编辑： 这个问题看起来可能是同一个问题，但没有回应... 
 
 编辑：在测试用例5中，任务似乎被卡在WaitingForActivation状态。 
 
 我使用.NET 4.5中的System.Net.Http.HttpClient遇到了一些奇怪的行为 - 其中“等待”调用结果（例如） httpClient.GetAsync(...)将永远不会返回。 
 
 这仅在使用新的异步/等待语言功能和Tasks API的某些情况下发生 - 代码总是似乎在仅使用延续时才起作用。 
 
 以下是一些重现问题的代码 - 将其放入Visual Studio 11中的一个新的“MVC 4 WebApi项目”，以显示以下GET端点： 
 
/api/test1
/api/test2
/api/test3
/api/test4
/api/test5 <--- never completes
/api/test6
 
 这里的每个端点返回相同的数据（来自stackoverflow.com的响应头），除了/api/test5 ，从未完成。 
 
 我在HttpClient类中遇到了错误，还是以某种方式滥用API？ 
 
 复制代码： 
 
public class BaseApiController : ApiController
{
    /// <summary>
    /// Retrieves data using continuations
    /// </summary>
    protected Task<string> Continuations_GetSomeDataAsync()
    {
        var httpClient = new HttpClient();

        var t = httpClient.GetAsync("http://stackoverflow.com", HttpCompletionOption.ResponseHeadersRead);

        return t.ContinueWith(t1 => t1.Result.Content.Headers.ToString());
    }

    /// <summary>
    /// Retrieves data using async/await
    /// </summary>
    protected async Task<string> AsyncAwait_GetSomeDataAsync()
    {
        var httpClient = new HttpClient();

        var result = await httpClient.GetAsync("http://stackoverflow.com", HttpCompletionOption.ResponseHeadersRead);

        return result.Content.Headers.ToString();
    }
}

public class Test1Controller : BaseApiController
{
    /// <summary>
    /// Handles task using Async/Await
    /// </summary>
    public async Task<string> Get()
    {
        var data = await Continuations_GetSomeDataAsync();

        return data;
    }
}

public class Test2Controller : BaseApiController
{
    /// <summary>
    /// Handles task by blocking the thread until the task completes
    /// </summary>
    public string Get()
    {
        var task = Continuations_GetSomeDataAsync();

        var data = task.GetAwaiter().GetResult();

        return data;
    }
}

public class Test3Controller : BaseApiController
{
    /// <summary>
    /// Passes the task back to the controller host
    /// </summary>
    public Task<string> Get()
    {
        return Continuations_GetSomeDataAsync();
    }
}

public class Test4Controller : BaseApiController
{
    /// <summary>
    /// Handles task using Async/Await
    /// </summary>
    public async Task<string> Get()
    {
        var data = await AsyncAwait_GetSomeDataAsync();

        return data;
    }
}

public class Test5Controller : BaseApiController
{
    /// <summary>
    /// Handles task by blocking the thread until the task completes
    /// </summary>
    public string Get()
    {
        var task = AsyncAwait_GetSomeDataAsync();

        var data = task.GetAwaiter().GetResult();

        return data;
    }
}

public class Test6Controller : BaseApiController
{
    /// <summary>
    /// Passes the task back to the controller host
    /// </summary>
    public Task<string> Get()
    {
        return AsyncAwait_GetSomeDataAsync();
    }
}

Edit: This question looks like it might be the same problem, but has no responses...
 
Edit: In test case 5 the task appears to be stuck in WaitingForActivation state.
 
I've encountered some odd behaviour using the System.Net.Http.HttpClient in .NET 4.5 - where "awaiting" the result of a call to (e.g.) httpClient.GetAsync(...) will never return.
 
This only occurs in certain circumstances when using the new async/await language functionality and Tasks API - the code always seems to work when using only continuations.
 
Here's some code which reproduces the problem - drop this into a new "MVC 4 WebApi project" in Visual Studio 11 to expose the following GET endpoints:
 
/api/test1
/api/test2
/api/test3
/api/test4
/api/test5 <--- never completes
/api/test6
 
Each of the endpoints here return the same data (the response headers from stackoverflow.com) except for /api/test5 which never completes.
 
Have I encountered a bug in the HttpClient class, or am I misusing the API in some way?
 
Code to reproduce:
 
public class BaseApiController : ApiController
{
    /// <summary>
    /// Retrieves data using continuations
    /// </summary>
    protected Task<string> Continuations_GetSomeDataAsync()
    {
        var httpClient = new HttpClient();

        var t = httpClient.GetAsync("http://stackoverflow.com", HttpCompletionOption.ResponseHeadersRead);

        return t.ContinueWith(t1 => t1.Result.Content.Headers.ToString());
    }

    /// <summary>
    /// Retrieves data using async/await
    /// </summary>
    protected async Task<string> AsyncAwait_GetSomeDataAsync()
    {
        var httpClient = new HttpClient();

        var result = await httpClient.GetAsync("http://stackoverflow.com", HttpCompletionOption.ResponseHeadersRead);

        return result.Content.Headers.ToString();
    }
}

public class Test1Controller : BaseApiController
{
    /// <summary>
    /// Handles task using Async/Await
    /// </summary>
    public async Task<string> Get()
    {
        var data = await Continuations_GetSomeDataAsync();

        return data;
    }
}

public class Test2Controller : BaseApiController
{
    /// <summary>
    /// Handles task by blocking the thread until the task completes
    /// </summary>
    public string Get()
    {
        var task = Continuations_GetSomeDataAsync();

        var data = task.GetAwaiter().GetResult();

        return data;
    }
}

public class Test3Controller : BaseApiController
{
    /// <summary>
    /// Passes the task back to the controller host
    /// </summary>
    public Task<string> Get()
    {
        return Continuations_GetSomeDataAsync();
    }
}

public class Test4Controller : BaseApiController
{
    /// <summary>
    /// Handles task using Async/Await
    /// </summary>
    public async Task<string> Get()
    {
        var data = await AsyncAwait_GetSomeDataAsync();

        return data;
    }
}

public class Test5Controller : BaseApiController
{
    /// <summary>
    /// Handles task by blocking the thread until the task completes
    /// </summary>
    public string Get()
    {
        var task = AsyncAwait_GetSomeDataAsync();

        var data = task.GetAwaiter().GetResult();

        return data;
    }
}

public class Test6Controller : BaseApiController
{
    /// <summary>
    /// Passes the task back to the controller host
    /// </summary>
    public Task<string> Get()
    {
        return AsyncAwait_GetSomeDataAsync();
    }
}

原文：https://stackoverflow.com/questions/10343632