C# 在应用程序启动时自动保存和检索背景图像

在我的WindowsPhone7应用程序中，我创建了一个按钮，可以根据计数的数量改变回退。默认情况下，我有一个背景图像。我需要的是，如果用户更改了背景（使用上面的按钮），我希望该图像应该通过独立存储保存或记住。然后，当应用程序再次启动时，该特定图像（用户上次选择的）应该是我的应用程序的背景图像。现在的问题是，我无法检索图像（但我能够保存），并且不知道如何在应用程序启动时自动将该图像作为背景图像（不想使用任何按钮保存或检索，必须是自动的）。有人能帮我吗？非常感谢你的辛勤工作

private void button1_Click(object sender, RoutedEventArgs e)
    {
        string imguri = "";

        click_count = click_count % 4;
        switch (click_count)
        {
            case 0: imguri = "Images/image4.png"; break;
            case 1: imguri = "Images/image3.jpg"; break;
            case 2: imguri = "Images/image2.png"; break;
            case 3: imguri = "Images/image1.jpg"; break;

        }
        click_count++;

        BitmapImage bitmapImage = new BitmapImage(new Uri(imguri, UriKind.Relative));
        ImageBrush imageBrush = new ImageBrush();
        imageBrush.ImageSource = bitmapImage;
        var app = Application.Current as App;
        this.LayoutRoot.Background = imageBrush;
        app.appbrush = imageBrush;
        app.backchanged = true;

        string filename = "Images/app.jpg";
        StreamResourceInfo sr = Application.GetResourceStream(new Uri(filename, UriKind.Relative));

        bitmapImage.SetSource(sr.Stream);
        WriteableBitmap wb = new WriteableBitmap(bitmapImage);

        using (IsolatedStorageFile myIsolatedStorage = IsolatedStorageFile.GetUserStoreForApplication())
        {
            myIsolatedStorage.CreateDirectory("Images");

            if (myIsolatedStorage.FileExists(filename))
            {
                myIsolatedStorage.DeleteFile(filename);
            }
            IsolatedStorageFileStream filestream = myIsolatedStorage.CreateFile(filename);
            Extensions.SaveJpeg(wb, filestream, wb.PixelWidth, wb.PixelHeight, 0, 100);
            filestream.Close();
        }
    }

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请尝试以下代码加载图像并将其指定给背景：

IsolatedStorageFile Store = IsolatedStorageFile.GetUserStoreForApplication();

if (Store.FileExists("Images/app.jpg"))
{
    try
    {
        using (IsolatedStorageFileStream Stream = new IsolatedStorageFileStream(Path, FileMode.Open, FileAccess.Read, Store))
        {
            if (Stream.Length > 0)
            {
                BitmapImage Image = new BitmapImage();
                Image.SetSource(Stream);
                this.LayoutRoot.Background = Image;
            }
        }
    }
    catch (Exception)
    {            
    }
}

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当应用程序启动时，在哪里放置代码以自动加载该图像？