Question

I have a List<string> AllowedList and a Dictionary<string,List<string>> MyDictionary.

For each key in the dictionary, I want to check if it’s on the AllowedList, if not I want to delete the key and value from the dictionary.

I intuitively tried this which seems to be what I wanted:

foreach (string key in MyDictionary.Keys)
{
    if (!AllowedList.Contains(key)) MyDictionary.Remove(key);
}

However, I encountered an InvalidOperationException:

Collection was modified; enumeration operation may not execute.

I’m sure there’s probably a simple way around this, but I don’t immediately see it.

Answer 1

You could use Enumerable.Except to find the keys that are not in the dictionary:

foreach (var key in MyDictionary.Keys.Except(AllowedList).ToList())
    MyDictionary.Remove(key);

The ToList() creates a new list of the set difference and prevents the exception.