Silverlight 4 and data-bound combobox - the answer

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Update: 10th August 2010; Sorry, folks, this continues to be as complex before. What follows works ~95% of the time but will occasionally fail because the lookup data source is not available when the main data source is queried.

This really should not be difficult, but it appears that it really is.

I think that the solution is to move to an MVVM model, where the ViewModel presents the primary data source as well as the lookup data sources. Better; it provides dependency linking so that when the lookup lists change, then dependant views are updated.

You may now be thinking "WHAT!? It can't be that complicated, surely?!?!?!?". Well, I thought the same, and it seems that it really is that complex

It's all to do with two 21st century concepts;

1. Asynchronous (late) binding; As we display the UI, we don't know when all the data will be loaded from the database. What happens if the core dataset (Employees) is loaded before dependant datasets (Employee Position)?
2. Multi-threaded / multi-user / multi-context updates; What if we are quite happily wandering around our employee edit screen and somebody else adds a new Employee Position record? How do we reflect that into our (fairly single user) model of the world?

Unfortunately, and I really looked for shortcuts, these are enterprise-level discussions.

I believe that the answer is the MVVM model and I'm currently trying to catch up on my learning, through the following resources;

• MVVM on Wikipedia
• Karl Shifflett on MVVM in WPF
• The excellent Silverlight In Action book by Pete Brown (not yet released)
• The Cinch MVVM framework from the inestimable Sasha Barber

Until I catch up, then I can only recommend the rest of this article as an educational exercise. It ain't much cop for real life.

Sorry!

   Scott

 

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OK, I figured it out. How to present a data-bound combobox with Silverlight 4. This isn't going to work with SL2 or SL3 because they are missing the SelectedValuePath property.

So, let's set some context. I have a CredentialsType which, for the sake of this example, contains types which are;

• SNMP
• Windows
• SQL Server
• …etc…

Then, I have a CredentialsSet, which has a variety of login and password identifiers. So, for a Windows set, we have;

• CredentialsTypeId = the id of the CredentialsType we mentioned above, that is set to a CredentialsType where the CredentialsType.Name = "Windows"
• DomainColumn = the Active Directory domain name.
• LoginColumn = AD username
• PasswordColumn = AD password

Another CredentialsSet for an SNMP set may look like this;

• CredentialsTypeId = this id of the CredentialsType whose CredentialsType.Name = "SNMP"
• LoginColumn = SNMP Community string

So, the combobox is going to be on the CredentialsSet entity, pointing at the master (CredentialsType) record.

So, we have a master record for CredentialsType (primary key = CredentialsTypeId, Friendly Name = Name)

And the child record (in other words, the form that is displaying the record that is to be edited, that has a drop down relating to the parent record) has a number of child columns.

The XAML ends up looking like this;

 <ComboBox Grid.Column="2" Grid.ColumnSpan="2" Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="0,3,0,0"

Name="comboBox1" VerticalAlignment="Top" Width="120"

ItemsSource="{Binding ElementName=credentialsTypeDomainDataSource, Path=Data}"

DisplayMemberPath="Name"

SelectedValue="{Binding ElementName=credentialsSetDomainDataSource, Path=Data.CurrentItem.CredentialsTypeId, Mode=TwoWay}"

SelectedValuePath="CredentialsTypeId" />

 Here, you'll see that;

• the ItemSource = the path to the data source that holds the complete list of elements (in our case, the CredentialTypes)
• DisplayMemberPath = The name of the property that will be displayed in the dropdown
• SelectedValue = the two way binding (i.e. ID) that links the child record to the parent record
• SelectedValuePath = this should be the same as the SelectedValue and points the caller to the correct value in the parent record