[英]In F#, how do I tell if an object is an Async<_>, and how can I cast it to an Async<_>?
I'm currently trying to create an IHttpActionInvoker
for use with ASP.NET Web API that will allow a result to be an Async<'T>
. 我目前正在尝试创建一个
IHttpActionInvoker
,用于ASP.NET Web API,允许结果为Async<'T>
。 At the moment, I'm ignoring conversion of IHttpActionResult
s and only care about HttpResponseMessage
and a value of type 'T
. 目前,我忽略了
IHttpActionResult
的转换,只关心HttpResponseMessage
和类型为'T
的值。 I currently have the following implementation: 我目前有以下实现:
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)
This works for Async<HttpResponseMessage>
only. 这仅适用于
Async<HttpResponseMessage>
。 If I try to cast to Async<_>
, I get an exception stating that I can't cast to Async<obj>
. 如果我尝试转换为
Async<_>
,我会得到一个异常,说明我无法转换为Async<obj>
。 I also cannot correctly detect whether or not the actionDescriptor.ReturnType
is an Async<_>
. 我也无法正确检测
actionDescriptor.ReturnType
是否为Async<_>
。 This does not surprise me, but I'm not sure how to get around the problem. 这并不让我感到惊讶,但我不确定如何解决这个问题。
as an option (browser-compiled code, may contain errors) 作为选项(浏览器编译的代码,可能包含错误)
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)
After several helpful tips from outside StackOverflow, I came up with the following solution that appears to work. 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决方案。 I'm not thrilled with it, but it does the job.
我并不为此感到兴奋,但它确实起到了作用。 I'd appreciate any tips or pointers:
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)
I hope this helps someone else! 我希望这有助于其他人!
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