令人困惑的WPF繪製行爲

我目前正試圖堅持一個畫布到位圖，我遇到了一些非常奇怪的行爲。以下三種情況的源代碼出現在帖子的末尾。令人困惑的WPF繪製行爲

情況1：如預期的那樣，輸出文件（test.png）中出現紅色矩形。

情況2：輸出文件中不顯示紅色矩形。

情況3：輸出文件中不顯示紅色矩形。

似乎將矩形添加到畫布（即使該畫布從未用於將矩形渲染到磁盤）是必要的。似乎按鈕點擊必須啓動繪圖 - 它不能出現在Window構造函數中。這些都對我沒有意義，我認爲我誤解了一些東西。

此外，我很抱歉代碼格式不正確。我摔了20分鐘，但現在我放棄了。

由於提前，

- 布雷克Fresen

用於所有3例XAML：

<Window x:Class="ScanOutlineCreator.Window1" 
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    Title="Window1" Height="250" Width="250"> 
     <Grid> 
      <Grid.RowDefinitions> 
       <RowDefinition Height="20" /> 
       <RowDefinition /> 
      </Grid.RowDefinitions> 
      <Button x:Name="btnGo" Grid.Row="0"> 
       <TextBlock Text="Go" /> 
      </Button> 
      <Canvas Grid.Row="1" x:Name="canvas" Width="200" Height="500"></Canvas> 
     </Grid> 
</Window>

情況1：

using System.IO; 
using System.Windows; 
using System.Windows.Media; 
using System.Windows.Media.Imaging; 
using System.Windows.Shapes; 
using Common.Drawing; 
namespace ScanOutlineCreator 
{ 
     public partial class Window1 
     { 
       static Rectangle r = new Rectangle { Width = 100, Height = 100, Fill = Brushes.Red }; 
       public Window1() 
       { 
         InitializeComponent(); 
         btnGo.Click += new RoutedEventHandler(btnGo_Click); 
         canvas.Children.Add(r); 
       } 

       static void btnGo_Click(object sender, RoutedEventArgs e) 
       { 
         using (Stream stm = File.Create("test.png")) 
         { 
           RenderTargetBitmap rtb = new RenderTargetBitmap(100, 100, 96, 96, PixelFormats.Pbgra32); 
           rtb.Render(r); 
           Util.RenderTargetBitmapToStream(rtb, stm); 
         } 
       } 
     } 
}

情況2：

using System.IO; 
using System.Windows; 
using System.Windows.Media; 
using System.Windows.Media.Imaging; 
using System.Windows.Shapes; 
using Common.Drawing; 

namespace ScanOutlineCreator  
{  
    public partial class Window1 
    { 
     static Rectangle r = new Rectangle { Width = 100, Height = 100, Fill = Brushes.Red }; 
     public Window1() 
     { 
      InitializeComponent(); 
      btnGo.Click += new RoutedEventHandler(btnGo_Click); 
     } 

     static void btnGo_Click(object sender, RoutedEventArgs e) 
     { 
      using (Stream stm = File.Create("test.png")) 
      { 
       RenderTargetBitmap rtb = new RenderTargetBitmap(100, 100, 96, 96, PixelFormats.Pbgra32); 
       rtb.Render(r); 
       Util.RenderTargetBitmapToStream(rtb, stm); 
      } 
     } 
    } 
}

案例3：

using System.IO; 
using System.Windows.Media; 
using System.Windows.Media.Imaging; 
using System.Windows.Shapes; 
using Common.Drawing; 
namespace ScanOutlineCreator 
{ 
    public partial class Window1 
    { 
     static Rectangle r = new Rectangle { Width = 100, Height = 100, Fill = Brushes.Red }; 
     public Window1() 
     { 
      InitializeComponent(); 
      canvas.Children.Add(r); 
      using (Stream stm = File.Create("test.png")) 
      { 
       RenderTargetBitmap rtb = new RenderTargetBitmap(100, 100, 96, 96, PixelFormats.Pbgra32); 
       rtb.Render(r); 
       Util.RenderTargetBitmapToStream(rtb, stm); 
      } 
     } 
    } 
}

來源

2010-02-05 Breck Fresen