Question

I query a hierarchical data source like this:

repository.LookupValue(IEnumerable<string> ascendants)

where ascendants is a list of strings representing the path to the node being looked up.

I would like to cache results from these lookups. I had envisioned using a Dictionary<IEnumerable<string>, long>, but it seems when using an IEnumerable<string> as a dictionary key, reference equality is used, which is no good to me.

I could of course define my own type wrapping IEnumerable<string> and override it’s .Equals method, but that seems extreme.

Am I missing a simpler solution to the caching problem? Can anyone advise?

Answer 1

Use a Dictionary but use the constructor that takes an IEqualityComparer. Write a custom implementation of this to compare your collection of ascendants and pass it in.

Here’s an example (comparing arrays in this case) that uses built in StructuralComparisons.

    public class StringArrayComparer : IEqualityComparer<string[]>
    {
        public bool Equals(string[] x, string[] y)
        {
            return StructuralComparisons.StructuralEqualityComparer.Equals(x, y);
        }

        public int GetHashCode(string[] obj)
        {
            return StructuralComparisons.StructuralEqualityComparer.GetHashCode(obj);
        }
    }