Question

I wonder if someone could provide a simple example of the following. (preferably in VB.Net):

I have an SQL database with related tables and I am successfully using LINQ to SQL in other areas of my application, but am a little stuck with the Heirarchical Treeview.

I would simply like to use a LINQ query from the database as a source for a WPF Treeview. If I can set the ItemsSource for the treeview as my LINQ result and just set the databinding for treeview items to the various columns that would make my day, but I cant seem to get it cooking.

After spending hours searching the net, I can t find many examples that show this very simply at all. I have found similar ideas but nothing simple and specific for a newbie like myself.

As far as I understand, the relationships defined in the DBML file should stay intact when executing the LINQ query. So, can I have something like this as the ItemsSource for the Treeview?

Dim pdc As New ProjectDataContext()  
Public Property Selection() As Integer

Dim tree = From c In pdc.Customers _
           Where c.CustomerID = _Selection _
           Select c

projecttreeview.ItemsSource = tree

Then, the databinding for the TreeView Items could just be {Binding CustomerName) for a parent node and say {Binding Orders.OrderName} as a child node. eg:

<TreeView Name="projecttreeview">
  <TreeView.ItemTemplate>
    <HierarchicalDataTemplate ItemsSource="{Binding Customers}">
      <TextBlock.Text="Binding CustomerName}"/>
        <HierarchicalDataTemplate.ItemTemplate>
          <DataTemplate>
            <TextBlock.Text="{Binding Orders.OrderName}"/>
          </DataTemplate>
        </HierarchicalDataTemplate.ItemTemplate>
      </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
</TreeView>

Obviously this is not quite working out as simply as I would like. Any pointers would be greatly appreciated.

Answer 1

First of all, let me to clarify one issue you probably will run into: when you use query as an ItemsSource, then the ItemsControl won t reflect any changes after you will rerun the query, so, assigning the ItemsSource to a query is very similar to assigning it to the query result converted to an array.

Second, to access related objects/collections (wherever you do it from - would it be a DataTemplate or simply plain code) you have to explicitly point it out when you creating a query. In Entity Framework you would probably do that by means of Include method like this: treeview.ItemsSource = tree.Include("Orders") - it will force the engine to make JOIN SQL query over the database.

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