Question

I would like to define the following two functions:

void Map<T>(Func<T, string> mapper);

T Call<T>(string value);

Map needs to store the function that turns a string into a result of type T so that when the “Call” function is called with a type T and a string the appropriate function can be looked up and called.

I was thinking that map could store the function in a dictionary of type Dictionary<Type, Func<object, string>> and then Call could do the casting to the appropriate type but I’m unable to get that to work. Does anyone know how to achieve this?

Answer 1

The first type argument of Func is the input, the second the output: Func<in T, out TResult> — so you need Func<string, T>.

(The MSDN reference here uses Func<string, string> a fair bit which is annoying.)

Also, the dictionary can’t use the type argument T as that’s different for each element in the dictionary. Rather, use the superclass of Func<T, TResult> which is Delegate.

This should work:

    Dictionary<Type, Delegate> dictionary = new Dictionary<Type, Delegate>();

    public void Map<T>(Func<string, T> mapper)
    {
        dictionary[typeof(T)] = mapper;
    }

    public T Call<T>(string value)
    {
        var func = dictionary[typeof(T)] as Func<string, T>;
        return func.Invoke(value);
    }