Get from IQueryable to IEnumerable

问题

this should be a quick one for all you f# rockers, but it's got me stuck at the moment.

I've got an F# type which I'm trying to get to implement a c# interface

public interface ICrudService<T> where T: DelEntity, new()
{
    IEnumerable<T> GetAll();
}

here's how it's implemnted in c#:

public IEnumerable<T> GetAll()
{
    return repo.GetAll();
}

repo.GetAll returns an IQueryable, but the c# compiler knows enough to convert to IEnumerable because IQueryable<T> : IEnumerable<T>. but In F# the compiler can't work it and I've tried a few attempt to cast it correctly

type CrudService<'T when 'T :(new : unit -> 'T) and 'T :> DelEntity>(repo : IRepo<'T>) = 
    interface ICrudService<'T> with
         member this.GetAll () = 
            repo.GetAll<'T>

this is giving me a type mismatch error

回答1:

You need to cast to IEnumerable<'T>:

type CrudService<'T when 'T :(new : unit -> 'T) and 'T :> DelEntity>(repo : IRepo<'T>) = 
        interface ICrudService<'T> with
             member this.GetAll () = 
                repo.GetAll() :> IEnumerable<'T>

回答2:

What if you used Seq.cast<'T? It sounds like it just needs to be casted to the correct Type. Since IEnumerables are implemented differently in F# than they are in C#.

let seqCast : seq<int> = Seq.cast repo.GetAll<'T>

来源：https://stackoverflow.com/questions/23973233/get-from-iqueryable-to-ienumerable