c# - 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 来等待异步

在我的应用程序中，我想按如下方式在 sempahore WaitAsync 调用上进行等待。

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

但是我遇到了以下编译错误。

'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 中是否存在 uisng WaitAsync？

Answer 1

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

然而，您可以按照 Stephen Toub 在 Building Async Coordination Primitives, Part 5: AsyncSemaphore 中的示例实现自己的 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);
    }
}