win32鼠标事件，没单击但是自动发出WM_LBUTTONDOWN，求解释

win32鼠标事件，没有单击但是自动发出WM_LBUTTONDOWN，求解释
我在做鼠标事件的测试时，没有单击窗口，但是用于测试的控制台却显示有了一个WM_LBUTTONDOWN信息，
当我单击后却又多出了一个WM_LBUTTONDOWN信息，求大神解释原因，小弟不胜感谢。
代码如下：

// WinMouse.cpp : Defines the entry point for the application.

//


#include <windows.h>

#include <stdio.h>


HINSTANCE g_hInst = NULL;

HANDLE g_hStdOut = NULL;

int g_nXpos = 0;

int g_nYpos = 0;

int g_nXupperLeftCorner = 0;

int g_nYupperLeftCorner = 0;

int g_nXdownrightCorner = 0;

int g_nYdownrightCorner = 0;

void PrintLog(LPSTR pszLog)

{

	WriteConsole(g_hStdOut,pszLog,strlen(pszLog),NULL,NULL);

} 

void OnPaint(HWND hWnd,

			UINT uMsg,

			WPARAM wParam,

			LPARAM lParam)

{

//	CHAR szText[] = "Hello Paint";

	PAINTSTRUCT ps = {0};

	HDC hDc = BeginPaint(hWnd,&ps);

//	TextOut(hDc,g_nXpos,g_nYpos,szText,strlen(szText));

//	Rectangle(hDc,g_nXupperLeftCorner,g_nYupperLeftCorner,g_nXdownrightCorner,g_nYdownrightCorner);

	EndPaint(hWnd,&ps);

}

LRESULT CALLBACK WndProc(HWND hWnd,

						 UINT uMsg,

						 WPARAM wParam,

						 LPARAM lParam)

{

	CHAR szText[100] = {0};

	switch (uMsg)

	{

	case WM_PAINT:

		OnPaint(hWnd,uMsg,wParam,lParam);

	case WM_LBUTTONDOWN:

		{

			PrintLog("WM_LBUTTONDOWN\n");

			g_nXupperLeftCorner = LOWORD(lParam);

			g_nYupperLeftCorner = HIWORD(lParam);

			sprintf(szText,"x1:%d y1:%d\n",g_nXupperLeftCorner,g_nYupperLeftCorner);

			PrintLog(szText);

			// 		InvalidateRect(hWnd,NULL,TRUE);

			//		UpdateWindow(hWnd);	

		}

			break;

	case WM_LBUTTONUP:

		{

			PrintLog("WM_LBUTTONUP\n");

			g_nXdownrightCorner = LOWORD(lParam);

			g_nYdownrightCorner = HIWORD(lParam);

			sprintf(szText,"x2:%d y2:%d\n",g_nXdownrightCorner,g_nYdownrightCorner);

			PrintLog(szText);

			InvalidateRect(hWnd,NULL,FALSE);

			UpdateWindow(hWnd);

		}


		break;

	case WM_RBUTTONUP:

		PrintLog("WM_RBUTTONUP\n");

		break;

	case WM_RBUTTONDOWN:

		PrintLog("WM_RBUTTONDOWN\n");

		break;

	case WM_MOUSEMOVE:

		{

// 			int nX = LOWORD(lParam);

// 			int nY = HIWORD(lParam);

// 			if(wParam & MK_LBUTTON)

// 			{

// // 				g_nXpos = nX;

// // 				g_nYpos = nY;

// 				g_nXdownrightCorner = LOWORD(lParam);

// 				g_nYdownrightCorner = HIWORD(lParam);

// 				InvalidateRect(hWnd,NULL,TRUE);

// 				UpdateWindow(hWnd);

// 			}


// 			LPPOINT ptScreen = new POINT;

// 			ptScreen->x = nX;

// 			ptScreen->y = nY;

// 			ClientToScreen(hWnd,ptScreen);

// 			sprintf(szText,"x:%d(%d) y:%d(%d)",nX,ptScreen->x,nY,ptScreen->y);

// 			PrintLog(szText);

// 			PrintLog("WM_MOUSEMOVE\n");

// 			delete(ptScreen);

// 			if(wParam & MK_CONTROL)

// 			{

// 				PrintLog("WM_MOUSEMOVE: MK_CONTROL\n");

// 			}

// 			if(wParam & MK_LBUTTON)

// 			{

// 				PrintLog("WM_MOUSEMOVE: MK_LBUTTON\n");

// 			}

		}


		break;

	case WM_DESTROY:

		PostQuitMessage(0);

	}

	return DefWindowProc(hWnd,uMsg,wParam,lParam);

}

BOOL MyRegister(LPSTR pszClassName)

{

	WNDCLASSEX wce = {0};

	wce.cbSize = sizeof(wce);

	wce.style = CS_VREDRAW|CS_HREDRAW;

	wce.lpfnWndProc = WndProc;

	wce.cbClsExtra = 0;

	wce.cbWndExtra = 0;

	wce.hInstance = g_hInst;

	wce.hIcon = NULL;

	wce.hCursor = NULL;

	wce.hbrBackground = HBRUSH(COLOR_BTNFACE);

	wce.lpszMenuName = NULL;

	wce.lpszClassName = pszClassName;

	wce.hIconSm = NULL;

	ATOM aTom = RegisterClassEx(&wce);

	if(0 == aTom)

	{

		return FALSE;

	}

	return TRUE;

}

HWND CreateWnd(LPSTR pszClassName)

{

	HWND hWnd = CreateWindowEx(0,pszClassName,"MyWnd",WS_OVERLAPPEDWINDOW,

		CW_USEDEFAULT,CW_USEDEFAULT,CW_USEDEFAULT,CW_USEDEFAULT,NULL,

		NULL,g_hInst,NULL);

	return hWnd;

}

void DisplayWnd(HWND hWnd)

{

	ShowWindow(hWnd,SW_SHOW);

	UpdateWindow(hWnd);

}

void Message()

{

	MSG msg = {0};

	while(GetMessage(&msg,NULL,0,0))

	{

		TranslateMessage(&msg);

		DispatchMessage(&msg);

	}

}

void NewConsole()

{

	AllocConsole();

	g_hStdOut = GetStdHandle(STD_OUTPUT_HANDLE);

}

int APIENTRY WinMain(HINSTANCE hInstance,

                     HINSTANCE hPrevInstance,

                     LPSTR     lpCmdLine,

                     int       nCmdShow)