C # Draw form customer non-client area to use WM_PAINT and Wm_ncpaint

The form is divided into two parts: the client area and the non-client zone (non-client area)
The WM_PAINT message, OnPaint () method, GetDC () API functions are all processed by the client area of the form.

The title bar is in the non-client area, so the WM_PAINT message, OnPaint () method, GetDC () API functions are not used

GETWINDOWDC () is the canvas handle that gets the entire form (device context translates as: equipment manifest, which I used to call the canvas handle), including non-client areas

GDI can be drawn without a DC, because the operating system must know where you want to draw the graphics

The system redraws the form when other forms are obscured or moved away. The WM_PAINT and wm_ncpaint messages are emitted to notify the form to redraw the interface.

Received Wm_ncpaint message (non-client area draw message) stating that the non-client area is in need of redrawing

Overloaded WndProc () method (Form message processing), handling messages such as Wm_ncpaint ....

The basic step is to intercept the wm_ncpaint message, get the form's full canvas handle, and draw the graphic on the full canvas.
Wm_ncpaint wm_nccalcsize wm_ncactivate wm_nchittest
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Tile

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[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 ();   The rectangular area of the title bar button.   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);   Converting DC to. NET graphics makes it easy to use the graphics GS = GRAPHICS.FROMHDC (HDC) provided by the framework. 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 ();   Frees GDI Resources 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;   }}}//The area of the button is updated in time when the window size changes. 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; }   }   }