WM_PANINT 与 WM_ERASEBKGND 的区别
当WM_PAINT不是由InvalidateRect产生时,即由最大化，最小化等产生时，或者移动产生（移动有时只会产生WM_ERASEBKGND消息)系统先发送WM_ERASEBKGND消息,再发送WM_PAINT消息.
如果处理WM_ERASEBKGND消息时返回FALSE，BeginPaint标记pt.fErase 为TRUE,如果处理WM_ERASEBKGND时返回TRUE,BeginPaint标记pt.fErase为FALSE.
当WM_PAINT由InvalidateRect产生时,先发送WM_PAINT消息（异步),如果InvalidateRect的bErase为TRUE,BeginPaint检查到更新区域需要删除背景,向窗口发送一个WM_ERASEBKGND消息,如果处理WM_ERASEBKGND消息时返回FALSE，BeginPaint标记pt.fErase 为TRUE,如果处理WM_ERASEBKGND时返回TRUE,BeginPaint标记pt.fErase为FALSE. 如果pt.fErase标记为TRUE,指示应用程序应该处理背景,但是应用程序不一定需要处理,pt.fErase只是作为一个标记.
补充：DefWindowProc(hWnd, message, wParam, lParam)处理WM_ERASEBKGND消息时默认用下面的画刷清除背景
wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
WM_ERASEBKGND返回TRUE和返回FALSE是一个规范，一般情况下没有什么区别，但是如果什么时候用到了，会根据函数返回值判断后续处理。因此最好按照要求返回数据.
最后再提供一些测试代码，让大家更好的理解上面的内容。
// WM_ERASEBKGNDTest.cpp : Defines the entry point for the application. // #include “stdafx.h” #include “WM_ERASEBKGNDTest.h” #define MAX_LOADSTRING 100 // Global Variables: HINSTANCE hInst; // current instance TCHAR szTitle[MAX_LOADSTRING]; // The title bar text TCHAR szWindowClass[MAX_LOADSTRING]; // the main window class name // Forward declarations of functions included in this code module: ATOM MyRegisterClass(HINSTANCE hInstance); BOOL InitInstance(HINSTANCE, int); LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM); INT_PTR CALLBACK About(HWND, UINT, WPARAM, LPARAM); int APIENTRY _tWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPTSTR lpCmdLine, int nCmdShow) { UNREFERENCED_PARAMETER(hPrevInstance); UNREFERENCED_PARAMETER(lpCmdLine); // TODO: Place code here. MSG msg; HACCEL hAccelTable; // Initialize global strings LoadString(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING); LoadString(hInstance, IDC_WM_ERASEBKGNDTEST, szWindowClass, MAX_LOADSTRING); MyRegisterClass(hInstance); // Perform application initialization: if (!InitInstance (hInstance, nCmdShow)) { return FALSE; } hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_WM_ERASEBKGNDTEST)); // Main message loop: while (GetMessage(&msg, NULL, 0, 0)) { if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg)) { TranslateMessage(&msg); DispatchMessage(&msg); } } return (int) msg.wParam; } // // FUNCTION: MyRegisterClass() // // PURPOSE: Registers the window class. // // COMMENTS: // // This function and its usage are only necessary if you want this code // to be compatible with Win32 systems prior to the ‘RegisterClassEx’ // function that was added to Windows 95. It is important to call this function // so that the application will get ‘well formed’ small icons associated // with it. // ATOM MyRegisterClass(HINSTANCE hInstance) { WNDCLASSEX wcex; wcex.cbSize = sizeof(WNDCLASSEX); wcex.style = CS_HREDRAW | CS_VREDRAW; wcex.lpfnWndProc = WndProc; wcex.cbClsExtra = 0; wcex.cbWndExtra = 0; wcex.hInstance = hInstance; wcex.hIcon = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_WM_ERASEBKGNDTEST)); wcex.hCursor = LoadCursor(NULL, IDC_ARROW); wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW+1); // wcex.hbrBackground = NULL; wcex.lpszMenuName = MAKEINTRESOURCE(IDC_WM_ERASEBKGNDTEST); wcex.lpszClassName = szWindowClass; wcex.hIconSm = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL)); return RegisterClassEx(&wcex); } // // FUNCTION: InitInstance(HINSTANCE, int) // // PURPOSE: Saves instance handle and creates main window // // COMMENTS: // // In this function, we save the instance handle in a global variable and // create and display the main program window. // BOOL InitInstance(HINSTANCE hInstance, int nCmdShow) { HWND hWnd; hInst = hInstance; // Store instance handle in our global variable hWnd = CreateWindow(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL); if (!hWnd) { return FALSE; } ShowWindow(hWnd, nCmdShow); UpdateWindow(hWnd); return TRUE; } // // FUNCTION: WndProc(HWND, UINT, WPARAM, LPARAM) // // PURPOSE: Processes messages for the main window. // // WM_COMMAND - process the application menu // WM_PAINT - Paint the main window // WM_DESTROY - post a quit message and return // // LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam) { int wmId, wmEvent; PAINTSTRUCT ps; HDC hdc; switch (message) { case WM_COMMAND: wmId = LOWORD(wParam); wmEvent = HIWORD(wParam); // Parse the menu selections: switch (wmId) { case IDM_ABOUT: DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About); break; case IDM_EXIT: DestroyWindow(hWnd); break; default: return DefWindowProc(hWnd, message, wParam, lParam); } break; case WM_LBUTTONDOWN: RECT rect; GetClientRect(hWnd, &rect); InvalidateRect(hWnd, &rect, TRUE); MessageBeep(2); break; //case WM_ERASEBKGND: // OutputDebugString(_T(“Enter WM_ERASEBKGND/n”)); // return 1; // break; case WM_PAINT: OutputDebugString(_T(“Enter WM_PAINT/n”)); hdc = BeginPaint(hWnd, &ps); if (ps.fErase == TRUE) OutputDebugString(_T(“ps.fErase == TRUE/n/n”)); // TODO: Add any drawing code here… EndPaint(hWnd, &ps); break; case WM_DESTROY: PostQuitMessage(0); break; default: if (WM_ERASEBKGND == message) OutputDebugString(_T(“Enter WM_ERASEBKGND/n”)); return DefWindowProc(hWnd, message, wParam, lParam); } // DefWindowProc(hWnd, message, wParam, lParam); return 0; } // Message handler for about box. INT_PTR CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam) { UNREFERENCED_PARAMETER(lParam); switch (message) { case WM_INITDIALOG: return (INT_PTR)TRUE; case WM_COMMAND: if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL) { EndDialog(hDlg, LOWORD(wParam)); return (INT_PTR)TRUE; } break; } return (INT_PTR)FALSE; } WM_PAINT和WM_ERASEBKGND补充：
在做窗口界面优化的时候，正确的处理WM_PAINT和WM_ERASEBKGND是很重要的，不管是先产生WM_ERASEBKGND还是WM_PAINT.
在处理OnPaint时采用如下方法：
PAINTSTRUCT PS;
HDC hdc = BeginPaint(&PS);
做相关GDI操作
EndPaint(&PS);
处理OnEraseBkgnd时采用如下方法：
return 1;
第一种情况（WM_PAINT消息产生在前，WM_ERASEBKGND消息产生在后）
BeginPaint会检测是否删除背景，如果是，发送WM_ERASEBKGND(当WM_PAINT手动（由InvalidateRect等函数）产生时）
第二种情况（WM_PAINT消息产生在后，WM_ERASEBKGND消息产生在前）
在处理WM_PAINT之前已经处理了WM_ERASEBKGND
如果处理WM_ERASEBKGND消息时返回FALSE，BeginPaint标记pt.fErase 为TRUE,如果处理WM_ERASEBKGND时返回TRUE,BeginPaint标记pt.fErase为FALSE.WM_ERASEBKGND返回TRUE和返回FALSE是一个规范，一般情况下没有什么区别，但是如果什么时候用到了，会根据函数返回值判断后续处理。因此最好按照要求返回数据.