Silverlike

Colin Eberhardt created a very useful Clipping behavior class which helps you do clipping easily. The behavior can detect the ActualHeight and ActualWidth at run time and create a RectangleGeometry() to Clip your Grid.

using System;
using System.Windows;

namespace Namespace
{

    public class Clip
    {
        public static bool GetToBounds(DependencyObject depObj)
        {
            return (bool)depObj.GetValue(ToBoundsProperty);
        }

        public static void SetToBounds(DependencyObject depObj, bool clipToBounds)
        {
            depObj.SetValue(ToBoundsProperty, clipToBounds);
        }

        /// <summary>
        /// Identifies the ToBounds Dependency Property.
        /// <summary>
        public static readonly DependencyProperty ToBoundsProperty =
        DependencyProperty.RegisterAttached("ToBounds", typeof(bool),
        typeof(Clip), new PropertyMetadata(false, OnToBoundsPropertyChanged));

        private static void OnToBoundsPropertyChanged(DependencyObject d,
        DependencyPropertyChangedEventArgs e)
        {
        FrameworkElement fe = d as FrameworkElement;
        if (fe != null)
        {
            ClipToBounds(fe);

            // whenever the element which this property is attached to is loaded
            // or re-sizes, we need to update its clipping geometry
            fe.Loaded += new RoutedEventHandler(fe_Loaded);
            fe.SizeChanged += new SizeChangedEventHandler(fe_SizeChanged);

        }
        }

        /// <summary>
        /// Creates a rectangular clipping geometry which matches the geometry of the
        /// passed element
        /// </summary>
        private static void ClipToBounds(FrameworkElement fe)
        {
        if (GetToBounds(fe))
        {
            fe.Clip = new RectangleGeometry()
            {
            Rect = new Rect(0, 0, fe.ActualWidth, fe.ActualHeight)
            };
        }
        else
        {
            fe.Clip = null;
        }
        }

        static void fe_SizeChanged(object sender, SizeChangedEventArgs e)
        {
            ClipToBounds(sender as FrameworkElement);
        }

        static void fe_Loaded(object sender, RoutedEventArgs e)
        {
            ClipToBounds(sender as FrameworkElement);
        }
    }
}

<UserControl x:Class="Namespace.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:this="clr-namespace:Namespace;assembly=Namespace"
    >
    <Grid x:Name="LayoutRoot"  this:Clip.ToBounds="True" >

    </Grid>
</UserControl>