WPF ComboBox LostFocus not working as expected, What do to?

Hi,

I put a ComboBox on my WPF forms named cboTest with a TextBox named txtTest

and what I want is, when the cbox LostFocus, Get the value selected and make over check

so I create a LostFocus Event like this

Private Sub cboTest_LostFocus(ByVal sender As Object, ByVal e As System.Windows.RoutedEventArgs) Handles cboTest.LostFocus
     'My code
     If(cboTesT.Text = "1") Then
         txtTest.text="Your boss 1"
     Else
         txtTest.Text ="Your boss unknow"
     End If

End Sub

At this stage everything work as expected

But I replace my code with this on, It work strangely

Private Sub cboTest_LostFocus(ByVal sender As Object, ByVal e As System.Windows.RoutedEventArgs) Handles cboTest.LostFocus
     'My code
     MessageBox.Show(cboTest.Text)

End Sub

when I use Keyboarb to select the combobox and change its value, it work perfectly.

But When I use the mouse to select it, the MessageBox shows continuously especially when I just click on the ComboBox, (it show two times, sure after click OK two times)  and three times when I succeed to finally change the value, again when I click on the TextBox and When I want to close the WPF Forms.

Please, someone can help me fix this issue, so that the MessgeBox only show after the user have selected a value (using his mouse)????

PS : I've already see all the related post, but they just do nothing about showing the value throw MessageBox

Thanks a lot in advance for your Help