面试题：Visual Basic 与 Windows API 深度集成问题

1. 声明 API 函数

在 Visual Basic 中调用 Windows API，需在模块中声明所需函数。对于屏幕截图，常用的函数有 GetDC、CreateCompatibleDC、CreateCompatibleBitmap、SelectObject、BitBlt、DeleteObject、DeleteDC 和 SaveBitmapToFile（自定义保存函数，需自行实现）。

Option Explicit

Private Declare Function GetDC Lib "user32" (ByVal hwnd As Long) As Long
Private Declare Function CreateCompatibleDC Lib "gdi32" (ByVal hdc As Long) As Long
Private Declare Function CreateCompatibleBitmap Lib "gdi32" (ByVal hdc As Long, ByVal nWidth As Long, ByVal nHeight As Long) As Long
Private Declare Function SelectObject Lib "gdi32" (ByVal hdc As Long, ByVal hObject As Long) As Long
Private Declare Function BitBlt Lib "gdi32" (ByVal hDestDC As Long, ByVal x As Long, ByVal y As Long, ByVal nWidth As Long, ByVal nHeight As Long, ByVal hSrcDC As Long, ByVal xSrc As Long, ByVal ySrc As Long, ByVal dwRop As Long) As Long
Private Declare Function DeleteObject Lib "gdi32" (ByVal hObject As Long) As Long
Private Declare Function DeleteDC Lib "gdi32" (ByVal hdc As Long) As Long

2. 处理相关数据结构

Windows 位图使用 BITMAPINFO 结构来描述其格式信息，需在 Visual Basic 中定义相应结构。

Private Type RGBQUAD
    rgbBlue As Byte
    rgbGreen As Byte
    rgbRed As Byte
    rgbReserved As Byte
End Type

Private Type BITMAPINFOHEADER
    biSize As Long
    biWidth As Long
    biHeight As Long
    biPlanes As Integer
    biBitCount As Integer
    biCompression As Long
    biSizeImage As Long
    biXPelsPerMeter As Long
    biYPelsPerMeter As Long
    biClrUsed As Long
    biClrImportant As Long
End Type

Private Type BITMAPINFO
    bmiHeader As BITMAPINFOHEADER
    bmiColors(0 To 255) As RGBQUAD
End Type

3. 实现屏幕截图功能

编写函数实现屏幕截图并保存为 BMP 文件。

Public Sub CaptureScreen(ByVal filePath As String)
    Dim hdcScreen As Long
    Dim hdcMem As Long
    Dim hBitmap As Long
    Dim hOldBitmap As Long
    Dim bmi As BITMAPINFO
    Dim lpBits As Long
    Dim fileNum As Integer
    Dim bytesWritten As Long
    Dim screenWidth As Long
    Dim screenHeight As Long
    
   '获取屏幕设备上下文
    hdcScreen = GetDC(0)
    
   '创建兼容设备上下文
    hdcMem = CreateCompatibleDC(hdcScreen)
    
   '获取屏幕尺寸
    screenWidth = GetSystemMetrics(0)
    screenHeight = GetSystemMetrics(1)
    
   '创建兼容位图
    hBitmap = CreateCompatibleBitmap(hdcScreen, screenWidth, screenHeight)
    
   '选择位图到内存设备上下文
    hOldBitmap = SelectObject(hdcMem, hBitmap)
    
   '执行 BitBlt 操作将屏幕内容复制到内存位图
    BitBlt hdcMem, 0, 0, screenWidth, screenHeight, hdcScreen, 0, 0, vbSrcCopy
    
   '填充 BITMAPINFO 结构
    With bmi.bmiHeader
       .biSize = Len(bmi.bmiHeader)
       .biWidth = screenWidth
       .biHeight = screenHeight
       .biPlanes = 1
       .biBitCount = 32
       .biCompression = 0
       .biSizeImage = 0
       .biXPelsPerMeter = 0
       .biYPelsPerMeter = 0
       .biClrUsed = 0
       .biClrImportant = 0
    End With
    
   '获取位图数据
    lpBits = GlobalAlloc(GMEM_FIXED, bmi.bmiHeader.biSizeImage)
    GetDIBits hdcMem, hBitmap, 0, bmi.bmiHeader.biHeight, ByVal lpBits, bmi, DIB_RGB_COLORS
    
   '保存位图到文件
    fileNum = FreeFile
    Open filePath For Binary As #fileNum
    Put #fileNum, , "BM"
    Put #fileNum, , bmi.bmiHeader.biSizeImage + 54
    Put #fileNum, , 0
    Put #fileNum, , 54
    Put #fileNum, , bmi.bmiHeader
    bytesWritten = Put(#fileNum, , ByVal lpBits, bmi.bmiHeader.biSizeImage)
    Close #fileNum
    
   '释放资源
    GlobalFree lpBits
    SelectObject hdcMem, hOldBitmap
    DeleteObject hBitmap
    DeleteDC hdcMem
    ReleaseDC 0, hdcScreen
End Sub

4. 可能遇到的错误及解决办法

API 函数调用失败：可能由于参数错误或系统资源不足导致。检查函数返回值，若为 0 则表示失败，可通过 GetLastError 函数获取详细错误信息并针对性解决。
文件保存失败：可能由于文件路径错误、权限不足等原因。确保文件路径正确且有写入权限，若保存 BMP 文件失败，可检查 Put 操作的返回值是否为预期写入字节数。

5. 性能优化和资源管理

性能优化：
- 尽量减少不必要的操作，如多次获取屏幕尺寸等信息，可在开始时获取并缓存。
- 使用合适的 BitBlt 光栅操作码（dwRop 参数），vbSrcCopy 适用于简单的复制操作，对于复杂需求可选择其他更高效的操作码。
资源管理：
- 及时释放不再使用的资源，如 DeleteObject 删除位图对象，DeleteDC 删除设备上下文，GlobalFree 释放分配的内存。
- 在函数结束前确保所有资源都已正确释放，避免资源泄漏。