This is a very common scenario most developers stumble across - they are building an app for Windows Phone and have to target all 3 screen resolutions available with Windows Phone 8.
The 3 screen resolutions for your reference are:
WVGA - 480x800 or 800x480 (15:9 ratio)
WXGA - 768x1280 or 1280x768 (15:9 ratio)
720p - 720x1280 or 1280x720 (16:9 ratio)
There may be many scenarios in which you would want to know the screen resolution of the device your app is running on, a popular one being that you want to show higher quality images for higher resolution devices. You can accomplish this very easily by checking for the ScaleFactor as follows:
if(App.Current.Host.Content.ScaleFactor == 100)
{
// WVGA
}
else if (App.Current.Host.Content.ScaleFactor == 160)
{
// WXGA
}
else if (App.Current.Host.Content.ScaleFactor == 150)
{
// 720p
}
More at http://paraswadehra.blogspot.com/2012/12/how-to-determine-screen-resolution-on.html
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Paras Wadehra
Twitter: @ParasWadehra
FB Group: http://www.facebook.com/WindowsPhoneDeveloper