Archives / 2009 / August
  • .NET RIA Services Exception handling

    Note: This post is based on the July preview of the .NET RIA Services, so changes may happen in a future release of the framework

    At the moment I’m updating a code example for my Silverlight 3.0 course and have added some exception handling to the code. I also got a question about how to handle exception while calling the Load operation of the DomainService class. So I decided to write a blog post about it, maybe someone more have the same question.

    By default if you make a call to a DomainService query method, you will not get an exception on the client-side, if you don’t adding some extra code. Lets pretend that the following code is your DomainService and Query method:

    public class MyDomainService : DomainService
          public IEnumerable<Customer> GetCustomers()
               //Code that can throw an exception;

    The method above will throw an exception. If you on the client-side want to “call/use” the GetCustomers method, you will use the generated DomainContext class’s Load method:

    var d = new MyDomainContext(); var loadOperation = d.Load<Customer>(d.GetCustomersQuery(), MyCallBack, null); LayoutRoot.DataContext = d.Customers;

    When this code is executed, you will not get any exception, so the user or you may think that no exception have occurred. But that isn’t the case. So what I’m going to write about now, is different way you can check if an exception occurred (Wouldn’t it be nice if we only could get an exception instead of adding extra code to check if an exception is thrown ;)).

    The LoadOperation class has two properties we can use, HasError and Error. To check if we got an exception we can use the HasError property, if we want to get information about an exception, we can use the Error property. The first example I’m going to use, will only display an exception for the client.

    On the client-side I have a StackPanel called myErrorPanel, it also have a TextBlock where the Text property is bound to a property path “Error.Message”.

    StackPanel x:Name="myErrorPanel"> <TextBlock Text="{Binding Path=Error.Message}"></TextBlock> </StackPanel>

    In the code-behind we can now write the following code which will bound the LoadOperation to the myErrorPanel:

    var d = new MyDomainContext(); var loadOperation = d.Load<Customer>(d.GetCustomersQuery()); LayoutRoot.DataContext = d.Customers; myErrorPanel.DataContext = loadOperation;

    If the Load method of our DominContext has an error, the Error Message will be displayed for the User. By using this solution, we will not have any idea if an error has occurred during the Load operation, if it does it will only be shown for the User. The StackPanel will also take up some space on the user interface, so to solve it we can for example add a Converter, and bound the HasError to the StackPanel’s Visibility property. The following two other solution will instead show us, how we can use a callback method and the LoadOperation’s Complteted event to have a better control if an exception is thrown during the Load operation.

    The DomainContext’s Load method can take a callback as an parameter, by using a callback, we will get a notification when the Load operation is completed, and we can also see if there was any error during the Load operation. The following code will use a callback and check if an error has occurred:

    var d = new MyDomainContext(); var loadOperation = d.Load<Customer>(d.GetCustomersQuery(), MyCallBack, null); ... void MyCallBack(LoadOperation<Customer> loadOperation) { if (loadOperation.HasError) //Handle the error }

    The following solution will use the LoadOptions’s Completed event to check if an error has occurred:

    var d = new MyDomainContext(); var loadOperation = d.Load<Customer>(d.GetCustomersQuery()); loadOption.Completed += new EventHandler(loadOption_Completed); ... void loadOption_Completed(object sender, EventArgs e) { if (((LoadOperation)sender).HasError) //handle the error }

    If you wonder what solution I prefer, it would be the one that uses a callback. I do want to see some changes to the .NET RIA Services Load operations, like specifying a callback just for handling exceptions, and a default generated callback added to the DominaContext, which will be called if no callback is used, by doing so, we could get an exception when the Load fails without adding some extra code.


  • Different ways to keep state when building RIA with Silverlight

    When building RIA with Silverlight, we will have some state issues we need to take care of. A Silverlight app by default is running in a browser, the browser have 3 bad buttons, Refresh, back and forward. If a user will press the Refresh button, the whole Silverlight app will be restarted. Same if they navigate from a Silverlight app with the back and forward buttons. If we build a RIA, our app will mostly hold the state for us on the client-side to reduce the number of round trip to the server etc. Lets assume a user will work with customer data. A user want to update the customer and information related to the customer. After 10 minutes of changes, the user suddenly by mistake refresh the page and all the data will be lost. So what we need to do when building RIA, is to remember the Refresh, back and forward buttons. This blog post will cover some of the Silverlight 3.0 features we can use to hold state and handle the Refresh, back and forward issues. This is something that I will also take up in my new updated Silverlight 3.0 course at Cornerstone (Where I work as an instructor, mentor, coach and consultant).


  • How SubmitChanges works in .NET RIA Servies

    Now I’m back from my 4 week long vacation. I have spend a lot of time with my new apartment, took about 2 month to get most of it ready, I still have a lot of things to do, but all the painting and stuff are done :) I got a question on my e-mail regarding .NET RIA Services and the Business Application project’s UserRegistartionService. It was about the AddUser method in the Service and when and where it’s called. I thought it would be great to write a post about it.


  • Moved to a new apartment

    I haven't blog for a while, the reason to this is that I moved to a new apartment three month ago, and have spend a lot of time with some decoration and finding new furniture etc. I have also been really busy with work until my vacation. I still have one week left out of four.