在 Func 中通过 Lembda 传递子类型<Parent, Parent>

Question

I have a method defined like this:

 public async Task<Parent> UpdateDataAsync(Func<Parent, Parent> updateExisting)
 {
   return await this.UpdateAsync<Parent>(existing => (updateExisting(existing)));
   
 }

This can be called like this with Parent's Child class

return await this.UpdateDataAsync(
            chld=>
            {
                return chld.UpdateState(State);
            });

Compiler complains about not being able to find UpdateState . UpdateState is defined on Child class of Parent . How can I make chld to be passed or inferred as Child . I tried to explicitly say (Child) infront of chld and after the lembda that doesnt work.

Answer 1

I've read your question a couple of times and while its still not clear, I think I get what your asking.

I assume that Child inherits from Parent right?

public class Parent
{
}

public class Child : Parent
{
   public Child UpdateState(State state)
   {
        //whatever this does
        return this;
   }
}

If I'm correct please update your question with this information.

So assuming the above is correct you need to make your UpdateDataAsync method generic with a constraint

public async Task<TParent> UpdateDataAsync<TParent>(Func<TParent, TParent> updateExisting)
where TParent : Parent
{
   return await this.UpdateAsync<TParent>(existing => (updateExisting(existing)));   
}

This allows C# to know that the item is a Parent while allowing you to work with a subclass

which means you can then do this

return await this.UpdateDataAsync<Child>(
    chld=>
    {
        return chld.UpdateState(State);
    });

NB the type parameter here is probably unneeded, since the compiler can infer it, but I've included it for clarity