What's the meaning of "UseTaskFriendlySynchronizationContext"?

There is a new app setting in asp.net 4.5

<add key="aspnet:UseTaskFriendlySynchronizationContext" value="true" />

code like this can run in asp.net 4.0

protected void Button1_Click(object sender, EventArgs e)
{
    CallAysnc();
}

public void CallAysnc()
{
    AsyncOperation asyncOp = AsyncOperationManager.CreateOperation(Guid.NewGuid().ToString());

    WebClient client = new WebClient();
    client.DownloadStringCompleted += (object sender, DownloadStringCompletedEventArgs e) =>
    {
        asyncOp.PostOperationCompleted(CallCompleted, e.Result);
    };
    client.DownloadStringAsync(new Uri("http://www.google.com"));
}

private void CallCompleted(object args)
{
    Response.Write(args.ToString());
}

But it doesn't work in asp.net 4.5,and when I remove the new appsetting,it works again!

So what's the meaning of "UseTaskFriendlySynchronizationContext" ?

Regarding UseTaskFriendlySynchronizationContext, from Microsoft Forums: > That tells ASP.NET to use an entirely new asynchronous pipeline which > follows CLR conventions for kicking off asynchronous operations, > including returning threads to the ThreadPool when necessary. ASP.NET > 4.0 and below followed its own conventions which went against CLR guidelines, and if the switch is not enabled it is > very easy for asynchronous methods to run synchronously, deadlock the request, or otherwise not behave as expected.

Also, I think AsyncOperationManager is intended for desktop applications. For ASP.NET apps you should be using RegisterAsyncTask and setting <%@ Page Async="true", see here for more details.

So using the new c# keywords your example would be:

protected void Button1_Click(object sender, EventArgs e)
{
    RegisterAsyncTask(new PageAsyncTask(CallAysnc));
}

private async Task CallAysnc()
{
    var res = await new WebClient().DownloadStringTaskAsync("http://www.google.com");
    Response.Write(res);
}

The aim is to support the following by release but is not currently supported in the beta:

protected async void Button1_Click(object sender, EventArgs e)
{
    var res = await new WebClient().DownloadStringTaskAsync("http://www.google.com");
    Response.Write(res);
}

More details, quoted from ASP.NET 4.5.1 documentation for appSettings on MSDN:

>aspnet:UseTaskFriendlySynchronizationContext > >Specifies how asynchronous code paths in ASP.NET 4.5 behave. > > ... > > If this key value is set to false [default], asynchronous code paths in ASP.NET 4.5 behave as they did in ASP.NET 4.0. If this key > value is set to true, ASP.NET 4.5 uses code paths that are optimized > for Task-returning APIs. Setting this compatibility switch is > mandatory for WebSockets-enabled applications, for using Task-based > asynchrony in Web Forms pages, and for certain other asynchronous > behaviors.