I recently needed a dictionary for a small project. Nothing special there but on this occasion, I needed a dictionary that was keyed by two sub-keys. I thought about using an array but you loose strong typing. I thought about using  a dictionary of dictionaries but the code is messy, complicated and difficult to maintain. Here is a better solution. (I also coded structures that allow for compound keys with 3 and 4 values. If you need that, just expand on the 2 key version.):

[Serializable, StructLayout(LayoutKind.Sequential)]

public struct CompoundKey<T1, T2>

{

    private T1 key1;

    private T2 key2;

    public CompoundKey(T1 key1, T2 key2)

    {

        this.key1 = key1;

        this.key2 = key2;

    }

    public T1 Key1

    {

        get { return this.key1; }

    }

    public T2 Key2

    {

        get { return this.key2; }

    }

    public override string ToString()

    {

        StringBuilder builder = new StringBuilder();

        builder.Append('[');

        if (this.Key1 != null)

        {

            builder.Append(this.Key1.ToString());

        }

        builder.Append(", ");

        if (this.Key2 != null)

        {

            builder.Append(this.Key2.ToString());

        }

        builder.Append(']');

        return builder.ToString();

    }

}

My dictionary:

Dictionary<CompoundKey<int, string>, string> dictionary = new Dictionary<CompoundKey<int, string>, string>();

dictionary.Add(new CompoundKey<int, string>(1, "A"), "X123");

dictionary.Add(new CompoundKey<int, string>(2, "B"), "M394");

dictionary.Add(new CompoundKey<int, string>(3, "B"), "M395");

if (dictionary.ContainsKey(new CompoundKey<int, string>(3, "B")))

{

    Fred = dictionary[new CompoundKey<int, string>(3, "B")];

}