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