在F＃中，如何判斷對象是否為Async <_>，如何將其轉換為Async <_>？

Question

我目前正在嘗試創建一個IHttpActionInvoker ，用於ASP.NET Web API，允許結果為Async<'T> 。 目前，我忽略了IHttpActionResult的轉換，只關心HttpResponseMessage和類型為'T的值。 我目前有以下實現：

type AsyncApiActionInvoker() =
    inherit Controllers.ApiControllerActionInvoker()

    override x.InvokeActionAsync(actionContext, cancellationToken) =
        if actionContext = null then
            raise <| ArgumentNullException("actionContext")

        let actionDescriptor = actionContext.ActionDescriptor
        Contract.Assert(actionDescriptor <> null)

        if actionDescriptor.ReturnType = typeof<Async<HttpResponseMessage>> then

            let controllerContext = actionContext.ControllerContext
            Contract.Assert(controllerContext <> null)

            let task = async {
                let! asyncResult = Async.AwaitTask <| actionDescriptor.ExecuteAsync(controllerContext, actionContext.ActionArguments, cancellationToken)
                // For now, throw if the result is an IHttpActionResult.
                if typeof<IHttpActionResult>.IsAssignableFrom(actionDescriptor.ReturnType) then
                    raise <| InvalidOperationException("IHttpResult is not supported when returning an Async")
                let! result = asyncResult :?> Async<HttpResponseMessage>
                return actionDescriptor.ResultConverter.Convert(controllerContext, result) }

            Async.StartAsTask(task, cancellationToken = cancellationToken)

        else base.InvokeActionAsync(actionContext, cancellationToken)

這僅適用於Async<HttpResponseMessage> 。 如果我嘗試轉換為Async<_> ，我會得到一個異常，說明我無法轉換為Async<obj> 。 我也無法正確檢測actionDescriptor.ReturnType是否為Async<_> 。 這並不讓我感到驚訝，但我不確定如何解決這個問題。

Answer 1

作為選項（瀏覽器編譯的代碼，可能包含錯誤）

let (|Async|_|) (ty: Type) =
    if ty.IsGenericType && ty.GetGenericTypeDefinition() = typedefof<Async<_>> then
        Some (ty.GetGenericArguments().[0])
    else 
        None

type AsyncApiActionInvoker() =
    inherit Controllers.ApiControllerActionInvoker()

    static let AsTaskMethod = typeof<AsyncApiActionInvoker>.GetMethod("AsTask")

    static member AsTask<'T> (actionContext: Controllers.HttpActionContext, cancellationToken: CancellationToken) =
        let action = async {
            let task = 
                actionContext.ActionDescriptor.ExecuteAsync(
                    actionContext.ControllerContext, 
                    actionContext.ActionArguments, 
                    cancellationToken
                )
            let! result = Async.AwaitTask task
            let! asyncResult = result :?> Async<'T>
            return actionContext.ActionDescriptor.ResultConverter.Convert(actionContext.ControllerContext, box asyncResult)
        }

        Async.StartAsTask(action, cancellationToken = cancellationToken)

    override x.InvokeActionAsync(actionContext, cancellationToken) =
        if actionContext = null then
            raise <| ArgumentNullException("actionContext")

        match actionContext.ActionDescriptor.ReturnType with
        | Async resultType ->
            let specialized = AsTaskMethod.MakeGenericMethod(resultType)
            downcast specialized.Invoke(null, [|actionContext, cancellationToken|])
        | _ -> base.InvokeActionAsync(actionContext, cancellationToken)

Answer 2

在StackOverflow外部提供了一些有用的提示之后，我想出了以下可行的解決方案。 我並不為此感到興奮，但它確實起到了作用。 我很感激任何提示或指示：

type AsyncApiActionInvoker() =
    inherit Controllers.ApiControllerActionInvoker()

    static member internal GetResultConverter(instanceType: Type, actionDescriptor: HttpActionDescriptor) : IActionResultConverter =
        if instanceType <> null && instanceType.IsGenericParameter then
            raise <| InvalidOperationException()

        if instanceType = null || typeof<HttpResponseMessage>.IsAssignableFrom instanceType then
            actionDescriptor.ResultConverter
        else
            let valueConverterType = typedefof<ValueResultConverter<_>>.MakeGenericType instanceType
            let newInstanceExpression = Expression.New valueConverterType
            let ctor = Expression.Lambda<Func<IActionResultConverter>>(newInstanceExpression).Compile()
            ctor.Invoke()

    static member internal StartAsTask<'T>(task, resultConverter: IActionResultConverter, controllerContext, cancellationToken) =
        let computation = async {
            let! comp = Async.AwaitTask task
            let! (value: 'T) = unbox comp
            return resultConverter.Convert(controllerContext, value) }
        Async.StartAsTask(computation, cancellationToken = cancellationToken)

    override this.InvokeActionAsync(actionContext, cancellationToken) =
        if actionContext = null then
            raise <| ArgumentNullException("actionContext")

        let actionDescriptor = actionContext.ActionDescriptor
        Contract.Assert(actionDescriptor <> null)

        let returnType = actionDescriptor.ReturnType
        // For now, throw if the result is an IHttpActionResult.
        if typeof<IHttpActionResult>.IsAssignableFrom(returnType) then
            raise <| InvalidOperationException("IHttpResult is not supported when returning an Async")

        if returnType.IsGenericType && returnType.GetGenericTypeDefinition() = typedefof<Async<_>> then
            let controllerContext = actionContext.ControllerContext
            Contract.Assert(controllerContext <> null)

            let computation = actionDescriptor.ExecuteAsync(controllerContext, actionContext.ActionArguments, cancellationToken)
            let innerReturnType = returnType.GetGenericArguments().[0]
            let converter = AsyncApiActionInvoker.GetResultConverter(innerReturnType, actionDescriptor)
            this.GetType()
                .GetMethod("StartAsTask", BindingFlags.NonPublic ||| BindingFlags.Static)
                .MakeGenericMethod(innerReturnType)
                .Invoke(null, [| computation; converter; controllerContext; cancellationToken |])
                |> unbox

        else base.InvokeActionAsync(actionContext, cancellationToken)

我希望這有助於其他人！