CODEWITHSUNDEEP
c#.net-4.0semaphoreasync-await
whoami
whoami

Reputation: 1899

Using await SemaphoreSlim.WaitAsync in .NET 4

My application is using .NET 4. I am using await async using nuget package

In my application I want to do an await on sempahore WaitAsync call as follows.

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

However I am getting following compilation error.

'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?)

Could there be anyway of uisng WaitAsync in .NET 4.0?

Upvotes: 5

Views: 7092

Answers (6)

i3arnon
i3arnon

Reputation: 116548

You can't use SemaphoreSlim.WaitAsync in .Net 4.0 since this method was added to SemaphoreSlim in .Net 4.5.

You can however implement your own AsyncSemaphore following Stephen Toub's example in Building Async Coordination Primitives, Part 5: 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);
    }
}

Upvotes: 6

Phil Jollans
Phil Jollans

Reputation: 3759

The class AysncSemaphore, posted in two other answers, does not compile with Framework 4.0, because Task.FromResult is not defined.

This is my alternative version:

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);
    }
}

Upvotes: 1

Yuval Itzchakov
Yuval Itzchakov

Reputation: 149538

As the WaitAsync isn't avaliable in .NET 4.0, You can use the code from Stephan Toub's series of async synchronization primitives:

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); 
    }
}

Upvotes: 1

Stephen Cleary
Stephen Cleary

Reputation: 456507

As others have mentioned, .NET 4.5 introduced SemaphoreSlim.WaitAsync.

For .NET 4.0, you can either write your own async-compatible lock or use one from my Nito.AsyncEx NuGet package.

Upvotes: 3

Sriram Sakthivel
Sriram Sakthivel

Reputation: 73462

WaitAsync is introduced with .Net 4.5. You either need to implement yourself as an extension method by looking into source(not sure if that is possible), or you can use StephenToub's AsyncSemaphore.

Upvotes: 2

Servy
Servy

Reputation: 203829

No, you'll need to upgrade to .NET 4.5 (or write a WaitAsync extension (or inherently asynchronous semaphore) yourself).

The async extensions for .NET 4.0 are there to allow for the support of the actual await keyword. Much of the work of .NET 4.5 is adding asynchronous operations to various BCL types that can be awaited. If you need those operations, you'll need to upgrade the version of the framework you use.

Upvotes: 1

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