在 .NET 4 中使用 await SemaphoreSlim.WaitAsync

Question

我的应用程序正在使用 .NET 4。我正在使用 nuget package

等待异步

在我的应用程序中，我想对 sempahore WaitAsync 调用进行如下等待。

SemaphoreSlim semphore = new SemaphoreSlim(100);
await semphore.WaitAsync();

但是我遇到以下编译错误。

'System.Threading.SemaphoreSlim' does not contain a definition for 'WaitAsync' and no extension method 'WaitAsync' accepting a first argument of type 'System.Threading.SemaphoreSlim' could be found (are you missing a using directive or an assembly reference?)

.NET 4.0 中是否有 uisng WaitAsync？

Answer 1

不，您需要升级到 .NET 4.5（或自己编写 WaitAsync 扩展（或固有的异步信号量））。

.NET 4.0 的异步扩展可以支持实际的 await 关键字。 .NET 4.5 的大部分工作是向可以等待的各种 BCL 类型添加异步操作。如果您需要这些操作，您需要升级您使用的框架版本。

Answer 2

WaitAsync 是在 .Net 4.5 中引入的。您要么需要通过查看 source(not sure if that is possible), or you can use StephenToub's AsyncSemaphore.

将自己实现为扩展方法

Answer 3

您不能在 .Net 4.0 中使用 SemaphoreSlim.WaitAsync，因为此方法已添加到 .Net 4.5 中的 SemaphoreSlim。

但是，您可以按照 Stephen Toub 在 Building Async Coordination Primitives, Part 5: AsyncSemaphore 中的示例实施自己的 AsyncSemaphore：

public class AsyncSemaphore
{
    private readonly static Task s_completed = Task.FromResult(true);
    private readonly Queue<TaskCompletionSource<bool>> m_waiters = new Queue<TaskCompletionSource<bool>>();
    private int m_currentCount; 

    public AsyncSemaphore(int initialCount)
    {
        if (initialCount < 0) throw new ArgumentOutOfRangeException("initialCount");
        m_currentCount = initialCount;
    }

    public Task WaitAsync()
    {
        lock (m_waiters)
        {
            if (m_currentCount > 0)
            {
                --m_currentCount;
                return s_completed;
            }
            else
            {
                var waiter = new TaskCompletionSource<bool>();
                m_waiters.Enqueue(waiter);
                return waiter.Task;
            }
        }
    }
    public void Release()
    {
        TaskCompletionSource<bool> toRelease = null;
        lock (m_waiters)
        {
            if (m_waiters.Count > 0)
                toRelease = m_waiters.Dequeue();
            else
                ++m_currentCount;
        }
        if (toRelease != null)
            toRelease.SetResult(true);
    }
}

Answer 4

由于 WaitAsync 在 .NET 4.0 中不可用，您可以使用 Stephan Toub's series of async synchronization primitives:

中的代码

public class AsyncSemaphore 
{ 
    private readonly static Task s_completed = Task.FromResult(true); 
    private readonly Queue<TaskCompletionSource<bool>> m_waiters = 
                                            new Queue<TaskCompletionSource<bool>>(); 
    private int m_currentCount; 

    public AsyncSemaphore(int initialCount)
    {
        if (initialCount < 0) 
        {
            throw new ArgumentOutOfRangeException("initialCount"); 
        }
        m_currentCount = initialCount; 
    }

    public Task WaitAsync() 
    { 
        lock (m_waiters) 
        { 
            if (m_currentCount > 0) 
            { 
                --m_currentCount; 
                return s_completed; 
            } 
            else 
            { 
                var waiter = new TaskCompletionSource<bool>(); 
                m_waiters.Enqueue(waiter); 
                return waiter.Task; 
            } 
        } 
    }

    public void Release() 
    { 
        TaskCompletionSource<bool> toRelease = null; 
        lock (m_waiters) 
        { 
            if (m_waiters.Count > 0) 
                toRelease = m_waiters.Dequeue(); 
            else 
                ++m_currentCount; 
        } 
        if (toRelease != null) 
            toRelease.SetResult(true); 
    }
}

Answer 5

正如其他人所提到的，.NET 4.5 引入了 SemaphoreSlim.WaitAsync。

对于 .NET 4.0，您可以 write your own async-compatible lock or use one from my Nito.AsyncEx NuGet package.

Answer 6

另外两个答案中发布的 class AysncSemaphore 无法使用 Framework 4.0 进行编译，因为 Task.FromResult 未定义。

这是我的替代版本：

public class AsyncSemaphore
{
    private readonly static Task s_completed ;
    private readonly Queue<TaskCompletionSource<bool>> m_waiters = new Queue<TaskCompletionSource<bool>>();
    private int m_currentCount; 

    static AsyncSemaphore()
    {
      var tcs = new TaskCompletionSource<bool>();
      tcs.SetResult(true);
      s_completed = tcs.Task ;
    }

    public AsyncSemaphore(int initialCount)
    {
        if (initialCount < 0) throw new ArgumentOutOfRangeException("initialCount");
        m_currentCount = initialCount;
    }

    public Task WaitAsync()
    {
        lock (m_waiters)
        {
            if (m_currentCount > 0)
            {
                --m_currentCount;
                return s_completed;
            }
            else
            {
                var waiter = new TaskCompletionSource<bool>();
                m_waiters.Enqueue(waiter);
                return waiter.Task;
            }
        }
    }
    public void Release()
    {
        TaskCompletionSource<bool> toRelease = null;
        lock (m_waiters)
        {
            if (m_waiters.Count > 0)
                toRelease = m_waiters.Dequeue();
            else
                ++m_currentCount;
        }
        if (toRelease != null)
            toRelease.SetResult(true);
    }
}

在 .NET 4 中使用 await SemaphoreSlim.WaitAsync

Using await SemaphoreSlim.WaitAsync in .NET 4

c#

semaphore

.net-4.0

async-await