You have to use FocusManager
bool b = FocusManager.GetFocusedElement() == myTextbox;
Question
How to know whether a control such as TextBox has the input focus in a Windows Phone Silverlight app?
Solution
You have to use FocusManager
bool b = FocusManager.GetFocusedElement() == myTextbox;
OTHER TIPS
There are events like GotFocus and LostFocus for controls.
If you subscribe to these events they automatically get called when your input receives or looses focus
you can use those events for your purpose.
XAML Declaration
<TextBox Name="myTextbox" GotFocus="myTextbox_GotFocus" />
and inside the cs
private void myTextbox_GotFocus(object sender, RoutedEventArgs e)
{
}
private void ContentPanel_LostFocus(object sender, RoutedEventArgs e)
{
}