WCF数据服务BeginSaveChanges仅运行一次

Question

I have the following WCF Data Service code in a Xamarin Forms application. It updates a simple row in a table.

    Static.Dialogs.Alert("Starting");
    DataServiceQuery<SRef.SimpleObject> query = (DataServiceQuery<SRef.SimpleObject>)Entities.SimpleObject.Where(x => x.ID == Guid.Parse("DEF47A0F-AF1E-4043-B8C8-56084841E80B"));
    query.BeginExecute((result) =>
             {
                 try
                 {
                     Static.Dialogs.Alert("Getting the object");
                     var actData = query.EndExecute(result).FirstOrDefault();
                     if (actData != null)
                     {
                         actData.Info = "Info"+randomNumber;
                         Entities.UpdateObject(actData);
                         Entities.ChangeState(actData, EntityStates.Modified);
                         Static.Dialogs.Alert("Before the update");
                         Entities.BeginSaveChanges(SaveChangesOptions.BatchWithIndependentOperations, (result2) =>
                         {
                             try
                             {
                                 Static.Dialogs.Alert("BeginSaveChanges starts");
                                 var r = Entities.EndSaveChanges(result2);
                                 Static.Dialogs.Alert("Update done ");
                             }
                             catch (Exception ex2)
                             {
                                 Static.Dialogs.Alert("Error:" + ex2.Message);
                             }
                         }, null);
                     }
                     else
                         Static.Dialogs.Alert("No object");
                 }
                 catch (Exception ex1)
                 {
                     Static.Dialogs.Alert("Error:" + ex1.Message);
                 }
             }, null);
         }
         catch (Exception ex)
         {
             Static.Dialogs.Alert("Error:" + ex.Message);
         }            
     });

I have tested it on an emulator and a physical device.

Sometimes it works perfectly .

Sometimes I get only these messages:

• Starting
• Getting the object
• Before the update

Sometimes only these:

• Starting

It mostly gets wrong when I get a perfect update and I try again. Like it has 'used up' the only connection and after that it doesn't work.

On the server side I log every error and nothing is catched there. Also, no exception on the client side.

The DataServiceContext MergeOption is set to PreserveChanges .

What could affect it? When I send out a request, I have to wait some time? Should I close the connection somehow?

I reckon it is some kind of cache problem.

UPDATE :

I tried again, with a simpler approach (I only save a new item now):

private DataServiceReference.DataEntities entities;

public DataServiceReference.DataEntities Entities
{
  get
  {
    if (entities == null)
    {
      entities = new DataServiceReference.DataEntities(Static.BaseURI);
      entities.MergeOption = MergeOption.OverwriteChanges;
      entities.SaveChangesDefaultOptions = SaveChangesOptions.ReplaceOnUpdate;
    }
    return entities;
  }
}

var newItem = new DataServiceReference.Info()
{
  Name = "Name " + DateTime.Now.Second,
  ID = Guid.NewGuid(),
  Role = "1"
};
Entities.AddToInfo(newItem);
try
{
  foreach (var item in Entities.Entities)
  {
    System.Diagnostics.Debug.WriteLine(item.Identity + " " + item.State);
  }
  var res = Entities.BeginSaveChanges(SaveChangesOptions.Batch,
      (result) =>
      {
        //var s = 3; //debug point - only hit once
        try
        {
          //back to the UI thread
          Xamarin.Forms.Device.BeginInvokeOnMainThread(() =>
          //(result.AsyncState as DataServiceReference.DataEntities).EndSaveChanges(result));
             Entities.EndSaveChanges(result));
        }
        catch (Exception ex)
        {
          System.Diagnostics.Debug.WriteLine(ex.Message);
          throw;
        }
      }, Entities);
  //res.AsyncWaitHandle.WaitOne(1000); //it only blocks the main thread, no use

  resultList.Add(newItem.Name);
}
catch (Exception ex2)
{
   System.Diagnostics.Debug.WriteLine(ex2.Message);
   throw;
}

I also read (and it was pointed out in the first answer) that the result is provided on a different thread, so I added a dispatcher call to get the results (note the UI thread call: Xamarin.Forms.Device.BeginInvokeOnMainThread ).

In an application where the callback must be invoked on a specific thread, you must explicitly marshal the execution of the End method, which handles the response, to the desired thread. For example, in Windows Presentation Foundation (WPF)-based applications and Silverlight-based applications, the response must be marshaled back to the UI thread by using the BeginInvoke method on the Dispatcher object.

Note the mentioning of the End method!

I added the following debug message:

foreach (var item in Entities.Entities)
{
  System.Diagnostics.Debug.WriteLine(item.Identity + " " + item.State);
}

The result:

[0:]http://192.168.1.100/InfoDataService/InfoDataService.svc/Info(guid'f1057131-90ee-11d7-9812-0040f6cc1384') Unchanged
[0:]http://192.168.1.100/InfoDataService/InfoDataService.svc/Info(guid'f1057133-90ee-11d7-9812-0040f6cc1384') Unchanged
[0:]http://192.168.1.100/InfoDataService/InfoDataService.svc/Info(guid'f6cfce91-90ef-11d7-9812-0040f6cc1384') Unchanged
[0:]http://192.168.1.100/InfoDataService/InfoDataService.svc/Info(guid'a6c2d822-91a7-11d7-9813-0040f6cc1384') Unchanged
[0:]http://192.168.1.100/InfoDataService/InfoDataService.svc/Info(guid'a6c2d823-91a7-11d7-9813-0040f6cc1384') Unchanged
[0:]http://192.168.1.100/InfoDataService/InfoDataService.svc/Info(guid'a6c2d824-91a7-11d7-9813-0040f6cc1384') Unchanged
[0:]http://192.168.1.100/InfoDataService/InfoDataService.svc/Info(guid'b750e561-91b8-11d7-9813-0040f6cc1384') Unchanged
[0:]http://192.168.1.100/InfoDataService/InfoDataService.svc/Info(guid'b750e562-91b8-11d7-9813-0040f6cc1384') Unchanged
[0:]http://192.168.1.100/InfoDataService/InfoDataService.svc/Info(guid'b750e563-91b8-11d7-9813-0040f6cc1384') Unchanged
[0:]http://192.168.1.100/InfoDataService/InfoDataService.svc/Info(guid'eee2d1f7-17cb-4283-a053-01f6cf7bb2fd') Unchanged
[0:]  Added
[0:]  Added
[0:]  Added

It seems, that the context keeps gathering the objects but it sends only the first new object to the service and the other objects keep piling up.

The article emphasizes the thread issues of the End method, but the callback is the real issue here (it is not fired), so we never get to the End call and the UI thread.

This seems like a serious bug and I can't tell what to do...

Answer 1

The reason is that BeginExecute will start another thread, and your main thread returned even when the sub-thread doesn't finished. Then this code may even not execute the BeginSaveChanges .

So to make sure all the sub threads finished. The main thread need to wait for the sub threads.

The simplest way is to System.Threading.Thread.Sleep(5000) ; Or you can use IAsyncResult.AsyncWaitHandle.WaitOne()