Accessing a Dictionary.Keys Key through a numeric index

As @Falanwe points out in a comment, doing something like this is incorrect:

int LastCount = mydict.Keys.ElementAt(mydict.Count -1);

You should not depend on the order of keys in a Dictionary. If you need ordering, you should use an OrderedDictionary, as suggested in this answer. The other answers on this page are interesting as well.

You can use an OrderedDictionary.

Represents a collection of key/value pairs that are accessible by the key or index.

A Dictionary is a Hash Table, so you have no idea the order of insertion!

If you want to know the last inserted key I would suggest extending the Dictionary to include a LastKeyInserted value.

E.g.:

public MyDictionary<K, T> : IDictionary<K, T>
{
    private IDictionary<K, T> _InnerDictionary;

    public K LastInsertedKey { get; set; }

    public MyDictionary()
    {
        _InnerDictionary = new Dictionary<K, T>();
    }

    #region Implementation of IDictionary

    public void Add(KeyValuePair<K, T> item)
    {
        _InnerDictionary.Add(item);
        LastInsertedKey = item.Key;

    }

    public void Add(K key, T value)
    {
        _InnerDictionary.Add(key, value);
        LastInsertedKey = key;
    }

    .... rest of IDictionary methods

    #endregion

}

You will run into problems however when you use .Remove() so to overcome this you will have to keep an ordered list of the keys inserted.

Why don't you just extend the dictionary class to add in a last key inserted property. Something like the following maybe?

public class ExtendedDictionary : Dictionary<string, int>
{
    private int lastKeyInserted = -1;

    public int LastKeyInserted
    {
        get { return lastKeyInserted; }
        set { lastKeyInserted = value; }
    }

    public void AddNew(string s, int i)
    {
        lastKeyInserted = i;

        base.Add(s, i);
    }
}

You could always do this:

string[] temp = new string[mydict.count];
mydict.Keys.CopyTo(temp, 0)
int LastCount = mydict[temp[mydict.count - 1]]

But I wouldn't recommend it. There's no guarantee that the last inserted key will be at the end of the array. The ordering for Keys on MSDN is unspecified, and subject to change. In my very brief test, it does seem to be in order of insertion, but you'd be better off building in proper bookkeeping like a stack--as you suggest (though I don't see the need of a struct based on your other statements)--or single variable cache if you just need to know the latest key.

I think you can do something like this, the syntax might be wrong, havent used C# in a while To get the last item

Dictionary<string, int>.KeyCollection keys = mydict.keys;
string lastKey = keys.Last();

or use Max instead of Last to get the max value, I dont know which one fits your code better.

I agree with the second part of Patrick's answer. Even if in some tests it seems to keep insertion order, the documentation (and normal behavior for dictionaries and hashes) explicitly states the ordering is unspecified.

You're just asking for trouble depending on the ordering of the keys. Add your own bookkeeping (as Patrick said, just a single variable for the last added key) to be sure. Also, don't be tempted by all the methods such as Last and Max on the dictionary as those are probably in relation to the key comparator (I'm not sure about that).

The way you worded the question leads me to believe that the int in the Dictionary contains the item's "position" on the Dictionary. Judging from the assertion that the keys aren't stored in the order that they're added, if this is correct, that would mean that keys.Count (or .Count - 1, if you're using zero-based) should still always be the number of the last-entered key?

If that's correct, is there any reason you can't instead use Dictionary<int, string> so that you can use mydict[ mydict.Keys.Count ]?

I don't know if this would work because I'm pretty sure that the keys aren't stored in the order they are added, but you could cast the KeysCollection to a List and then get the last key in the list... but it would be worth having a look.

The only other thing I can think of is to store the keys in a lookup list and add the keys to the list before you add them to the dictionary... it's not pretty tho.