c# - 具有 in 修饰符的事件参数的 Observable.FromEvent 会导致异常

Question

我正在使用一个第三方库，它有一个事件\事件处理程序，我想将其转换为 一个可观察的。事件处理程序委托(delegate)需要一个带有 in 关键字的结构。这会导致异常:

System.ArgumentException - 无法绑定(bind)到目标方法，因为其签名与委托(delegate)类型的签名不兼容。

来自

System.Private.CoreLib.dll!System.Reflection.RuntimeMethodInfo.CreateDelegateInternal(System.Type delegateType, object firstArgument, System.DelegateBindingFlags bindingFlags) Line 556    C#
    System.Private.CoreLib.dll!System.Reflection.RuntimeMethodInfo.CreateDelegate(System.Type delegateType, object target) Line 534 C#
    System.Reactive.dll!System.Reactive.ReflectionUtils.CreateDelegate<Test.TheDelegate>(object o, System.Reflection.MethodInfo method) Line 18 C#

如果我从参数中删除 in，它会按预期工作。 我不明白为什么？甚至可以做到吗？

using System;
using System.Diagnostics;
using System.Reactive.Linq;

Test test = new();

Start();
test.TriggerEvent();
test.TriggerEvent();

void Start()
{
    Debug.WriteLine("Start");
    var o = Observable.FromEvent<Test.TheDelegate, SomeStruct>
        (h => test.TheEvent += h, 
         h => test.TheEvent -= h)
                      .Subscribe(OnNext);
}

void OnNext(SomeStruct s) => Debug.WriteLine("OnNext");

public class Test
{
    public delegate void TheDelegate(in SomeStruct o);

    // Without the in (or ref) keyword, no exception
    //public delegate void TheDelegate(SomeStruct o);

    public event TheDelegate? TheEvent;

    public void TriggerEvent()
    {
        SomeStruct s = new();
        TheEvent?.Invoke(s);
        Debug.WriteLine("Event Fired");
    }
}

public struct SomeStruct
{ }

Answer 1

我能够重现ArgumentException在运行时。要解决这个问题，您可以使用 Observable.FromEvent具有额外 conversion 的过载参数:

public static IObservable<TEventArgs> FromEvent<TDelegate, TEventArgs>(
    Func<Action<TEventArgs>, TDelegate> conversion,
    Action<TDelegate> addHandler,
    Action<TDelegate> removeHandler);

用法:

var observable = Observable.FromEvent<Test.TheDelegate, SomeStruct>
(
    action => (in SomeStruct arg) => action(arg),
    h => test.TheEvent += h,
    h => test.TheEvent -= h
);
var subscription = observable.Subscribe(OnNext);

conversion转换 Action<SomeStruct>到 Test.TheDelegate 。与更简单的重载相反，转换的有效性是在编译时检查的。