RIAlity Issues

Recently I had to face a very frustrating issue regarding the integration of a RIA Service as a LightSwitch Datasource. I tried to find references for it in the web. I did. But none of them (at least the ones I managed to find) didn’t seem to match my case. So after solving my issue I thought sharing was a good idea…
After having implemented and tested my RIA service using a small Silverlight application I decided to add a datasource and use it. Which I did. Focusing at the part of the RIA service that caused the problem, I had a class called ImageAttachment and 2 queries exposing it. Below you can see the initial code:

   1:      [Query(IsDefault = true)]

   2:      public IQueryable<ImageAttachment> GetImageAttachments() {

   3:        return new List<ImageAttachment>().AsQueryable();

   4:      }

   5:   

   6:      public IQueryable<ImageAttachment> GetFolderItemImageAttachments(Guid? providerId, string folderId, string mailItemId) {

   7:        if (!providerId.HasValue || folderId == null || mailItemId == null)

   8:          return GetImageAttachments();

   9:        return GetAttachments(providerId.Value, folderId, mailItemId);

  10:      }

Importing the RIA Service GetFolderItemImageAttachments was properly imported as an optional query for ImageAttachment.



I added an ImageAttachment visual collection in a screen fetching results from GetFolderItemImageAttachments query. Everything worked smoothly. But when the time came to load the visual collection (i.e. run the query) I got an exception I hadn’t come across before neither as a LS developer nor as a Silverlight one:

Load operation failed for query ‘GetFolderItemImageAttachments’.  The remote server returned an error: NotFound.

The behavior was very strange. Although the query was recognized and imported from LS for some reason at runtime it looked like the method was not found. I placed a breakpoint at the method and it didn’t hit. The method was never called. Before doing this I was sure it was some bug of mine in the implementation that caused the RIA service to throw this generic exception. But no! The execution never reached the web method. It was then that I decided to “google” it. Looked like it was a RIA Services+LightSwitch(?) issue. All references where either not appropriate in my case or had no effect. Then I did what I usually do when I have problem I cannot solve: I scrolled to the part of code listed above and started to just stare at the code waiting for an answer. And I came to me! Why not make sure the query method IS a query? So what I did was just add the QueryAttribute to my method with no parameters:

   1:      [Query(IsDefault = true)]

   2:      public IQueryable<ImageAttachment> GetImageAttachments() {

   3:        return new List<ImageAttachment>().AsQueryable();

   4:      }

   5:   

   6:      [Query]

   7:      public IQueryable<ImageAttachment> GetFolderItemImageAttachments(Guid? providerId, string folderId, string mailItemId) {

   8:        if (!providerId.HasValue || folderId == null || mailItemId == null)

   9:          return GetImageAttachments();

  10:        return GetAttachments(providerId.Value, folderId, mailItemId);

  11:      }

I thought that if it didn’t solve the problem I surely would not cause a new one. It IS a query method after all. Why let LS infer it? And it worked.

I am quite sure I was lucky. I mean, the query was recognized either way. I cannot explain what exactly was the bug the QueryAttribute fixed. To stress my point more, I have to admit that went back to the RIA Service and I removed the attribute. Still worked! Like a deadlock resolved. Maybe only LightSwitch Team can explain. Anyhow I thought of sharing in case anyone else runs into something like this.