C# Collection was modified; enumeration operation may not execute [duplicate]

Answer 1

Any collection that you iterate over with foreach may not be modified during iteration.

So while you're running a foreach over rankings, you cannot modify its elements, add new ones or delete any.

Answer 2

As others have pointed out, you are modifying a collection that you are iterating over and that's what's causing the error. The offending code is below:

foreach (KeyValuePair<int, int> kvp in rankings)
{
    .....

    if((double)(similarModules/modules.Count)>0.6)
    {
        rankings[kvp.Key] = rankings[kvp.Key] + 4;  // <--- This line is the problem
    }
    .....

What may not be obvious from the code above is where the Enumerator comes from. In a blog post from a few years back about Eric Lippert provides an example of what a foreach loop gets expanded to by the compiler. The generated code will look something like:

{
    IEnumerator<int> e = ((IEnumerable<int>)values).GetEnumerator(); // <-- This
                                                       // is where the Enumerator
                                                       // comes from.
    try
    { 
        int m; // OUTSIDE THE ACTUAL LOOP in C# 4 and before, inside the loop in 5
        while(e.MoveNext())
        {
            // loop code goes here
        }
    }
    finally
    { 
      if (e != null) ((IDisposable)e).Dispose();
    }
}

If you look up the MSDN documentation for IEnumerable (which is what GetEnumerator() returns) you will see:

Enumerators can be used to read the data in the collection, but they cannot be used to modify the underlying collection.

Which brings us back to what the error message states and the other answers re-state, you're modifying the underlying collection.

Answer 3

The error tells you EXACTLY what the problem is (and running in the debugger or reading the stack trace will tell you exactly where the problem is):

C# Collection was modified; enumeration operation may not execute.

Your problem is the loop

foreach (KeyValuePair<int, int> kvp in rankings) {
    //
}

wherein you modify the collection rankings. In particular, the offensive line is

rankings[kvp.Key] = rankings[kvp.Key] + 4;

Before you enter the loop, add the following line:

var listOfRankingsToModify = new List<int>();

Replace the offending line with

listOfRankingsToModify.Add(kvp.Key);

and after you exit the loop

foreach(var key in listOfRankingsToModify) {
    rankings[key] = rankings[key] + 4;
}

That is, record what changes you need to make, and make them without iterating over the collection that you need to modify.

Answer 4

I suspect the error is caused by this:

foreach (KeyValuePair<int, int> kvp in rankings)

rankings is a dictionary, which is IEnumerable. By using it in a foreach loop, you're specifying that you want each KeyValuePair from the dictionary in a deferred manner. That is, the next KeyValuePair is not returned until your loop iterates again.

But you're modifying the dictionary inside your loop:

rankings[kvp.Key] = rankings[kvp.Key] + 4;

which isn't allowed...so you get the exception.

You could simply do this

foreach (KeyValuePair<int, int> kvp in rankings.ToArray())

Answer 5

The problem is where you are executing:

rankings[kvp.Key] = rankings[kvp.Key] + 4;

You cannot modify the collection you are iterating through in a foreach loop. A foreach loop requires the loop to be immutable during iteration.

Instead, use a standard 'for' loop or create a new loop that is a copy and iterate through that while updating your original.