Introduction
In this short tip, I explain how I made an OData
sample using the async extension of .NET 4.5.
Using the Code
Today, I watched this video about using OData
in a WinRT app.
To summarize, he (Jerry Nixon) adds a service reference to the Netflix OData API at http://odata.netflix.com/ and then proceeds to do something like:
protected override void OnNavigatedTo(NavigationEventArgs e)
{
var _uri = new Uri("http://odata.netflix.com/Catalog/");
var ctxt = new NetFlixService.NetflixCatalog(_uri);
var data = new DataServiceCollection<NetFlixService.Title>(ctxt);
var query = from t in ctxt.Titles
where t.Name.Contains("Star Trek")
select t;
data.LoadCompleted += delegate
{
this.DataContext = data;
};
data.LoadAsync(query);
}
Basically, he creates an OData
service reference, runs a database query against it, and shows the results.
What I will show is how to improve this, how to use the new async
/ await
keywords instead of the LoadCompleted
delegate.
There seems to be no obvious way of doing that! Time to Google!
From the await (C# Reference) reference, it seems that await
can (only) be applied to a method returning a Task
, and a Task
(from Googling) can be created from IAsyncResult
.
So first, I started by creating a very simple and reusable IAsyncResult
implementation class:
public class SimpleAsyncResult : IAsyncResult, IDisposable
{
ManualResetEvent waitHandle = new ManualResetEvent(false);
public void Finish()
{
IsCompleted = true;
waitHandle.Set();
waitHandle.Dispose();
}
public void Dispose() { waitHandle.Dispose(); }
public bool IsCompleted { get; private set; }
public object AsyncState { get; set; }
public bool CompletedSynchronously { get; set; }
public WaitHandle AsyncWaitHandle { get { return waitHandle; } }
}
With that, I can easily create an extension method for my OData
classes returning a Task
:
public static class OData
{
public static Task<DataServiceCollection<T>> AsyncQuery<T>(
this DataServiceCollection<T> data, IQueryable<T> query = null)
{
var asyncr = new SimpleAsyncResult();
Exception exResult = null;
data.LoadCompleted += delegate(object sender, LoadCompletedEventArgs e)
{
exResult = e.Error;
asyncr.Finish();
};
if (query == null)
data.LoadAsync();
else
data.LoadAsync(query);
return Task<DataServiceCollection<T>>.Factory.FromAsync(asyncr
, r =>
{
if (exResult != null)
throw new AggregateException("Async call problem", exResult);
return data;
}
);
}
}
Remark: Here, I wrap the exception because I don’t want to lose the stack trace (with “throw exResult
”).
And voila, I can update my NavigatedTo
method to use the .NET4.5 async extension!
protected async override void OnNavigatedTo(NavigationEventArgs e)
{
var _uri = new Uri("http://odata.netflix.com/Catalog/");
var ctxt = new NetFlixService.NetflixCatalog(_uri);
var data = new DataServiceCollection<NetFlixService.Title>(ctxt);
var query = from t in ctxt.Titles
where t.Name.Contains("Star Trek")
select t;
await data.AsyncQuery(query);
this.DataContext = data;
}
Points of Interest
This is a nice way to explore some internals of the async
extension in .NET4.5.
History
The Australia born French man who went back to Australia later in life...
Finally got over life long (and mostly hopeless usually, yay!) chronic sicknesses.
Worked in Sydney, Brisbane, Darwin, Billinudgel, Darwin and Melbourne.