我认为您只能使用CustomRenderer来实现:

I think you can only achieve this with a CustomRenderer:

iOS:

protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
{
       base.OnElementPropertyChanged(sender, e);

       Control.Layer.BorderColor = UIColor.Red.CGColor;
       Control.Layer.BorderWidth = 1;
}

在Android上，我认为没有CustomRender是不可能的(实际上，如果是...我不知道〜对不起):

On Android, I think it's not possible without a CustomRender (Actually, if it is... I don't know how ~ Sorry):

使用CustomRenderer就像这样:

Using the CustomRenderer would be something like this:

    [assembly: ExportRenderer(typeof(Entry), typeof(SuperEntryRenderer))]
    namespace Bla{
    public class SuperEntryRenderer : EntryRenderer
        {
            protected override void OnElementChanged(ElementChangedEventArgs<Entry> e)
            {
                base.OnElementChanged(e);
                if (e.OldElement == null)
                {
                    var nativeEditText = (global::Android.Widget.EditText)Control;
                    var shape = new ShapeDrawable(new Android.Graphics.Drawables.Shapes.RectShape());
                    shape.Paint.Color = Xamarin.Forms.Color.Red.ToAndroid();
                    shape.Paint.SetStyle(Paint.Style.Stroke);
                    nativeEditText.Background = shape;
                }
            }
        }