Private Sub HandleCheck(ByVal sender As Object, ByVal e As RoutedEventArgs)
     Dim cb As CheckBox = CType(sender, CheckBox)
     If (cb.Name = "cb1") Then
         text1.Text = "Two state CheckBox checked."
     Else
         text2.Text = "Three state CheckBox checked."
     End If
 End Sub

 Private Sub HandleUnchecked(ByVal sender As Object, ByVal e As RoutedEventArgs)
     Dim cb As CheckBox = CType(sender, CheckBox)
     If (cb.Name = "cb1") Then
         text1.Text = "Two state CheckBox unchecked."
     Else
         text2.Text = "Three state CheckBox unchecked."
     End If
 End Sub

 Private Sub HandleThirdState(ByVal sender As Object, ByVal e As RoutedEventArgs)
     Dim cb As CheckBox = CType(sender, CheckBox)
     text2.Text = "Three state CheckBox indeterminate."
 End Sub

private void HandleCheck(object sender, RoutedEventArgs e)
{
    CheckBox cb = sender as CheckBox;
    if (cb.Name == "cb1")
        text1.Text = "Two state CheckBox checked.";
    else
        text2.Text = "Three state CheckBox checked.";
}

private void HandleUnchecked(object sender, RoutedEventArgs e)
{
    CheckBox cb = sender as CheckBox;
    if (cb.Name == "cb1")
        text1.Text = "Two state CheckBox unchecked.";
    else
        text2.Text = "Three state CheckBox unchecked.";
}

private void HandleThirdState(object sender, RoutedEventArgs e)
{
    CheckBox cb = sender as CheckBox;
    text2.Text = "Three state CheckBox indeterminate.";
}

<!-- two state CheckBox -->
<CheckBox x:Name="cb1" Content="Two State CheckBox" 
          Checked="HandleCheck" Unchecked="HandleUnchecked" Margin="5" />
<TextBlock x:Name="text1" Margin="5" />

<!-- three state CheckBox -->
<CheckBox x:Name="cb2" Content="Three State CheckBox" 
    IsThreeState="True" Checked="HandleCheck" 
    Indeterminate="HandleThirdState" Unchecked="HandleUnchecked" Margin="5" />
<TextBlock x:Name="text2" Margin="5" />