C# WinForm 在視窗標題列新增自訂按鈕

在視窗標題列加入按鈕並非新技術（VC++ 時代即可實現），但有朋友詢問 C# WinForm 如何做，順手以 C# 實作一個範例。核心原理相同：覆寫視窗程序 (WndProc) 處理非用戶區訊息 (WM_NCxxxx)，但過程中學到兩個 C# 技巧：

1. C# 覆寫 WndProc 即可攔截視窗訊息，無需呼叫 SetWindowLong API。
2. 透過 Graphics.FromHdc() 將 Windows API 的 HDC 轉為 System.Drawing.Graphics，即可使用 GDI+ 繪圖功能，避開繁瑣的 GDI API 呼叫。

完整程式碼示範：建立自訂標題列按鈕區域、處理 WM_NCPAINT 繪製漸層按鈕、處理 WM_NCLBUTTONDOWN 偵測點擊、處理 SizeChanged 更新按鈕位置。

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

namespace WindowsApplication2
{
    public class Form1 : System.Windows.Forms.Form
    {
        private Rectangle m_rect = new Rectangle(205, 4, 16, 16);

        [DllImport("User32.dll")]
        private static extern IntPtr GetWindowDC(IntPtr hwnd);
        [DllImport("User32.dll")]
        private static extern int ReleaseDC(IntPtr hwnd, IntPtr hdc);

        protected override void WndProc(ref Message m)
        {
            base.WndProc(ref m);
            switch (m.Msg)
            {
                case 0x85: // WM_NCPAINT
                case 0x86: // WM_NCACTIVATE
                    IntPtr hDC = GetWindowDC(m.HWnd);
                    Graphics gs = Graphics.FromHdc(hDC);
                    gs.FillRectangle(new LinearGradientBrush(m_rect, Color.Pink, Color.Purple, LinearGradientMode.BackwardDiagonal), m_rect);
                    StringFormat strFmt = new StringFormat { Alignment = StringAlignment.Center, LineAlignment = StringAlignment.Center };
                    gs.DrawString("✓", this.Font, Brushes.BlanchedAlmond, m_rect, strFmt);
                    gs.Dispose();
                    ReleaseDC(m.HWnd, hDC);
                    break;
                case 0xA1: // WM_NCLBUTTONDOWN
                    Point mousePoint = new Point((int)m.LParam);
                    mousePoint.Offset(-this.Left, -this.Top);
                    if (m_rect.Contains(mousePoint)) MessageBox.Show("hello");
                    break;
            }
        }

        private void Form1_SizeChanged(object sender, EventArgs e)
        {
            m_rect.X = this.Bounds.Width - 95;
            m_rect.Y = 4;
            m_rect.Width = m_rect.Height = 16;
        }

        [STAThread]
        static void Main() { Application.Run(new Form1()); }
    }
}