Why is Dictionary.First() so slow?

Question

Not a real question because I already found out the answer, but still interesting thing.

I always thought that hash table is the fastest associative container if you

Answer 1

Reflector shows that Dictionary maintains a Entry array that it's KeyCollection.Enumerator uses. Normally, the lookup should be relatively fast, as it can just index into the array (assuming you don't want a sorted First):

// Dictionary
private Entry[] entries;

However, if you're removing the first elements of that array, then you end up walking the array until you find a non-empty one:

// Dictionary.KeyCollection.Enumerator
while (this.index < this.dictionary.count) {
    if (this.dictionary.entries[this.index].hashCode >= 0) {
        this.currentKey = this.dictionary.entries[this.index].key;
        this.index++;
        return true;
    }
    this.index++;
}

As you remove your entries, you start getting more and more empties at the front of the entries array, and it becomes slower to retrieve First next time.