Silverlight Asynchronous operation Foreach loop

Question

I have a query for getting a list of prescriptions as below:

 var PRSCRPTSQuery = GV.dbContext.Load(GV.dbContext.GetPRSCRPTQuery(GV.curCustomer.CustCode,
                                          oOrdritemEdited.ProdCode, oOrdritemEdited.MedCode));
              PRSCRPTSQuery.Completed += new EventHandler(PRSCRPTSQuery_Completed);

In the query completed event, I have the following code :

 void PRSCRPTSQuery_Completed(object sender, EventArgs e)
  {          
      lstPRSCRPT = GV.dbContext.PRSCRPTs.Where(p=>p.Status =="Activated").ToList();
      if (lstPRSCRPT.Count > 0)
      {
          foreach (var rec in lstPRSCRPT)
          {
               var OrderItemQuery = GV.dbContext.Load(GV.dbContext.GetOrdritemsQuery(rec.PresNo));
               OrderItemQuery.Completed += new EventHandler(OrderItemQuery_Completed);               
          }                            
      } 
  }       

The list lstPRSCRPT can contain more than one record. I presume, the foreach loop will advance to the next item in the loop without waiting for the OrderItemQuery_Completed event which is below:

  void OrderItemQuery_Completed(object sender, EventArgs e)
  {
      lstOrderItem = GV.dbContext.OrderItems.ToList();
      if (lstOrderItem.Count > 0)
      {
          foreach (var OrdrItemRec in lstOrderItem)
          {
              TotTonnes = (double)(TotTonnes + OrdrItemRec.Quantity);
          }              
      }
   }

Is there any work around for this situation? I am new to the asynchronous type of programming in SL

Answer 1

I see where your coming from and when i first started Silverlight programming i gripped to my preconceptions of synchronous execution so i know what i have when ive finished calling a query and also i know exactly where it's errored.

Silverlight however takes this concept and tries to rip it from you yelling "This way is better trust me!" and for the purpose it serves of enriching client side interactivity it certainly succeeds. It just takes time. You just need to learn more about the style of how to link it all together.

The link previously shown by Faster Solutions shows where C# is going in terms of asynchronous coding but it pays to know what its actually accomplishing for you. Some of which you've already grasped in the code you've linked in the question.

When i've faced the same situation you have where you have back to back async callbacks is to raise an event when i've finished doing what i'm doing. For example:

public event EventHandler<EventArgs> LoadComplete;
public int QueryCount {get;set;}
public int QuerysCompleted {get;set;}

public void GetItems()
{
    var PRSCRPTSQuery = GV.dbContext.Load(GV.dbContext.GetPRSCRPTQuery
        (GV.curCustomer.CustCode, oOrdritemEdited.ProdCode, oOrdritemEdited.MedCode));
    PRSCRPTSQuery.Completed += new EventHandler(PRSCRPTSQuery_Completed);
    LoadComplete += loader_LoadComplete;
}


void PRSCRPTSQuery_Completed(object sender, EventArgs e)
{          
  lstPRSCRPT = GV.dbContext.PRSCRPTs.Where(p=>p.Status =="Activated").ToList();
  if (lstPRSCRPT.Count > 0)
  {
      QueryCount = lstPRSCRPT.Count;
      foreach (var rec in lstPRSCRPT)
      {
           var OrderItemQuery = GV.dbContext.Load(GV.dbContext.GetOrdritemsQuery(rec.PresNo));
           OrderItemQuery.Completed += new EventHandler(OrderItemQuery_Completed);               
      }                            
  } 

 }

  void OrderItemQuery_Completed(object sender, EventArgs e)
  {
      QueryCompleted++;
      lstOrderItem = GV.dbContext.OrderItems.ToList();
      if (lstOrderItem.Count > 0)
      {
          foreach (var OrdrItemRec in lstOrderItem)
          {
              TotTonnes = (double)(TotTonnes + OrdrItemRec.Quantity);
          }              
      }

     if(QueryCompleted == QueryCount)
     {
         RaiseLoadComplete();
     } 
  }



 public void RaiseLoadComplete()
 {
      if(LoadComplete != null)
      {
           LoadComplete(this, new EventArgs());
      }
 }


 void loader_LoadComplete(object sender, EventArgs e)
 {
      //Code to execute here
 }    

I attach an event when starting the first query of what code to execute when i'm done. In the first query call back i initialise a count of how many responses i am expecting. Then in the second query callback i increment until i get the right amount and call the event to say im done.

The only caution with this approach is if one of the queries error, The final code will never get executed.

Answer 2

You might find that the VS Async CTP of interest. It introduces the new "async" keyword for handling asyncronous events. He's a blog explaining it: VS Async