C# list.remove throwing enumeration operation error in foreach loop

Question

I have looked at multiple SO questions related to mine, as well as Googling, and I have been unable to find a solution that works for me.

Given a list of files, I am trying to cull all the non-cs ones, for reasons. I take the string array, convert to list, and iterate over the list, removing all the files I don't want by using list.Remove().

After the first removal, it errors out with

Collection was modified; enumeration operation might not execute

The code is:

string[] files = null;
            try
            {
                files = System.IO.Directory.GetFiles(currentDir);

                //Converting to list, to use list.Remove rather than rewriting entire array at each delete
                var list = new List<string>(files);
                foreach (string readFile in list)
                {
                    if (Path.GetExtension(readFile) != ".cs") //|| Path.GetExtension(readFile) != ".dll")
                    {
                        //remove, as we don't currently care about non cs files.
                        list.Remove(readFile);
                    }
                }

                //Converting back to string array for use in the rest of the program
                files = list.ToArray();
            }

I have also tried RemoteAt(), which produces the same error.

string[] files = null;
            try
            {
                files = System.IO.Directory.GetFiles(currentDir);

                //Converting to list, to use list.Remove rather than rewriting entire array at each delete
                var list = new List<string>(files);
                int i = 0;
                for(int i=0; i<list.Count();i++)
                {
                    if (Path.GetExtension(readFile) != ".cs") //|| Path.GetExtension(readFile) != ".dll")
                    {
                        //remove, as we don't currently care about non cs files.
                        list.RemoveAt(i);
                    }
                    i++;
                }

                //Converting back to string array for use in the rest of the program
                files = list.ToArray();
            }

Any recommendations for overcoming this error, as some of the directories will have over 500 files in them, and I want to avoid rewriting the string array as much as possible.

I have read the following SO questions:

How to remove item from list in C#?

C# error Collection was modified; enumeration operation might not execute

Enumerations Foreach Loop C#

"List.Remove" in C# does not remove item?

https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.list-1.remove?view=netframework-4.8

https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.list-1.removeat?view=netframework-4.8

If you only want `*.cs`, you can pass that to [`GetFiles`](https://learn.microsoft.com/en-us/dotnet/api/system.io.directoryinfo.getfiles?view=netframework-4.8#System_IO_DirectoryInfo_GetFiles_System_String_). — Blorgbeard, Apr 29 '19 at 16:25
Or you can replace your for-loop with a quick LINQ expression, something like `return files.Where(t=>Path.GetExtension(t) == ".cs").ToArray();` — Blorgbeard, Apr 29 '19 at 16:26

score 2 · Answer 1 · answered Apr 29 '19 at 16:31

2

You can also use a for loop in reverse order so that removed entries do not effect the loop.

for (int x = list.Count-1; x >= 0; x--)
{
  string readFile = list[x];
  // ...
  list.Remove(readFile);
}

answered Apr 29 '19 at 16:31

Troy Mac1ure

637
4
9

score 1 · Accepted Answer · answered Apr 29 '19 at 16:21

1

If I change

foreach (string readFile in list)

to

foreach (string readFile in list.ToList())

it works.

My guess is that the list.Remove alters the original list, and invalidates the enumeration.

answered Apr 29 '19 at 16:21

GokuMizuno

493
2
5
14

2

"*My guess is that the list.Remove alters the original list, and invalidates the enumeration.*" -- this is correct – canton7 Apr 29 '19 at 16:22

score 1 · Answer 3 · answered Apr 29 '19 at 16:27

1

You can use the RemoveAll method on List:

list.RemoveAll(readFile => Path.GetExtension(readFile) != ".cs");

This will remove everything that matches your predicate, and will return the number of items it removed.

answered Apr 29 '19 at 16:27

Sean

60,939
11
97
136

C# list.remove throwing enumeration operation error in foreach loop

3 Answers3