C＃无法将T转换为T.

Question

I have a generic collection MyCollection<T> that I've made, and everything works fine except this new function Apply that I'm adding:

class MyCollection<T> {
    T value;
    public MyCollection(T starter) { value = starter; }
    public MyCollection<S> Apply<T, S>(Func<T, S> function) {
        return new MyCollection<S>(function(value));  // error in function(value)
    }
}

This gives me an error I've never seen before:

Argument 1: cannot convert from 'T' to 'T [C:\folder\code.cs (line number)]'

What are the two T types? What's wrong with the conversion I'm attempting?

Answer 1

You problem is that the type parameter T in

class MyCollection<T>

is not the same type parameter as T in

Apply<T, S>

so your function takes another type than the type of value

if you change

Apply<T, S>

to

Apply<S>

your code will compile

Answer 2

One T comes from MyCollection<T> and one from Apply<T, S> . Apply<S> should suffice.

Answer 3

It's trying to cast the generic type of the class to the generic type of the method. Remove the T from the signature of the Apply method.

class MyCollection<T> {
    T value;
    public MyCollection(T starter) { value = starter; }
    public MyCollection<S> Apply<S>(Func<T, S> function) {
        return new MyCollection<S>(function(value));
    } 
}