c# - Windows Phone 8.1 Web Browser launcher LaunchUriAsync Not working -

i want open web browser , navigate predefined website button click. find has windows phone 8 , silverlight microsoft.phone.tasks not work wp8.1. perhaps im searching wrong phrases cannot figure out proper way task.

i should have clarified earlier, tried using this

var uri = new uri(uritolaunch); 

found here launcher.launchuriasync vs gives me error message using var. implmented exaclt msdn said, there still errors.

it has been simplified, have write

launcher.launchuriasync(new uri(url));  

the namespace should windows.system