使用Bitmap创建Caret无显示且无报错问题求助（附代码）

我参考微软文档中「处理键盘输入」章节的WM_SETFOCUS代码段，修改为用Bitmap创建Caret，但屏幕上看不到任何Caret图像，且CreateCaret()函数未返回错误信息。我已确认Bitmap已成功创建，最初的代码如下：

case WM_SETFOCUS:
{
     int caret_width = (nCharX/5)>=2 ? nCharX/5 : 2;
     RECT rcCaret = { 0, 0, caret_width, nCharY };
     hdc = GetDC(hwnd);
     HBITMAP hBitmapNew = CreateCompatibleBitmap(hdc, caret_width, nCharY);
     HDC hdcMemNew = CreateCompatibleDC(hdc);
     SelectObject(hdcMemNew, hBitmapNew);
     HBRUSH caretBrush = CreateSolidBrush(RGB(255,0,0));
     FillRect(hdcMemNew, &rcCaret, caretBrush);

     if(CreateCaret(hwnd, (HBITMAP) hBitmapNew, 0, 0)==0) {
        MessageBox(NULL, "CreateCaret() error!", "Error", MB_OK) ;
     }

     // CreateCaret(hwnd, 0, nCharX, nCharY); // Adjust the caret position.
     SetCaretPos(nCaretPosX * nCharX, nCaretPosY * nCharY); // Display the caret.
     ShowCaret(hwnd);

     DeleteObject(caretBrush);
     DeleteObject(hdcMemNew);
     DeleteObject(hBitmapNew);
     ReleaseDC(hwnd, hdc);
}
break;

我尝试修改Caret的Bitmap，但窗口上仍无Caret图像。后来意识到Bitmap变量不应被删除，将其改为全局变量后的最新代码如下，问题仍未解决，恳请帮忙：

HBITMAP hBitmap; // Global variable

case WM_SETFOCUS:
{
     int caret_width = (nCharX/5)>=2 ? nCharX/5 : 2;
     RECT rcCaret = { 0, 0, caret_width, nCharY };
     hdc = GetDC(hwnd);
     HDC hdcMem = CreateCompatibleDC(hdc);
     hBitmap = CreateCompatibleBitmap(hdc, caret_width, nCharY);
     HBITMAP hBitmapOld = (HBITMAP) SelectObject(hdcMem, hBitmap);
     // To get a red colored caret, we can use the result of XOR operation
     // between the background color and the red color as the bitmap color.
     HBRUSH caretBrush = CreateSolidBrush(RGB(236, 236, 236) ^ RGB(255, 0, 0));
     FillRect(hdcMem, &rcCaret, caretBrush);

     if(CreateCaret(hwnd, (HBITMAP) hBitmap, 0, 0)==0) {
        MessageBox(NULL, "CreateCaret() error!", "Error", MB_OK) ;
     }

     SetCaretPos(nCaretPosX * nCharX, nCaretPosY * nCharY);
     ShowCaret(hwnd);

     DeleteObject(caretBrush);
     DeleteObject(hdcMem);
     SelectObject(hdcMem, hBitmapOld);
     ReleaseDC(hwnd, hdc);
}

内容的提问来源于stack exchange，提问作者Niteya