extracting keys and values from disctionary using an index

I'm not sure about the index here. What happens if you add the items into the list in different order? Do you want to get the third smallest key? Why I'm asking this is because SortedDictionary[^] guarantees that the items are sorted by the key.

So one possibility, if you want to use Dictionary, is to query the dictionary using LINQ and define the sorting in the statement. Something like:

var q = from b in
               (from a in ListPPM
                orderby a.Key
                select a).Skip(2)
            select b;

q.First().Key;

you can access a dictionary's key by its ordinal in the key collection with the following:

int index = 3;//or whatever you want
ListPPM.Keys.ElementAt<int>(index);

requires System.Linq namespace

EDIT : post format was messing up generic type

public sealed class CustomDictionary<tkey,TValue> : KeyedCollection<tkey,TValue>>
{
    protected override TKey GetKeyForItem(KeyValuePair<tkey,TValue> item)
    {
        return item.Key;
    }
    
}

CustomDictionary<string,int> custDict = new CustomDictionary<string,int>(); 
CustDict.Add(new KeyValuePair<string,>("key", 7));
CustDict.Add(new KeyValuePair<string,>("key1", 8));
KeyValuePair<string,> i = CustDict[1];
int valueByIndex = CustDict[1].Value;
int valueByKey = CustDict["key"].Value;
string keyByIndex = CustDict[0].Key;

// Your requirment 
string skeyByIndex = CustDict[0].Key+","+CustDict[0].Value;