我刚开始学API，遇到了一个问题
#include<windows.h>
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
    HWND hWnd;
    MSG Msg;
    WNDCLASS wndclass;
    wndclass.style=CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc=WndProc;
    wndclass.cbClsExtra=0;
    wndclass.cbWndExtra=0;
    wndclass.hInstance=hInstance;
    wndclass.hIcon=LoadIcon(NULL,IDI_APPLICATION);
    wndclass.hCursor=LoadCursor(NULL, IDC_ARROW);
    wndclass.hbrBackground=(HBRUSH)GetStockObject(WHITE_BRUSH);
    wndclass.lpszMenuName=NULL;
    wndclass.lpszClassName="WinKeyboard";
    if(!RegisterClass(&wndclass))
    {
        MessageBeep(0);
        return FALSE;
    }
    hWnd=CreateWindow(
        "WinKeyboard",
        "键盘操作例程",
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT,
        0,
        CW_USEDEFAULT,
        0,
        NULL,
        NULL,
        hInstance,
        NULL
        );
    ShowWindow(hWnd, nCmdShow);
    UpdateWindow(hWnd);
    while(GetMessage(&Msg, NULL, 0,0))
    {
        TranslateMessage(&Msg);
        DispatchMessage(&Msg);
    }
    return Msg.wParam;
}


#define BufSize 15  //数组长度
LONG WINAPI WndProc(HWND hWnd, UINT iMessage, WPARAM wParam, LPARAM lParam) 
{
    static char cCharBuf[BufSize]; //字符数组
    static int nNumChar = 0; //字符个数
    static int nArrayPos = 0; //脱字符号位置
    static int nLnHeight; //行高
    static int nCharWidth; //字符平均宽度
    int x;
    HDC hDC;
    TEXTMETRIC tm; //字体信息的结构
    PAINTSTRUCT PtStr;
    switch (iMessage)
    {
    case WM_LBUTTONDOWN:
        MessageBox(hWnd, "鼠标已坏，请更换", NULL, MB_ABORTRETRYIGNORE);
        break;
    case WM_CHAR:
        switch(wParam)
        {
        case 0x08: //退格
            if(nArrayPos == 0) //脱字符号已处于最前面
                MessageBox(hWnd, "当前位置是文本的起始位置,不能回退", NULL, MB_OK);
            else
            {
                nArrayPos--; //脱字符号前移一位
                nNumChar--; //字符数减1
                InvalidateRect(hWnd, NULL, TRUE);
            }
            break;
        case 0x1B: //Esc
            MessageBox(hWnd, "您现在不能按Esc键,请继续其他操作", NULL, MB_OK);
            break;
/*
        case 0x0A: ...; break; //换行
        case 0x09: ...; break; //Tab
        case 0x0D: ...; break; //回车
*/
        default: //其他可显示字符
            if(nNumChar >= BufSize) //字符数>=数组长度
            {
                MessageBox(hWnd, "缓冲区已满,不能再输入字符了\n若需要删除字符,请用BackSpace键", NULL, MB_OK);
                break;
            }
            //如果脱字符号不在串尾,脱字符号后面的字符都向后移一位
            for(x=nNumChar; x>nArrayPos; x--)
                cCharBuf[nArrayPos] = cCharBuf[x-1];
            //插入字符
            cCharBuf[nArrayPos] = (unsigned char)wParam;
            nArrayPos++; //脱字符号后移
            nNumChar++; //字符数+1
            InvalidateRect(hWnd, NULL, TRUE);
        }
        break;
    case WM_CREATE:
        hDC = GetDC(hWnd);
        GetTextMetrics(hDC, &tm);
        nLnHeight = tm.tmHeight+tm.tmExternalLeading;
        nCharWidth = tm.tmAveCharWidth;
        ReleaseDC(hWnd, hDC);
        break;
    case WM_KEYDOWN:
        {
            switch(wParam)
            {
            case VK_END:
                nArrayPos = nNumChar;
                break;
            case VK_HOME:
                nArrayPos = 0;
                break;
            case VK_DELETE:
                if(nArrayPos == nNumChar) //脱字符号在串尾
                    MessageBox(hWnd, "缓冲区已空,没有字符可供删除", NULL, MB_OK);
                else
                {
                    //从脱字符号后面第2个字符开始依次前移一位
                    for(x=nArrayPos; x<nNumChar; x++)
                        cCharBuf[x] = cCharBuf[x+1];
                    nNumChar--; //字符数减1
                    InvalidateRect(hWnd, NULL, TRUE);
                }
                break;
            case VK_LEFT:
                if(nArrayPos>0)
                    nArrayPos--;
                else
                    MessageBox(hWnd, "您已经移动到起始位置,不能再往左移动了", NULL, MB_OK);
                break;
            case VK_RIGHT:
                if(nArrayPos < nNumChar)
                    nArrayPos++;
                else
                    MessageBox(hWnd, "已经到缓冲区的末尾,不能再向右移动了", NULL, MB_OK);
                break;
            }
        }
        break;
    case WM_PAINT:
        hDC = BeginPaint(hWnd, &PtStr);
        TextOut(hDC, nCharWidth, nLnHeight, cCharBuf, nNumChar);
        EndPaint(hWnd, &PtStr);
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hWnd, iMessage, wParam, lParam);
    }
    return NULL;
}
这个程序在运行后，出现了这样一个问题：在WM_LBUTTONDOWN消息处理时如果不加BREAK就会在运行时输出一个英镑符号,但是加了BREAK后就 没 这个问题了,这是为什么呢?