I could find solution to this problem.
step 1
Need to set lostFocus event to txt_Value1.
<TextBox x:Name="txt_Value1" Text="{Binding Path=Value1}" LostFocus="txt_Value1_LostFocus" />
Steep 2
In C# code,
private void txt_Value1_LostFocus(object sender, RoutedEventArgs e)
{
string textValue= ((TextBox)sender).Text;
}
Thanks.