Multi-key dictionary in c#? [duplicate]

I know there isn't one in the BCL but can anyone point me to a good opensource one?

By Multi I mean 2 keys. ;-)

I've also used tuples as jason in his answer does. However, I suggest you simply define a tuple as a struct:

public struct Tuple<T1, T2> {
    public readonly T1 Item1;
    public readonly T2 Item2;
    public Tuple(T1 item1, T2 item2) { Item1 = item1; Item2 = item2;} 
}

public static class Tuple { // for type-inference goodness.
    public static Tuple<T1,T2> Create<T1,T2>(T1 item1, T2 item2) { 
        return new Tuple<T1,T2>(item1, item2); 
    }
}

You get immutability, .GetHashcode and .Equals for free, which (while you're waiting for C# 4.0) is nice 'n simple...

One warning however: the default GetHashcode implementation (sometimes) only considers the first field so make sure to make the first field the most discriminating or implement GetHashcode yourself (e.g. using FieldwiseHasher.Hash(this) from ValueUtils), otherwise you'll likely run into scalability issues.

Also, you get to avoid nulls which tend to complicate matters (and if you really want nulls, you just make your Tuple<> nullable). Slightly offtopic, am I the only one annoyed at the framework-level lack of support for non-null references? I work on large project, and occasionally a null creeps in somewhere it really shouldn't -- and hey presto, you get a nullreference exception -- but with a stack trace that points you to the reference's first usage, not the actually faulty code.

Of course, .NET 4.0 is pretty old by now; most of us can just use .NET 4.0's tuple.

Edit: to workaround the poor GetHashCode implementation that .NET provides for structs I've written ValueUtils, which also allows you to use real names for your multi-field keys; that means you might write something like:

sealed class MyValueObject : ValueObject<MyValueObject> {
    public DayOfWeek day;
    public string NamedPart;
    //properties work fine too
}

...which hopefully makes it easier to have human-readable names for data with value semantics, at least until some future version of C# implements proper tuples with named members; hopefully with decent hashcodes ;-).

I use a Tuple as the keys in a Dictionary.

public class Tuple<T1, T2> {
    public T1 Item1 { get; private set; }
    public T2 Item2 { get; private set; }

    // implementation details
}

Be sure to override Equals and GetHashCode and define operator!= and operator== as appropriate. You can expand the Tuple to hold more items as needed. .NET 4.0 will include a built-in Tuple.

Tuples will be (are) in .Net 4.0 Until then, you can also use a

 Dictionary<key1, Dictionary<key2, TypeObject>>

or, creating a custom collection class to represent this...

 public class TwoKeyDictionary<K1, K2, T>: 
        Dictionary<K1, Dictionary<K2, T>> { }

or, with three keys...

public class ThreeKeyDictionary<K1, K2, K3, T> :
    Dictionary<K1, Dictionary<K2, Dictionary<K3, T>>> { }

Many good solutions here, What I am missing here is an implementation based on the build in Tuple type, so I wrote one myself.

Since it just inherits from Dictionary<Tuple<T1,T2>, T> you can always use both ways.

var dict = new Dictionary<int, int, Row>();
var row = new Row();
dict.Add(1, 2, row);
dict.Add(Tuple.Create(1, 2, row));
dict.Add(new Tuple<int, int>(1, 2));

here is the code.

public class Dictionary<TKey1,TKey2,TValue> :  Dictionary<Tuple<TKey1, TKey2>, TValue>, IDictionary<Tuple<TKey1, TKey2>, TValue>
{

    public TValue this[TKey1 key1, TKey2 key2]
    {
        get { return base[Tuple.Create(key1, key2)]; }
        set { base[Tuple.Create(key1, key2)] = value; }
    }

    public void Add(TKey1 key1, TKey2 key2, TValue value)
    {
        base.Add(Tuple.Create(key1, key2), value);
    }

    public bool ContainsKey(TKey1 key1, TKey2 key2)
    {
        return base.ContainsKey(Tuple.Create(key1, key2));
    }
}

Please be aware that this implementation depends on the Tuple.Equals() implementation itself:

http://msdn.microsoft.com/en-us/library/dd270346(v=vs.110).aspx

The obj parameter is considered to be equal to the current instance under the following conditions:

It is a Tuple object.
Its two components are of the same types as the current instance.
Its two components are equal to those of the current instance. Equality is determined by the default object equality comparer for each component.

Multi-key dictionary in c#? [duplicate]

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