窗体分为两部分：客户区(Client area)和非客户区(Non-Client area)
WM_PAINT消息、OnPaint()方法、GetDC()API函数都是处理窗体客户区绘制的
而标题栏处于非客户区中，所以WM_PAINT消息、OnPaint()方法、GetDC()API函数都用不上
GetWindowDC()是获得整个窗体的画布句柄(Device Context翻译为：设备清单，我习惯称为画布句柄)，包括非客户区
GDI的绘制都离不开DC，因为操作系统必须知道你要在什么地方绘制图形
当其他窗体遮挡或者移开，系统都会重新绘制窗体。这时就会发出WM_PAINT和WM_NCPAINT消息通知窗体重绘界面。
收到WM_NCPAINT消息(非客户区绘制消息)说明非客户区正需要重新绘制
重载WndProc()方法(窗体消息处理过程)，处理WM_NCPAINT等消息即可....
“基本的步骤” 就是：截获WM_NCPAINT消息、得到窗体完整画布句柄、在完整画布上绘制图形。
WM_NCPAINT WM_NCCALCSIZE WM_NCACTIVATE WM_NCHITTEST
建议楼主搜索关键词：“C# WM_NCPAINT WndProc Graphics”得到更多的参考资料
贴
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
[DllImport ( "User32.dll ")]
private static extern IntPtr GetWindowDC(IntPtr hwnd);
[DllImport ( "User32.dll ")]
private static extern int ReleaseDC(IntPtr hwnd, IntPtr hdc);
[DllImport ( "Kernel32.dll ")]
private static extern int GetLastError();
//标题栏按钮的矩形区域。
Rectangle m_rect = new Rectangle(205, 6, 20, 20);
protected override void WndProc(ref Message m)
{
base.WndProc(ref m);
switch(m.Msg)
{
case 0x86://WM_NCACTIVATE
goto case 0x85;
case 0x85://WM_NCPAINT
{
IntPtr hDC = GetWindowDC(m.HWnd);
//把DC转换为.NET的Graphics就可以很方便地使用Framework提供的绘图功能了
Graphics gs = Graphics.FromHdc(hDC);
gs.FillRectangle(new LinearGradientBrush(m_rect, Color.Pink, Color.Purple, LinearGradientMode.BackwardDiagonal), m_rect);
StringFormat strFmt = new StringFormat();
strFmt.Alignment = StringAlignment.Center;
strFmt.LineAlignment = StringAlignment.Center;
gs.DrawString( "√ ", this.Font, Brushes.BlanchedAlmond, m_rect, strFmt);
gs.Dispose();
//释放GDI资源
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;
}
}
}
//在窗口大小改变时及时更新按钮的区域。
private void Form1_SizeChanged(object sender, System.EventArgs e)
{
m_rect.X = this.Bounds.Width - 95;
m_rect.Y = 6;
m_rect.Width = m_rect.Height = 20;
}
}
}