Dictionary with Func as key

Question

I am wondering if this is a sane choice of key for a dictionary? What I want to do is use an expression as the key in a dictionary, something like:

    var map3 = new Dictionary<Func<int, bool>, int>();
    map3.Add((x) => x % 2 == 0, 1);
    map3.Add((x) => x % 10 == 0, 2);
    // ...

    var key = map3.Keys.SingleOrDefault(f => f(2));
    // key = (x) => x % 2
    // map3[key] = 1

The idea being this is a cleaner way than having big if-else or switch statements.

Does this make sense? Will it work? Is there an simpler way?

Answer 1

Considering the way that you use your map, you will be better off with a List<Tuple<Func<int,bool>,int>>, because the order of checking the lambdas will no longer be arbitrary, as in a hash-based dictionary. This approach also lets you skip the lookup step:

var map3 = new List<Tuple<Func<int,bool>,int>> {
    new Tuple<Func<int,bool>,int>((x) => x % 2 == 0, 1)
,   new Tuple<Func<int,bool>,int>((x) => x % 10 == 0, 2)
};
var t = map3.SingleOrDefault(t => t.Item1(2));
if (t != null) {
    var v = t.Item2;
}

Answer 2

Rewriting @dasblinkenlight's answer with latest syntax:

void Main()
{
    var map3 = new List<(Func<int, bool> Key, int Value)> {
        (Key: (x) => x * 2 == 4, Value: 1),
        (Key: (x) => x * 10 == 100, Value: 2)
    };

    var result = map3.SingleOrDefault(x => x.Key(10));
    Console.WriteLine(result.Value);
}

When Key evaluates to a Func which is not there on the List SingleOrDefault returns an element with key null and value 0.

Key and Value above are for readability, they can be removed and in that case result.Intem2 will produce output