Complex Keys In Generic Dictionary

Let us start with the quiz about generic dictionary.

Dictionary simpleDict = new Dictionary(StringComparer.OrdinalIgnoreCase);
simpleDict["name1"] = "value";
simpleDict["Name1"] = "value2";

What value will simpleDict["name1"] return?

Let's get back to using complex keys in generic dictionary.

.NET Framework provides IEqualityComparer<T> interface that can be used by dictionary to distinguish between different keys.

Imagine we have a complex class that we want to serve as a key in our dictionary.

public class ComplexKey
{
    public int Part1 { get; set; }
    public string Part2 { get; set; }
}

The implentation of the comparer will be the following:

public class ComplexKeyComprarer : IEqualityComparer
{
    public bool Equals(ComplexKey x, ComplexKey y)
    {
        return x.Part1.Equals(y.Part1) && x.Part2.Equals(y.Part2);
    }

    public int GetHashCode(ComplexKey obj)
    {
        return obj.Part1.GetHashCode() ^ obj.Part2.GetHashCode();
    }
}

Having created the comparer we can now instantiate dictionary and operate with complex keys in the same way as with simple ones.

Dictionary<ComplexKey, string> complexDict = 
    new Dictionary<ComplexKey, string>(new ComplexKeyComprarer());

ComplexKey ck1 = new ComplexKey() { Part1 = 1, Part2 = "name1" };
ComplexKey ck2 = new ComplexKey() { Part1 = 1, Part2 = "name2" };

complexDict[ck1] = "value1";
complexDict[ck2] = "value2";

Very convenient by the way :)