Using Where to specify different generics.

Question

I'm writing a bijective dictionary class, but I want to ensure the two generic types are not the same type for two reasons.

Firstly, I would like it to implement the IDictionary interface in both directions, but

public class BijectiveDictionary<TKey, TValue>
    : IDictionary<TKey, TValue>, IDictionary<TValue, TKey>

gives me " 'BijectiveDictionary<TKey,TValue>' cannot implement both 'IDictionary<TKey,TValue>' and 'IDictionary<TValue,TKey>' because they may unify for some type parameter substitutions " (which is understandable, but undesirable.)

Secondly, I would like to write an optimized solution if both types are the same.

public class BijectiveDictionary<TKey, TValue>
    : IDictionary<TKey, TValue> where TValue : TKey
{
    // Optimized solution
}

public class BijectiveDictionary<TKey, TValue>
    : IDictionary<TKey, TValue>, IDictionary<TValue, TKey> where TValue : !TKey
{
    // Standard solution
}

Is this possible?

If not, I can consider not implementing IDictionary, but I couldn't guarantee TValue this[TKey key] and TKey this[TValue key] would be different, which would be unfortunate.

---

It looks like the problem here is that when the two types are the same, the special cases arise.

My original intent was to create a dictionary which maps exactly one key to exactly one value, and visa versa, such that for every KeyValuePair<TKey, TValue>(X, Y), a KeyValuePair<TValue, TKey>(Y, X) exists as well.

When TKey = TValue, then this can be simplified down to a single dictionary:

public T this[T key]
{
    get { return this[key]; }
    set
    {
        base.Add(key, value);
        base.Add(value, key);
    }
}

In this case, you cannot Add(2,3); Add(3,4) because Add(2,3) maps 3 to 2 as well, and [3] would return 2.

However, Jaroslav Jandek's solution proposed using a second dictionary to do this for cases when TKey != TValue. And although this works wonderfully for those cases, (and what I decided to implement, in the end) it doesn't quite follow my original intent when TKey = TValue, by allowing Add(2,3); Add(3,4) to map a single key 3 to two values (2 in one direction, and 4 in the other,) though I believe strictly speaking is still a valid bijective function.

Answer 1

In short, no. Re optimising it, one option there might be a strategy based on a static initializer, that chooses an appropriate concrete implementation for the same-type case, but otherwise; i.e.

public class BijectiveDictionary<TKey, TValue>
    : IDictionary<TKey, TValue>
{
    static readonly ISomeStrategy strategy;
    static BijectiveDictionary() {
        if(typeof(TKey) == typeof(TValue)) {
            strategy = ...
        } else {
            strategy = ...
        }
    }
}

but you will almost certainly find yourself using MakeGenericType and reflection to create the instance (but once created it should be fine - and you only do that once).

Answer 2

How about this (different approach):

public class BijectiveDictionary<TKey, TValue> : Dictionary<TKey, TValue>
{
    public BijectiveDictionary<TValue, TKey> Reversed { get; protected set; }

    public BijectiveDictionary()
    {
        this.Reversed = new BijectiveDictionary<TValue,TKey>(true);
        this.Reversed.Reversed = this;
    }

    protected BijectiveDictionary(bool reversedWillBeSetFromTheCallingBiji) { }

    protected void AddRaw(TKey key, TValue value)
    {
        base.Add(key, value);
    }

    // Just for demonstration - you should implement the IDictionary interface instead.
    public new void Add(TKey key, TValue value)
    {
        base.Add(key, value);
        this.Reversed.AddRaw(value, key);
    }

    public static explicit operator BijectiveDictionary<TValue, TKey>(BijectiveDictionary<TKey, TValue> biji)
    {
        return biji.Reversed;
    }
}

and in code:

BijectiveDictionary<int, bool> a = new BijectiveDictionary<int, bool>();

a.Add(5, true);
a.Add(6, false);

Console.WriteLine(a[5]);// => True
Console.WriteLine(((BijectiveDictionary < bool, int>)a)[true]);// => 5
//or
Console.WriteLine(a.Reversed[true]);// => 5

Answer 3

Even if you could, what would be the output of this code?

var dict = new BijectiveDictionary<int, int>();

dict.Add(2,3);
dict.Add(3,4);

Console.WriteLine(dict[3]); // Do I get 2 or 4?

Maybe you can restructure your code in another way which isn't ambiguous?