有没有办法在 C# 中简化这种反射委托代码？

Question

在this 响应中，我提出了以下帮助方法，不同的Task 创建者可以重用它来将事件转换为任务完成源。

// Helper method
static Task<T> TaskFromEventHelper<T>(object target, string eventName, Func<TaskCompletionSource<T>, object> resultSetterFactory) {
    var tcs = new TaskCompletionSource<T>();
    var addMethod = target.GetType().GetEvent(eventName).GetAddMethod();
    var delegateType = addMethod.GetParameters()[0].ParameterType;
    var d = Delegate.CreateDelegate(delegateType, resultSetterFactory(tcs), "Invoke");
    addMethod.Invoke(target, new object[] {d});
    return tcs.Task;
}

// Empty events (Action style)
static Task TaskFromEvent(object target, string eventName) {
    return TaskFromEventHelper(target, eventName, (Func<TaskCompletionSource<object>, object>)(tcs => (Action)(() => tcs.SetResult(null))));
}

// One-value events (Action<T> style)
static Task<T> TaskFromEvent<T>(object target, string eventName) {
    return TaskFromEventHelper(target, eventName, (Func<TaskCompletionSource<T>, object>)(tcs => (Action<T>)(tcs.SetResult)));
}

// Two-value events (Action<T1, T2> or EventHandler style)
static Task<Tuple<T1, T2>> TaskFromEvent<T1, T2>(object target, string eventName) {
    return TaskFromEventHelper(target, eventName, (Func<TaskCompletionSource<Tuple<T1, T2>>, object>)(tcs => (Action<T1, T2>)((t1, t2) => tcs.SetResult(Tuple.Create(t1, t2)))));
}

在我给出的使用辅助方法的三个示例中，都有一个 tcs.SetResult 组件，这让我认为也有一种方法可以将它移到辅助方法中，这可能会简化签名，所以也许辅助方法只需要接受Func<?, T>，其中Func 将获取event 的输出并将其转换为tcs.SetResult 的输出。

也就是说，我认为必须有一种方法可以创建一个助手，这样我就可以将它写成

// Empty events (Action style)
static Task TaskFromEvent(object target, string eventName) {
    return TaskFromEventHelper<object>(target, eventName, new Func<object>(() => null));
}

// One-value events (Action<T> style)
static Task<T> TaskFromEvent<T>(object target, string eventName) {
    return TaskFromEventHelper<T>(target, eventName, new Func<T, T>(t => t));
}

// Two-value events (Action<T1, T2> or EventHandler style)
static Task<Tuple<T1, T2>> TaskFromEvent<T1, T2>(object target, string eventName) {
    return TaskFromEventHelper<Tuple<T1, T2>>(target, eventName, new Func<T1, T2, Tuple<T1, T2>>(Tuple.Create));
}

，但这就是为什么我不知道上面Func<?, T> 中的?。这一个例如需要? 是两个 参数。它可以以某种方式作为object 传递吗？我觉得这是可能的，但如果是这样的话，它需要一些真正的反射魔法。

Answer 1

你可以使用Expression:

static Task<T> TaskFromEventHelper<T>(object target, string eventName, Delegate resultSetter)
{
    var tcs             = new TaskCompletionSource<T>();
    var addMethod       = target.GetType().GetEvent(eventName).GetAddMethod();
    var delegateType    = addMethod.GetParameters()[0].ParameterType;
    var methodInfo      = delegateType.GetMethod("Invoke");
    var parameters      = methodInfo.GetParameters()
                                    .Select(a => Expression.Parameter(a.ParameterType))
                                    .ToArray();

    // building method, which argument count and
    // their types are not known at compile time
    var exp = // (%arguments%) => tcs.SetResult(resultSetter.Invoke(%arguments%))
        Expression.Lambda(
            delegateType,
            Expression.Call(
                Expression.Constant(tcs),
                tcs.GetType().GetMethod("SetResult"),
                Expression.Call(
                    Expression.Constant(resultSetter),
                    resultSetter.GetType().GetMethod("Invoke"),
                    parameters)),
            parameters);

    addMethod.Invoke(target, new object[] { exp.Compile() });
    return tcs.Task;
}