Это будет работать: XAML:
<StackPanel>
<ComboBox Name="cbo1" Margin="3">
<ComboBoxItem>
<TextBlock>First Item</TextBlock>
</ComboBoxItem>
<ComboBoxItem>
<TextBlock>Second Item</TextBlock>
</ComboBoxItem>
</ComboBox>
<TextBox Name="txtBox1" Margin="3" TextChanged="textBox1_textchanged"/>
</StackPanel>
Код:
Private Sub textBox1_textchanged(ByVal sender As Object, ByVal e As TextChangedEventArgs)
If cbo1.SelectedIndex > -1 Then
Dim i As ComboBoxItem = cbo1.SelectedItem
i.Content.text = sender.text
End If
End Sub