Silverlight 4 Ria Services and multiple threads

Question

I have a very long running query that takes too long to keep my client connected. I want to make a call into my DomainService, create a new worker thread, then return from the service so that my client can then begin polling to see if the long running query is complete.

The problem I am running into is that since my calling thread is exiting right away, I am getting exceptions thrown when my worker tries to access any entities since the ObjectContext gets disposed when the original thread ends.

Here is how I create the new context and call from my Silverlight client:

MyDomainContext context = new MyDomainContext();
context.SearchAndStore(_myParm, SearchQuery,
    p => {
        if (p.HasError) { // Do some work and return to start 
        }                 // polling the server for completion...
    }, null);

The entry method on the server:

[Invoke]
public int SearchAndStore(object parm)
{
    Thread t = new Thread(new ParameterizedThreadStart(SearchThread));
    t.Start(parms);     
    return 0;
    // Once this method returns, I get ObjectContext already Disposed Exceptions
}

Here is the WorkerProc method that gets called with the new Thread. As soon as I try to iterate through my query1 object, I get the ObjectContext already Disposed exception.

private void WorkerProc(object o)
{
    HashSet<long> excludeList = new HashSet<long>();

    var query1 = from doc in this.ObjectContext.Documents
        join filters in this.ObjectContext.AppliedGlobalFilters
            .Where(f => f.FilterId == 1)  
        on doc.FileExtension equals filters.FilterValue
        select doc.FileId;

    foreach (long fileId in query1) // Here occurs the exception because the
    {                               // Object Context is already disposed of.
        excludeList.Add(fileId);
    }
}

How can I prevent this from happening? Is there a way to create a new context for the new thread? I'm really stuck on this one.

Thanks.

Answer 1

Since you're using WCF RIA. I have to assume that you're implementing two parts:

• A WCF Web Service
• A Silverlight client which consumes the WCF Service.

So, this means that you have two applications. The service running on IIS, and the Silverlight running on the web browser. These applications have different life cycles. The silverlight application starts living when it's loaded in the web page, and it dies when the page is closed (or an exception happens). On the other hand (at server side), the WCF Web Service life is quite sort. You application starts living when the service is requested and it dies once the request has finished.

In your case your the server request finishes when the SearchAndStore method finishes. Thus, when this particular method starts ,you create an Thread which starts running on background (in the server), and your method continues the execution, which is more likely to finishes in a couple of lines.

If I'm right, you don't need to do this. You can call your method without using a thread, in theory it does not matter if it takes awhile to respond. this is because the Silvelight application (on the client) won't be waiting. In Silverlight all the operations are asynchronous (this means that they're running in their own thread). Therefore, when you call the service method from the client, you only have to wait until the callback is invoked.

If it's really taking long time, you are more likely to look for a mechanism to keep the connection between your silverlight client and your web server alive for longer. I think by modifying the service configuration.

Here is a sample of what I'm saying: https://github.com/hmadrigal/CodeSamples/tree/master/wcfria/SampleWebApplication01 In the sample you can see the different times on client and server side. You click the button and have to wait 30 seconds to receive a response from the server.

I hope this helps, Best regards, Herber