 0; i <= 10; i++) {
            b.y = i;
            b.z = i * 2;
            a.items = new List<structItem>();
            a.items.Add(b);
        }

        testStruct c = new testStruct();
        c = a;

        int counter = 0;

        //exception thrown on line below
        foreach (var item in a.items) {
            structItem d = item;
            d.z = 3;

            c.items[counter] = d;
            counter++;
        }

        a = c;
    }
}
}

I originally tried to simply put the following in the second foreach:

item.z = 3;

But that caused the following error:

Cannot modify members of "item" because it is a "foreach iteration" 

I tried to create a temporary object in order to be able to modify the struct data inside the foreach, but I recieve the exception above. My best guess is because my temporary struct is saving a reference to the original struct rather than the value itself – which is leading my original struct to be updated when my temporary struct is.

So my question is:
How do I pass this struct by value rather than reference? Or is there an entirely different way to get around this issue?

Thanks in advance for the help.

EDIT: Thanks for all the answers guys. I’m aware that the list is a reference type, but does that make it impossible to pass by value instead of reference then?

Answer 1

I’m a little confused by what your example code is trying to do, but I think perhaps part of your confusion may be that when you set c = a, you expect it to make a copy of the list. It does not. While the structure itself is a value type, the items property it contains is not. List<> is a reference type, so when you set c = a, it copies that items reference to c. So, when you enter the loop, both a and c contain a reference to the same list object. Therefore, when you modify the list while you are enumerating through it, it will always fail.

A simple way to avoid this is to iterate through a static copy of your list:

foreach (var item in a.items.ToArray())