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CSharp · Console

/Program.cs

using System;
using System.Text;
using System.Collections.Generic;

public class Program
{
	public static void Main()
	{
		var myDict = new MultiKeyDictionary<string, double, double, string>
		{
			{"Goodbye", 0.55, 9.00, "yaya"} // collection initializer works fine
		};
		
		myDict.Add("Hello", 1.11, 2.99, "hi");
		
		Console.WriteLine(myDict.ContainsKey("Hello", 1.11, 2.99));  // true
		Console.WriteLine(myDict.ContainsKey("a", 1.11, 2.99));      // false
		Console.WriteLine(myDict["Hello", 1.11, 2.99]);              // hi
		
		// myDict.Add(TripleKey.Create("Hello", 1.11, 2.99), "gh");     // bang! exception, 
                                                             // key already exist		
	}
}

[Serializable]
public struct TripleKey<TKeyA, TKeyB, TKeyC>
{
    private readonly TKeyA _keyA;
    private readonly TKeyB _keyB;
    private readonly TKeyC _keyC;
    public TKeyA KeyA => _keyA;
    public TKeyB KeyB => _keyB;
    public TKeyC KeyC => _keyC;

    public TripleKey(TKeyA keyA, TKeyB keyB, TKeyC keyC)
    {
        this._keyA = keyA;
        this._keyB = keyB;
        this._keyC = keyC;
    }

    // this code is almost the same as it is in Microsoft implementation
    public override string ToString()
    {
        var sBuilder = new StringBuilder();
        sBuilder.Append('(');
        if (KeyA != null)
        {
            sBuilder.Append(KeyA.ToString());
        }
        sBuilder.Append(", ");
        if (KeyB != null)
        {
            sBuilder.Append(KeyB.ToString());
        }
        sBuilder.Append(", ");
        if (KeyC != null)
        {
            sBuilder.Append(KeyC.ToString());
        }
        sBuilder.Append(')');
        return sBuilder.ToString();
    }
}

public static class TripleKey
{
    public static TripleKey<TKeyA, TKeyB, TKeyC> Create<TKeyA, TKeyB, TKeyC>(TKeyA keyA, TKeyB keyB, TKeyC keyC)
    {
        return new TripleKey<TKeyA, TKeyB, TKeyC>(keyA, keyB, keyC);
    }
}

public class MultiKeyDictionary<TKeyA, TKeyB, TKeyC, TValue> : Dictionary<TripleKey<TKeyA, TKeyB, TKeyC>, TValue>
{
    public TValue this[TKeyA keyA, TKeyB keyB, TKeyC keyC]
    {
        get
        {
            var key = TripleKey.Create(keyA, keyB, keyC);
            return base.ContainsKey(key) ? base[key] : default(TValue);
        }
        set
        {
            var key = TripleKey.Create(keyA, keyB, keyC);
            if (!ContainsKey(key))
                base.Add(key, value);

            this[key] = value;
        }
    }

    public bool ContainsKey(TKeyA keyA, TKeyB keyB, TKeyC keyC)
    {
        var key = TripleKey.Create(keyA, keyB, keyC);

        return base.ContainsKey(key);
    }

    public void Add(TKeyA keyA, TKeyB keyB, TKeyC keyC, TValue value)
    {
        base.Add(TripleKey.Create(keyA, keyB, keyC), value);
    }
}