如何解决类型约束不匹配的问题，从C＃到F＃ [英] how to solve type constraint mismatch, C# to F#

问题描述

我正在尝试将某些C＃代码作为p＃p应用程序的F＃实现。.我必须承认，我对p2p实现没有完全的了解，我希望它与解决类型问题无关。 p2p库在C＃中实现。

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#.

C＃实现：

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

...

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

基本思想是， p对等方现在订阅 TestMessage类型的消息，然后有一种类似的发布消息的方法。
'Subscribe'方法的签名为：

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)

ISubscription接口的定义：

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);
}

F＃实现：

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

最后一行导致以下错误：

The last line results in following errors:

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

和

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>'

我曾尝试用：>和：？>进行上下转换，但没有成功。.

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

我当然试图搜索解决方案，但是，要么没有解决方案，要么我不明白如何将其应用于我的问题...

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...

是否有解决方法？还是我应该放弃并为其创建一个C＃库项目（并使用F＃中的lib）？ ：）

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

推荐答案

正如Ganesh所说，F＃不支持协方差，这意味着您不能使用 ISubscription< DerivedType> ，其中在F＃代码中需要 ISubscription< BaseType> 。但是，对协方差的支持已包含在运行时中，因此您可以通过一组强制转换来解决此问题：

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<_>)

在这里您正在投射 sub 一直到 obj ，然后向下转换回 ISubscription< Message> ，应该可以。

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

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