c# - 临时变量对删除 GroupBox 背景的影响

Question

上下文:

考虑绘制一个带有渐变的 GroupBox 作为其背景的一部分。

示例:

---

让我们执行以下操作:

• 创建一个继承GroupBox 的类。
  • 将其 FlatStyle property 设置为 FlatStyle.System。
  • override 它是 WndProc 方法。
  • 处理 WM_ERASEBKGND 消息，我们在其中绘制渐变。
  • 处理 WM_PRINTCLIENT 消息，我们在其中调用 DefWndProc 并return。
    _{(将需要稍后。)}

• 添加一个 Label 作为子 Control。
  _{( Label 的背景必须是透明的才能看到它后面的渐变 Text。}

  • 创建一个继承Label 的类。
  • 覆盖 WndProc 方法。
  • “模拟透明度”通过调用 DrawThemeParentBackground 函数在 Label 上绘制 GroupBox 的背景的图形。

问题:

Depending on whether a temporary variable is used to hold the Graphics object, the end result varies, depicted with the code sample and image below:

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace MCVE
{
    class GroupBox : System.Windows.Forms.GroupBox
    {
        const int WM_ERASEBKGND = 0x14;
        const int WM_PRINTCLIENT = 0x318;

        protected override void WndProc(ref Message m)
        {
            switch (m.Msg)
            {
                case WM_ERASEBKGND:
                    base.WndProc(ref m);
                    using (var g = Graphics.FromHdc(m.WParam))//CASE 1
                    //using (var e = new PaintEventArgs(Graphics.FromHdc(m.WParam), ClientRectangle))//CASE 2
                    {
                        var e = new PaintEventArgs(g, ClientRectangle);//CASE 1
                        var r = new Rectangle(2, 12, Width - 4, Height - 2);
                        using (var b = new LinearGradientBrush(r, BackColor, SystemColors.Window, LinearGradientMode.Vertical))
                        {
                            e.Graphics.FillRectangle(b, r);//Draw the gradient.
                        }
                    }
                    m.Result = new IntPtr(1);//Signal that no further drawing of the background is necessary by WM_PAINT.
                    return;
                case WM_PRINTCLIENT:
                    DefWndProc(ref m);//Bypass GroupBox's internal handling so that actual painting is handled by Windows.
                    return;
            }
            base.WndProc(ref m);//Default processing of the rest of the messages.
        }
    };

    class Label : System.Windows.Forms.Label
    {
        const int WM_ERASEBKGND = 0x14;
        const int WM_PAINT = 0xF;

        [DllImport("user32.dll")] static extern IntPtr BeginPaint(IntPtr hWnd, out PAINTSTRUCT lpPaint);
        [DllImport("user32.dll")] static extern IntPtr EndPaint(IntPtr hWnd, ref PAINTSTRUCT lpPaint);
        //Ask Windows to send a message to the parent to draw it's background in the current device context.
        [DllImport("uxtheme.dll")] extern static int DrawThemeParentBackground(IntPtr hWnd, IntPtr hdc, ref Rectangle pRect);

        [StructLayout(LayoutKind.Sequential)]
        struct PAINTSTRUCT
        {
            public IntPtr hdc;
            public bool fErase;
            public Rectangle rcPaint;
            public bool fRestore;
            public bool fIncUpdate;
            [MarshalAs(UnmanagedType.ByValArray, SizeConst = 32)] public byte[] rgcReserved;
        };

        protected override void WndProc(ref Message m)
        {
            switch (m.Msg)
            {
                case WM_ERASEBKGND:
                    var r = ClientRectangle;
                    DrawThemeParentBackground(Handle, m.WParam, ref r);
                    m.Result = new IntPtr(1);//Signal that no further drawing of the background is necessary by WM_PAINT.
                    return;
                case WM_PAINT:
                    PAINTSTRUCT ps;
                    var hdc = BeginPaint(Handle, out ps);
                    EndPaint(Handle, ref ps);//Don't paint any text so that the gradient remains visible.
                    m.Result = IntPtr.Zero;
                    return;
            }
            base.WndProc(ref m);//Default processing of the rest of the messages.
        }
    };

    static class Program
    {
        [STAThread] static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);

            var form = new Form() { BackColor = SystemColors.Highlight };
            var groupbox = new GroupBox() { Anchor = (AnchorStyles)15, FlatStyle = FlatStyle.System, Location = new Point(10, 10), Text = "groupBox1" };

            form.Controls.Add(groupbox);
            groupbox.Controls.Add(new Label() { FlatStyle = FlatStyle.System, Location = new Point(50, 50) });

            Application.Run(form);
        }
    };
}

运行上面的 MCVE (案例 1) 会产生示例图像中所示的预期输出。

在注释掉标记为 CASE 1 的行并取消注释标记为 CASE 2 的行时，会产生以下不需要的输出:

问题:

为什么删除临时变量会产生如此不同的输出？

Answer 1

为了完整起见，将此作为答案发布。

• 正如 Ivan Stoev 指出的那样，非拥有 PaintEventArgs 不调用 Graphics Dispose 对象。
• 这有可见的副作用，因为 DC 在 WM_PRINTCLIENT Message 中被 Windows 重用，即在 WndProc 旁边发送。

在 Graphics object 上手动调用 Dispose 可以确认这一点。

using (var g = Graphics.FromHdc(m.WParam))
{
    using (var e = new PaintEventArgs(g, ClientRectangle))
    {
        var r = new Rectangle(2, 12, Width - 4, Height - 2);
        using (var b = new LinearGradientBrush(r, BackColor, SystemColors.Window, LinearGradientMode.Vertical))
        {
            e.Graphics.FillRectangle(b, r);//Draw the gradient.
        }
    }
}