GetPixel()函数出错

getpixel函数.....出现错误虽然使用了图像比较所需的所有名称空间.....我在visual studio 2013中制作Windows应用....我是否需要在任何地方定义getpixel()函数…

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Navigation;
using Microsoft.Phone.Controls;
using Microsoft.Phone.Shell;
using picture.Resources;
using System.Windows.Media.Imaging;
using System.Windows.Media;
using Microsoft.Expression.Drawing;
namespace picture {
    public partial class MainPage: PhoneApplicationPage {
        // Constructor
        public MainPage() {
            InitializeComponent();
            BitmapImage image1 = new BitmapImage();
            image1.SetSource(Application.GetResourceStream(new Uri(@
            "Assets/WP_20150925_09_20_03_Pro (2).jpg", UriKind.Relative)).Stream);
            BitmapImage image2 = new BitmapImage();
            image2.SetSource(Application.GetResourceStream(new Uri(@
            "Assets/WP_20150925_09_20_03_Pro.jpg", UriKind.Relative)).Stream);
            // Sample code to localize the ApplicationBar
            //BuildLocalizedApplicationBar();
        }

        private double get_Percentage_Method(WriteableBitmap wb1, WriteableBitmap wb2) {
            var img1 = wb1;
            var img2 = wb2;
            // Validation of image size code  
            int width1 = img1.PixelWidth;
            int width2 = img2.PixelWidth;
            int height1 = img1.PixelHeight;
            int height2 = img2.PixelHeight;
            if ((width1 != width2) || (height1 != height2)) {
                MessageBox.Show("Error: Images dimensions mismatch");
            }
            long diff = 0;
            for (int i = 0; i < width1; i++) {
                for (int j = 0; j < height1; j++) {
                    // get same position pixel of both WirtableBitmap Image object  
                    Color color1 = wb1.GetPixel(i, j);
                    Color color2 = wb2.GetPixel(i, j);
                    // converting each value iGetPixelnto Integer  
                    int r1 = (int) color1.R;
                    int g1 = (int) color1.G;
                    int b1 = (int) color1.B;
                    int r2 = (int) color2.R;
                    int g2 = (int) color1.G;
                    int b2 = (int) color1.B;
                    // calculating difference between RGB color value and adding it in Diff variable.  
                    diff += Math.Abs(r1 - r2);
                    diff += Math.Abs(g1 - g2);
                    diff += Math.Abs(b1 - b2);
                }
            }
            // Converting difference into percentage and then retun it.  
            double n = width1 * height1; //* 3;  
            double p = diff / n / 255.0;
            MessageBox.Show("diff percent: " + (p * 100.0));
            return (p * 100.0);
        }
    }
}