无法将类型转换为完全相同类型的约束泛型

Question

I'm curious as to why this is a compilation error:

public abstract class AbstractThingList<T> : List<T>
{
    protected QueryBuilder<TH> Query<TH>(string query) where TH : AbstractThingList<T>
    {
        // Argument 1: cannot convert from 'AbstractThingList<T>' to 'TH'
        return new QueryBuilder<TH>(this, query);
    }

    protected class QueryBuilder<TH> where TH : AbstractThingList<T>
    {
        private readonly TH Container;

        public QueryBuilder(TH container, string query)
        {
            Container = container;
        }

        public TH Execute()
        {
            return Container;
        }
    }
}

I receive the error message Argument 1: cannot convert from 'AbstractThingList<T>' to 'TH' , however QueryBuilder constrains TH to AbstractThingList<T> (the exact same T as used in the outer class), of which "this" is most definitely an instance of.

Have I missed something?

Answer 1

Within the Query method, while TH is constrained to be a subtype of AbstractThingList<T> , the type of this is AbstractThingList<T> and it is not necessarily a subtype of TH . A simple counterexample would be:

public class ThingList1 : AbstractThingList<string> { }

public class ThingList2 : AbstractThingList<string> {
   private void SomeMethod() {
       this.Query<ThingList1>("query")
   }
}

this is correctly disallowed since ThingList2 is not a subtype of ThingList1 .