{"id":56021,"date":"2012-02-11T21:37:30","date_gmt":"2012-02-11T21:37:30","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/pfxteam\/2012\/02\/11\/building-async-coordination-primitives-part-3-asynccountdownevent\/"},"modified":"2012-02-11T21:37:30","modified_gmt":"2012-02-11T21:37:30","slug":"building-async-coordination-primitives-part-3-asynccountdownevent","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/dotnet\/building-async-coordination-primitives-part-3-asynccountdownevent\/","title":{"rendered":"Building Async Coordination Primitives, Part 3: AsyncCountdownEvent"},"content":{"rendered":"

In my last two posts, I discussed building AsyncManualResetEvent<\/a> and AsyncAutoResetEvent<\/a> coordination primitives.  In this post, I’ll build on that to create a simple AsyncCountdownEvent.<\/p>\n

A countdown event is an event that will allow waiters to complete after receiving a particular number of signals.  The “countdown” comes from the common fork\/join pattern in which it’s often utilized: a certain number of operations participate, and as they complete they signal the event, which counts down from the original number to 0.  When it gets to 0, it becomes set, and all waiters can complete.<\/p>\n

The shape of our type will be as follows:<\/p>\n

\n

public class<\/font> AsyncCountdownEvent<\/font>
{
    public<\/font> AsyncCountdownEvent(int<\/font> initialCount);
    public<\/font> Task<\/font> WaitAsync();
    public void <\/font>Signal();
}<\/font><\/p>\n<\/blockquote>\n

At its core, a countdown event is really just a manual reset event and an integral count, so our AsyncCountdownEvent will have two members:<\/p>\n

\n

private readonly<\/font> AsyncManualResetEvent<\/font> m_amre = new<\/font> AsyncManualResetEvent<\/font>();
private int<\/font> m_count;<\/font>
<\/p>\n<\/blockquote>\n

The constructor of our type simply initializes m_count based on a supplied number of signals:<\/p>\n

\n

public<\/font> AsyncCountdownEvent(int<\/font> initialCount)
{
    if<\/font> (initialCount <= 0) throw new<\/font> ArgumentOutOfRangeException<\/font>(“initialCount”<\/font>);
    m_count = initialCount;
}<\/font><\/p>\n<\/blockquote>\n

The WaitAsync method is then trivial, as it’ll just delegate to the corresponding method on the AsyncManualResetEvent:<\/p>\n

\n

public<\/font> Task<\/font> WaitAsync() { return<\/font> m_amre.WaitAsync(); }<\/font><\/p>\n<\/blockquote>\n

Finally, our Signal method will decrement m_count, and if that brings it to 0, we’ll Set the m_amre:<\/p>\n

\n

public void <\/font>Signal()
{
    if<\/font> (m_count <= 0)
        throw new<\/font> InvalidOperationException<\/font>(); <\/p>\n

<\/font>    int<\/font> newCount = Interlocked<\/font>.Decrement(ref<\/font> m_count);
<\/font>    if<\/font> (newCount == 0)
        m_amre.Set();
    else if<\/font> (newCount < 0)
        throw new<\/font> InvalidOperationException<\/font>();
}<\/font><\/p>\n<\/blockquote>\n

One common usage of a type like AsyncCountdownEvent is using it as a form of a barrier: all participants signal and then wait for all of the other participants to arrive.  Given that, we could also add a simple SignalAndWait method to implement this common pattern:<\/p>\n

\n

public<\/font> Task<\/font> SignalAndWait()
{
    Signal();
    return<\/font> WaitAsync();
}<\/font><\/p>\n<\/blockquote>\n

Next time, we’ll take a look at implementing an actual asynchronous barrier<\/a>, one that can be used over and over by the participants to operate in lock step.<\/p><\/p>\n","protected":false},"excerpt":{"rendered":"

In my last two posts, I discussed building AsyncManualResetEvent and AsyncAutoResetEvent coordination primitives.  In this post, I’ll build on that to create a simple AsyncCountdownEvent. A countdown event is an event that will allow waiters to complete after receiving a particular number of signals.  The “countdown” comes from the common fork\/join pattern in which it’s […]<\/p>\n","protected":false},"author":360,"featured_media":58792,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[7908],"tags":[7925,7916,7909,7912],"class_list":["post-56021","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-pfxteam","tag-net-4-5","tag-coordination-data-structures","tag-parallel-extensions","tag-task-parallel-library"],"acf":[],"blog_post_summary":"

In my last two posts, I discussed building AsyncManualResetEvent and AsyncAutoResetEvent coordination primitives.  In this post, I’ll build on that to create a simple AsyncCountdownEvent. A countdown event is an event that will allow waiters to complete after receiving a particular number of signals.  The “countdown” comes from the common fork\/join pattern in which it’s […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/posts\/56021","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/users\/360"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/comments?post=56021"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/posts\/56021\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/media\/58792"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/media?parent=56021"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/categories?post=56021"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/tags?post=56021"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}