相关疑难解决方法(0)

asp.net 4.5中有一个新的应用程序设置

<add key="aspnet:UseTaskFriendlySynchronizationContext" value="true" />

像这样的代码可以在asp.net 4.0中运行

protected void Button1_Click(object sender, EventArgs e)
{
    CallAysnc();
}

public void CallAysnc()
{
    AsyncOperation asyncOp = AsyncOperationManager.CreateOperation(Guid.NewGuid().ToString());

    WebClient client = new WebClient();
    client.DownloadStringCompleted += (object sender, DownloadStringCompletedEventArgs e) =>
    {
        asyncOp.PostOperationCompleted(CallCompleted, e.Result);
    };
    client.DownloadStringAsync(new Uri("http://www.google.com"));
}

private void CallCompleted(object args)
{
    Response.Write(args.ToString());
}

但它在asp.net 4.5中不起作用,当我删除新的appsetting时,它再次起作用!

那么"UseTaskFriendlySynchronizationContext"的含义是什么？

下面的代码已添加到新创建的Visual Studio 2012 .NET 4.5 WebAPI项目中.

我正在尝试分配两者HttpContext.Current.User并Thread.CurrentPrincipal在异步方法中.Thread.CurrentPrincipal流量分配不正确,除非执行await Task.Yield();(或其他任何异步)(传递true到AuthenticateAsync()将导致成功).

这是为什么？

using System.Security.Principal;
using System.Threading.Tasks;
using System.Web.Http;

namespace ExampleWebApi.Controllers
{
    public class ValuesController : ApiController
    {
        public async Task GetAsync()
        {
            await AuthenticateAsync(false);

            if (!(User is MyPrincipal))
            {
                throw new System.Exception("User is incorrect type.");
            }
        }

        private static async Task AuthenticateAsync(bool yield)
        {
            if (yield)
            {
                // Why is this required?
                await Task.Yield();
            }

            var principal = new MyPrincipal();
            System.Web.HttpContext.Current.User …

在身份验证期间使用ASP.NET WebAPI进行Thread.CurrentPrincipal设置,以便控制器稍后可以使用该ApiController.User属性.

如果该身份验证步骤变为异步(以咨询另一个系统),CurrentPrincipal则会丢失任何变异(当调用者await恢复同步上下文时).

这是一个非常简化的示例(在实际代码中,身份验证发生在动作过滤器中):

using System.Diagnostics;
using System.Security.Principal;
using System.Threading;
using System.Threading.Tasks;

public class ExampleAsyncController : System.Web.Http.ApiController
{
    public async Task GetAsync()
    {
        await AuthenticateAsync();

        // The await above saved/restored the current synchronization
        // context, thus undoing the assignment in AuthenticateAsync(). 
        Debug.Assert(User is GenericPrincipal);
    }

    private static async Task AuthenticateAsync()
    {
        // Save the current HttpContext because it's null after await.
        var currentHttpContext = System.Web.HttpContext.Current;

        // Asynchronously determine identity.
        await Task.Delay(1000);
        var …

在使用ConfigureAwait(false)的引用库中使用Thread.CurrentPrincipal的声明会造成任何问题,或者ExecutionContext的逻辑调用上下文的流动会在那里照顾我吗？(到目前为止,我的阅读和测试表明它会).

示例WebAPI控制器操作:

[CustomAuthorizeThatSetsCurrentUsersClaimsToThreadCurrentContextAndHttpContextCurrentUser]
public async Task<Order> Get(int orderId)
{
    return await _orderBusinessLogicLibrary.LoadAsync(orderId); // defaults to .ConfigureAwait(true)
}

来自外部引用库的示例加载函数:

[ClaimsPrincipalPermission(
    SecurityAction.Demand,
    Operation="Read",
    Resource="Orders")]
[ClaimsPrincipalPermission(
    SecurityAction.Demand,
    Operation="Read",
    Resource="OrderItems")]
public async Task<Order> Load(int orderId)
{
    var order = await _repository.LoadOrderAsync(orderId).ConfigureAwait(false);

    // here's the key line.. assuming this lower-level function is also imposing
    // security constraints in the same way this method does, would
    // Thread.CurrentPrincipal still be correct inside the function below?
    order.Items = await _repository.LoadOrderItemsAsync(orderId).ConfigureAwait(false);
    return order;
}

此外,答案不能是"那么不要使用ConfigureAwait(false)!".这可能会导致其他问题,例如死锁(请勿在异步代码上阻止).