Multi-key dictionaries (of another kind) in C#?

This blog post seems to detail a rather decent implementation.

Multi-key generic dictionary class for C#

MultiKeyDictionary is a C# class that wraps and extends the Generic Dictionary object provided by Microsoft in .NET 2.0 and above. This allows a developer to create a generic dictionary of values and reference the value list through two keys instead of just the one provided by the Microsoft implementation of the Generic Dictionary<...>. You can see my article on CodeProject (here), however this code is more up-to-date and bug free.

Yes, define a class that adds the object to an internal hashtable with both keys,

 public MyClass<k1, k2, T>: Dictionary<object, T>
  {
      private Dictionary<k1, k2> keyMap;
      public new Add(k1 key1Val, k2 key2Val, T object)
      {
         keyMap.Add(key1Val, key2Val);
         base.Add(k2, object)
      }
      public Remove(k1 key1Val) 
      { 
          base.Remove(keyMap[key1Val]); 
          keyMap.Remove(key1Val);
      }
      public Remove(k2 key2Val) 
      { 
        base.Remove(key2Val);
        keyMap.Remove(key2Val);
      }
  }

There's nothing built into .NET BCL for this type of collection at the moment.

I see two options:

1. Use a two-level dictionary. The first level maps different keys to some common unique key (let's say a GUID), and the second level maps the GUID to the actual value.

2. Create a custom key class and implement Equals() and GetHashCode() so that any one component of the key is sufficient to find the entire key. You could then supply helper methods to construct instances of the key using only one of the values so that you could do lookups.