WPF

<Window x:Class="MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="400" Width="1200">


    <Grid Width="900" HorizontalAlignment="Left" DataContext="{Binding}">
        <Grid.RowDefinitions>
            <RowDefinition Height="44*" />
            <RowDefinition Height="44*" />
            <RowDefinition Height="44*" />
            <RowDefinition Height="256*" />
        </Grid.RowDefinitions>

        <StackPanel Grid.Row="0" Margin="5,0,0,0" FlowDirection="LeftToRight" Orientation="Horizontal" VerticalAlignment="Top"  HorizontalAlignment="Left" >
            <TextBlock Text="Select a Product" Margin="5"></TextBlock>
            <ComboBox Height="28" Name="cmbProducts2" Width="150" Margin="3"
                        IsEditable="False"
                        ItemsSource="{Binding Path=myProductList,  Mode=OneWay,  UpdateSourceTrigger=PropertyChanged}"
                        SelectedItem="{Binding mySelectedProduct}"
                        >
                <ItemsControl.ItemTemplate>
                    <DataTemplate>
                        <StackPanel Orientation="Horizontal" >
                            <TextBlock Text="{Binding Path=ProductName}" />
                            <TextBlock Text=" (" />
                            <TextBlock Text="{Binding Path=ProductID}" />
                            <TextBlock Text=") " />
                        </StackPanel>
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ComboBox>
        </StackPanel>

        <StackPanel Grid.Row="1" Margin="5,0,0,0" FlowDirection="LeftToRight" Orientation="Horizontal" VerticalAlignment="Top"  HorizontalAlignment="Left" >
            <TextBlock Text="Category Should Auto Select" Margin="5"></TextBlock>
            <ComboBox Height="28" Name="cmbCategory2" Width="150" Margin="3" IsEditable="False"
                        ItemsSource="{Binding Path=myCategoryList,  Mode=OneWay,  UpdateSourceTrigger=PropertyChanged}"
                        SelectedItem="{Binding mySelectedCategory}"
                        >
                <ItemsControl.ItemTemplate>
                    <DataTemplate>
                        <StackPanel Orientation="Horizontal" >
                            <TextBlock Text="{Binding Path=CategoryName}" />
                            <TextBlock Text=" (" />
                            <TextBlock Text="{Binding Path=CategoryID}" />
                            <TextBlock Text=") " />
                        </StackPanel>
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ComboBox>

        </StackPanel>
        <StackPanel Grid.Row="2" Margin="5,0,0,0" FlowDirection="LeftToRight" Orientation="Horizontal" VerticalAlignment="Top"  HorizontalAlignment="Left" >
            <TextBlock Text="mySelectedCategory.CategoryName=" />
            <TextBlock Text="{Binding mySelectedCategory.CategoryName}" Foreground="#FFFC0000"/>
            <TextBlock Text="  mySelectedCategory.CategoryID=" />
            <TextBlock Text="{Binding mySelectedCategory.CategoryID}" Foreground="#FFFC0000"/>
        </StackPanel>

        <StackPanel Grid.Row="3" Margin="5,0,0,0" FlowDirection="LeftToRight" Orientation="Horizontal" VerticalAlignment="Top"  HorizontalAlignment="Left" >
            <TextBlock Text="mySelectedProduct.myCategory.CategoryName=" />
            <TextBlock Text="{Binding mySelectedProduct.myCategory.CategoryName}" Foreground="#FFFC0000" ForceCursor="True"></TextBlock>
            <TextBlock Text="  mySelectedProduct.myCategory.CategoryID=" />
            <TextBlock Text="{Binding mySelectedProduct.myCategory.CategoryID}" Foreground="#FFFC0000"/>
        </StackPanel>

    </Grid>

</Window>

Imports System.Collections.ObjectModel
Public Class myDataRepository
    Private _mycateorylist As ObservableCollection(Of myCategory)
    Public Sub New()
        _mycateorylist = New ObservableCollection(Of myCategory)
        _mycateorylist = GetMyCategories()
    End Sub
    Public Function GetMyProducts() As ObservableCollection(Of myProduct)
        Dim a As New myProduct()
        Dim b As New myProduct
        Dim c As New myProduct
        Dim d As New myProduct
        a.ProductID = 1
        a.ProductName = "tire"
        a.CategoryID = 1
        b.ProductID = 2
        b.ProductName = "wipers"
        b.CategoryID = 1
        c.ProductID = 3
        c.ProductName = "tv"
        c.CategoryID = 2
        d.ProductID = 4
        d.ProductName = "radio"
        d.CategoryID = 2
        Dim list As New List(Of myProduct)
        list.Add(a)
        list.Add(b)
        list.Add(c)
        list.Add(d)
        Return New ObservableCollection(Of myProduct)(list)
    End Function
    Public Function GetMyCategories() As ObservableCollection(Of myCategory)
        Dim a As New myCategory
        Dim b As New myCategory
        a.CategoryID = 1
        a.CategoryName = "auto"
        b.CategoryID = 2
        b.CategoryName = "electronics"
        Dim list As New List(Of myCategory)
        list.Add(a)
        list.Add(b)
        Return New ObservableCollection(Of myCategory)(list)
    End Function
    Public Function GetMyCategory(ByVal id As Int32) As myCategory
        Dim results = From c In _mycateorylist
                Where c.CategoryID = id
                Select c
        Dim rs As IEnumerable(Of myCategory) = From x In results.AsEnumerable
                      Select New myCategory With
                             {
                                 .CategoryID = x.CategoryID,
                                 .CategoryName = x.CategoryName
                             }
        Return rs.FirstOrDefault
    End Function
End Class
Public Class myCategory
#Region "Primitive Properties"
    Public Property CategoryID() As Global.System.Int32
        Get
            Return _CategoryID
        End Get
        Set(value As Global.System.Int32)
            _CategoryID = value
        End Set
    End Property
    Private _CategoryID As Global.System.Int32
    Public Property CategoryName() As Global.System.String
        Get
            Return _CategoryName
        End Get
        Set(value As Global.System.String)
            _CategoryName = value
        End Set
    End Property
    Private _CategoryName As Global.System.String
#End Region
End Class
Public Class myProduct
#Region "Primitive Properties"
    Public Property ProductID() As Global.System.Int32
        Get
            Return _ProductID
        End Get
        Set(value As Global.System.Int32)
            _ProductID = value
        End Set
    End Property
    Private _ProductID As Global.System.Int32
    Public Property ProductName() As Global.System.String
        Get
            Return _ProductName
        End Get
        Set(value As Global.System.String)
            _ProductName = value
        End Set
    End Property
    Private _ProductName As Global.System.String
    Public Property CategoryID() As Nullable(Of Global.System.Int32)
        Get
            Return _CategoryID
        End Get
        Set(value As Nullable(Of Global.System.Int32))
            _CategoryID = value
        End Set
    End Property
    Private _CategoryID As Nullable(Of Global.System.Int32)
    Private _mycategory As myCategory
    Public Property myCategory As myCategory
        Get
            Return _mycategory
        End Get
        Set(value As myCategory)
            _mycategory = value
        End Set
    End Property
#End Region
End Class

Imports System.Collections.ObjectModel
Imports System.ComponentModel
Public Class MainWindowViewModel
    Implements INotifyPropertyChanged
    Public Property myProductList As ObservableCollection(Of myProduct)
    Public Property myCategoryList As ObservableCollection(Of myCategory)
    Private Property _context As myDataRepository
    Public Sub New(ByVal db As myDataRepository)
        _context = db
        myProductList = _context.GetMyProducts()
        myCategoryList = _context.GetMyCategories()
    End Sub
    Private Property _myselectedproduct As myProduct
    Public Property mySelectedProduct As myProduct
        Get
            Return _myselectedproduct
        End Get
        Set(value As myProduct)
            _myselectedproduct = value
            mySelectedProduct.myCategory = _context.GetMyCategory(mySelectedProduct.CategoryID)
            mySelectedCategory = mySelectedProduct.myCategory
            OnPropertyChanged(mySelectedProduct, New System.ComponentModel.PropertyChangedEventArgs("mySelectedProduct"))
        End Set
    End Property
    Private Property _myselectedcategory As myCategory
    Public Property mySelectedCategory As myCategory
        Get
            Return _myselectedcategory
        End Get
        Set(value As myCategory)
            _myselectedcategory = value
            OnPropertyChanged(mySelectedCategory, New System.ComponentModel.PropertyChangedEventArgs("mySelectedCategory"))
        End Set
    End Property
#Region "INotifyPropertyChanged Members"
    Private Sub OnPropertyChanged(ByVal sender As Object, ByVal e As PropertyChangedEventArgs)
        Dim propertyName As String = e.PropertyName
        Me.NotifyPropertyChanged(propertyName)
    End Sub
    Public Event PropertyChanged(sender As Object, e As System.ComponentModel.PropertyChangedEventArgs) Implements System.ComponentModel.INotifyPropertyChanged.PropertyChanged
    Private Sub NotifyPropertyChanged(ByVal propertyName As String)
        If Me.PropertyChangedEvent IsNot Nothing Then
            RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propertyName))
        End If
    End Sub
#End Region
End Class