Question

I know that the order of a dictionary is undefined, MSDN says so:

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

Thats fine, but if I have two instances of a dictionary, each with the same content, will the order be the same?

I’m guessing so because as I understand, the order is determined by the hash of the keys, and if the two dictionaries have the same keys, they have the same hashes, and therefore the same order…

… Right?

Thanks!

Andy.

Answer 1

No it is not guaranteed to be the same order. Imagine the scenario where you had several items in the Dictionary<TKey, TValue> with the same hash code. If they are added to the two dictionaries in different orders it will result in different orders in enumeration .

Consider for example the following (equality conforming) code

class Example
{
    public char Value;
    public override int GetHashCode()
    {
        return 1;
    }
    public override bool Equals(object obj)
    {
        return obj is Example && ((Example)obj).Value == Value;
    }
    public override string ToString()
    {
        return Value.ToString();
    }
}

class Program
{
    static void Main(string[] args)
    {
        var e1 = new Example() { Value = 'a' };
        var e2 = new Example() { Value = 'b' };
        var map1 = new Dictionary<Example, string>();
        map1.Add(e1, "1");
        map1.Add(e2, "2");

        var map2 = new Dictionary<Example, string>();
        map2.Add(e2, "2");
        map2.Add(e1, "1");

        Console.WriteLine(map1.Values.Aggregate((x, y) => x + y));
        Console.WriteLine(map2.Values.Aggregate((x, y) => x + y));
    }
}

The output of running this program is

12
21