WPF

internal ObservableCollection<INVCategory> GetCategoryList()
{
     DataLoadOptions dataLoadOptions = new DataLoadOptions();
     dataLoadOptions.LoadWith<INVCategory>(t => t.INVSubCategories);
     this.Context.LoadOptions = dataLoadOptions;

     var categories = from category in this.Context.INVCategories
                      orderby category.CatgeoryId descending
                      select category;
     return new ObservableCollection<INVCategory>(categories.ToList());
}

<ComboBox Grid.Column="1" Grid.Row="0" HorizontalAlignment="Stretch"                           VerticalAlignment="Stretch" Margin="0,2,10,2" TabIndex="0" x:Name="categoryComboBox"      
ItemsSource="{Binding CategoryList}" IsEditable="True" DisplayMemberPath="CategoryName"  SelectedValuePath="CatgeoryId" SelectedItem="{Binding CategoryList, Mode=TwoWay}" SelectedValue="{Binding Path=CurrentEntity.CategoryId, Mode=TwoWay}" >
</ComboBox>

<ComboBox Grid.Column="1" Grid.Row="1" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Margin="0,2,10,2" TabIndex="1"
ItemsSource="{Binding SelectedItem, ElementName=categoryComboBox, Mode=OneWay}" 
DisplayMemberPath="SubCategoryName" SelectedValuePath="SubCategoryId"
SelectedItem="{Binding INVSubCategories, Mode=TwoWay}">
</ComboBox>

C:\My Source 2010\PHDWebsite\PHDWebsite\obj\Debug\MainPage.g.i.cs(62,42): error CS0234: The type or namespace name 'DockPanel' does not exist in the namespace 'System.Windows.Controls' (are you missing an assembly reference?)

xmlns:controlsToolkit="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Toolkit"

C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1360,9): warning MSB3245: Could not resolve this reference. Could not locate the assembly "System.Windows.Controls.Toolkit, Version=2.0.5.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL". Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors.