Windows phone C++

I'm writing simple application for WP 8.1 using c++/cx. My problem starts when i try to do something in some kind of event. For example if i create simple button event "tapped" and i want to do something inside for example change the color of the button background, it doesn't execute in the correct time. I mean that for the code below it will first execute Somefunction() and then change the color of the button.Same happens for example when i try to show message box using message dialog and ShowAsync function.

but->Background = ref new SolidColorBrush(Windows::UI::Colors::Red);
Somefunciton();