OrderedDictionary and Dictionary

Question

I was looking for a way to have my Dictionary enumerate its KeyValuePair in the same order that they were added. Now, Dictionary's documentation clearly states that:

For purposes of enumeration, each item in the dictionary is treated as a KeyValuePair<TKey, TValue> structure representing a value and its key. The order in which the items are returned is undefined.

I found out that what I needed was an OrderedDictionary, but being the sceptic that I am, I decided to try it myself:

OrderedDictionary od = new OrderedDictionary(); Dictionary<String, String> d = new Dictionary<String, String>();  for (int i = 0; i < 10; i++) {     od.Add("key" + i, "value" + i);     d.Add("key" + i, "value" + i); }  System.Console.WriteLine("OrderedDictionary"); foreach (DictionaryEntry de in od) {     System.Console.WriteLine(de.Key + ", " + de.Value); }  System.Console.WriteLine("Dictionary"); foreach (var tmp in d) {     System.Console.WriteLine(tmp.Key + ", " + tmp.Value); } 

Output:

OrderedDictionary key0, value0 key1, value1 key2, value2 ...  Dictionary key0, value0 key1, value1 key2, value2 ... 

As you can see, both are ordered, and that raise two questions:

In which case does the Dictionary give a different order that the one in which the values are added? Does my first foreach loop assure me to retrieve my KeyValuePair in the same order, or do I have to use the index?

Answer 1

You are doing it wrong. You need not only to insert values sequentially into dictionary, but also remove some elements and see how the order has changed after this. The next code demonstrates this:

OrderedDictionary od = new OrderedDictionary(); Dictionary<String, String> d = new Dictionary<String, String>(); Random r = new Random();  for (int i = 0; i < 10; i++) {     od.Add("key" + i, "value" + i);     d.Add("key" + i, "value" + i);     if (i % 3 == 0)     {         od.Remove("key" + r.Next(d.Count));         d.Remove("key" + r.Next(d.Count));     } }  System.Console.WriteLine("OrderedDictionary"); foreach (DictionaryEntry de in od) {     System.Console.WriteLine(de.Key + ", " +de.Value); }  System.Console.WriteLine("Dictionary"); foreach (var tmp in d) {     System.Console.WriteLine(tmp.Key + ", " + tmp.Value); } 

prints something similar to (OrderedDictionary is always ordered):

OrderedDictionary key3, value3 key5, value5 key6, value6 key7, value7 key8, value8 key9, value9 Dictionary key7, value7 key4, value4 key3, value3 key5, value5 key6, value6 key8, value8 key9, value9