In C#: How to declare a generic Dictionary with a type as key and an IEnumerable<> of that type as value?

Question

I want to declare a dictionary that stores typed IEnumerable\'s of a specific type, with that exact type as key, like so: (Edited to follow johny g\'s comment)<

Answer 1

You may not even need a dictionary to be able to do this - but that depends on your needs. If you only ever need 1 such list per type per appdomain (i.e. the "dictionary" is static), the following pattern can be efficient and promotes type-inference nicely:

interface IBase {}

static class Container {
    static class PerType where T : IBase {
        public static IEnumerable list;
    }

    public static IEnumerable Get() where T : IBase 
        => PerType.list; 

    public static void Set(IEnumerable newlist) where T : IBase 
        => PerType.list = newlist;

    public static IEnumerable GetByExample(T ignoredExample) where T : IBase 
        => Get(); 
}

Note that you should think carefully before adopting this approach about the distinction between compile-time type and run-time type. This method will happily let you store a runtime-typed IEnumerable variable both under SomeType and -if you cast it- under any of SomeType's base types, including IBase, with neither a runtime nor compiletype error - which might be a feature, or a bug waiting to happen, so you may want an if to check that.

Additionally, this approach ignores threading; so if you want to access this data-structure from multiple threads, you probably want to add some locking. Reference read/writes are atomic, so you're not going to get corruption if you fail to lock, but stale data and race conditions are certainly possible.