how to solve type constraint mismatch, C# to F#

问题

I'm trying to implement some C# code as F# for a p2p application.. I must admit I don't have full understanding of the p2p implementation and I hope it is irrelevant for fixing the type problem.. All of the p2p library is implemented in C#.

The C# implementation:

public class TestMessage : Message
{
    public string Text { get; set; }
}

...

var p = new Peer();
p.Subscribe(new Subscription<TestMessage>
{
    Callback = x => Console.WriteLine(x.Text),
});

The basic idea is that the 'p' peer now subscribes to messages of the type 'TestMessage', and then there is a similar method for publishing messages.. The signature for the 'Subscribe' method is:

void Peer.Subscribe(ISubscription<Message> subscription)

The definition of the ISubscription interface:

public interface ISubscription<out T> where T : Message
{
    ICachingOptions CachingOptions { get; set; }
    IContact Contact { get; set; }
    string EntitySet { get; set; }
    string Key { get; }
    string Topic { get; }
    Type Type { get; }

    void InvokeCallback(Message message);
    ISerializableSubscription MakeSerializable();
    bool PredicateHolds(Message message);
}

The F# implementation:

type Xmsg(m:string) =
    inherit Message()
    member this.Text = m

let sub = Subscription<Xmsg>()
sub.Callback <- fun (m: Xmsg) -> System.Console.WriteLine(m.Text)
let p = new Peer()
p.Subscribe sub

The last line results in following errors:

The type 'Subscription<Xmsg>' is not compatible with the type 'ISubscription<Message>'

and

Type constraint mismatch. The type 
    Subscription<Xmsg>    
is not compatible with type
    ISubscription<Message>    
The type 'Subscription<Xmsg>' is not compatible with the type 'ISubscription<Message>'

I have tried messing around with :> and :?> for upcasting and downcasting, -but with no success..

Of course I have tried to search for a solution, however, either there are none or I don't understand how to apply them for my problem...

Is there a fix?, or should I just give up and make a C# library project for it (and use that lib from F#)? :)

回答1:

As Ganesh says, F# doesn't support covariance, which means that you can't use an ISubscription<DerivedType> where an ISubscription<BaseType> is needed in F# code. However, support for covariance is baked into the runtime, so you can probably get around this with a set of casts:

p.Subscribe(box sub :?> ISubscription<_>)

Here you're upcasting sub all the way to an obj and then downcasting back to an ISubscription<Message>, which ought to work.

来源：https://stackoverflow.com/questions/23018081/how-to-solve-type-constraint-mismatch-c-sharp-to-f

标签

covariance