IInputFilter is already part of the Mono for Android framework
It is part of the Android.Text namespace
http://androidapi.xamarin.com/?link=T:Android.Text.IInputFilter
class RegExInputFilter : Java.Lang.Object, IInputFilter
{
public Java.Lang.ICharSequence FilterFormatted(Java.Lang.ICharSequence source, int start, int end, ISpanned dest, int dstart, int dend)
{
//filter
}
}