由於歷史原因，很多時候我們的代碼並不完全是使用.NET寫成的。這時候和以往C++代碼的混合編程就顯得相當重要了。最近碰到了這樣的問題，將方法簡要記述如下。
調用簡單的C++函數
要在C#代碼中調用C++函數，大體的思路是這樣的：首先將C++函數寫成DLL形式的庫，然后在C#中導入DLL中的函數進行調用。具體的代碼類似這樣：
C++代碼：
1
int
StaticElementNumber
=
10
;
2 extern " C " AFX_API_EXPORT int GetArrayElementNumber()
3 {
4 return StaticElementNumber;
5 }
2 extern " C " AFX_API_EXPORT int GetArrayElementNumber()
3 {
4 return StaticElementNumber;
5 }
C#代碼：
(導入函數部分，寫在調用函數所在類中)
1
[DllImport(
"
MFCDll.dll
"
)]
2 public static extern int GetArrayElementNumber();
3 int ElementNumber = GetArrayElementNumber();
2 public static extern int GetArrayElementNumber();
3 int ElementNumber = GetArrayElementNumber();
s其中的細節，比如int和char等數據類型在C++和C#中占用的空間不同等等CLR會自動處理。(主要是通過Marshal類自動處理)
這樣的調用還支持調試。打開C#工程的Properties，在Debug選項卡中勾選Enable unmanaged code debugging即可啟用C++代碼調試。這樣在調試模式下，調用這個函數時可以繼續按F11跟進函數內部進行調試。
傳遞GDI對象
一些復雜的Windows對象可以通過句柄來傳送。比如下面的代碼就將一個GDI+ Bitmap對象轉換成GDI句柄進行傳送。
C++代碼(GDI+的聲明，引用等等省略)：
1
extern
"
C
"
AFX_API_EXPORT HBITMAP GetABitmap(WCHAR
*
strFileName)
2 {
3 Gdiplus::GdiplusStartupInput gdiplusStartupInput;
4 ULONG_PTR gdiplusToken;
5 GdiplusStartup( & gdiplusToken, & gdiplusStartupInput, NULL);
6 Bitmap * bitmap = Bitmap::FromFile(strFileName);
7 HBITMAP HBitmapToReturn;
8 bitmap -> GetHBITMAP(NULL, & HBitmapToReturn);
9 GdiplusShutdown(gdiplusToken);
10
11 return HBitmapToReturn;
12 }
2 {
3 Gdiplus::GdiplusStartupInput gdiplusStartupInput;
4 ULONG_PTR gdiplusToken;
5 GdiplusStartup( & gdiplusToken, & gdiplusStartupInput, NULL);
6 Bitmap * bitmap = Bitmap::FromFile(strFileName);
7 HBITMAP HBitmapToReturn;
8 bitmap -> GetHBITMAP(NULL, & HBitmapToReturn);
9 GdiplusShutdown(gdiplusToken);
10
11 return HBitmapToReturn;
12 }
C#代碼(用戶界面采用WPF，略去相關聲明和引用)：
1
[DllImport(
"
MFCDll.dll
"
)]
2 public static extern IntPtr GetABitmap([MarshalAs(UnmanagedType.LPWStr)] string strFileName);
3
4 private void MenuItemFileOpenOnClicked( object sender, RoutedEventArgs e)
5 {
6 OpenFileDialog dialog = new OpenFileDialog();
7 dialog.Title = " Load an image... " ;
8 dialog.Multiselect = false ;
9 if (dialog.ShowDialog() == true )
10 {
11 mainGrid.Children.Clear();
12
13 IntPtr hBitmap = GetABitmap(dialog.FileName);
14 Bitmap bitmap = Bitmap.FromHbitmap(hBitmap);
15 System.Windows.Controls.Image image = new Windows.Controls.Image();
16 image.Source = Windows.Interop.Imaging.CreateBitmapSourceFromHBitmap(hBitmap, ro, Int32Rect.Empty,
17 Windows.Media.Imaging.BitmapSizeOptions.FromEmptyOptions());
18 image.Stretch = System.Windows.Media.Stretch.Fill;
19 DeleteObject(hBitmap);
20 mainGrid.Children.Add(image);
21 }
22 }
2 public static extern IntPtr GetABitmap([MarshalAs(UnmanagedType.LPWStr)] string strFileName);
3
4 private void MenuItemFileOpenOnClicked( object sender, RoutedEventArgs e)
5 {
6 OpenFileDialog dialog = new OpenFileDialog();
7 dialog.Title = " Load an image... " ;
8 dialog.Multiselect = false ;
9 if (dialog.ShowDialog() == true )
10 {
11 mainGrid.Children.Clear();
12
13 IntPtr hBitmap = GetABitmap(dialog.FileName);
14 Bitmap bitmap = Bitmap.FromHbitmap(hBitmap);
15 System.Windows.Controls.Image image = new Windows.Controls.Image();
16 image.Source = Windows.Interop.Imaging.CreateBitmapSourceFromHBitmap(hBitmap, ro, Int32Rect.Empty,
17 Windows.Media.Imaging.BitmapSizeOptions.FromEmptyOptions());
18 image.Stretch = System.Windows.Media.Stretch.Fill;
19 DeleteObject(hBitmap);
20 mainGrid.Children.Add(image);
21 }
22 }
傳遞數組
傳遞定長數組很簡單，此處不述。下面的代碼實現變長數組的傳遞：
C++代碼：
1
int
StaticElementNumber
=
10
;
2 extern " C " AFX_API_EXPORT bool GetArray( int ElementNumber, double * BaseAddress)
3 {
4 if (ElementNumber < StaticElementNumber)
5 {
6 return false ;
7 }
8
9 for ( int i = 0 ; i < StaticElementNumber; ++ i)
10 {
11 BaseAddress[i] = 1 / (( double )i + 1 );
12 }
13
14 return true ;
15 }
16
17 extern " C " AFX_API_EXPORT int GetArrayElementNumber()
18 {
19 return StaticElementNumber;
20 }
2 extern " C " AFX_API_EXPORT bool GetArray( int ElementNumber, double * BaseAddress)
3 {
4 if (ElementNumber < StaticElementNumber)
5 {
6 return false ;
7 }
8
9 for ( int i = 0 ; i < StaticElementNumber; ++ i)
10 {
11 BaseAddress[i] = 1 / (( double )i + 1 );
12 }
13
14 return true ;
15 }
16
17 extern " C " AFX_API_EXPORT int GetArrayElementNumber()
18 {
19 return StaticElementNumber;
20 }
C#代碼：
1
[DllImport(
"
MFCDll.dll
"
)]
2 public static extern bool GetArray( int ElementNumber, [MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 0 )] double [] BaseAddress);
3
4 private void MenuItemFileGetArrayOnClicked( object sender, RoutedEventArgs e)
5 {
6 // Get array data.
7 int ElementNumber = GetArrayElementNumber();
8 double [] doubleArray = new double [ElementNumber];
9 GetArray(ElementNumber, doubleArray);
10
11 // Show the data.
12 mainGrid.Children.Clear();
13 ListBox listBox = new ListBox();
14 foreach ( double number in doubleArray)
15 {
16 listBox.Items.Add(number);
17 }
18 mainGrid.Children.Add(listBox);
19 }
20
2 public static extern bool GetArray( int ElementNumber, [MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 0 )] double [] BaseAddress);
3
4 private void MenuItemFileGetArrayOnClicked( object sender, RoutedEventArgs e)
5 {
6 // Get array data.
7 int ElementNumber = GetArrayElementNumber();
8 double [] doubleArray = new double [ElementNumber];
9 GetArray(ElementNumber, doubleArray);
10
11 // Show the data.
12 mainGrid.Children.Clear();
13 ListBox listBox = new ListBox();
14 foreach ( double number in doubleArray)
15 {
16 listBox.Items.Add(number);
17 }
18 mainGrid.Children.Add(listBox);
19 }
20
有了這三個功能，一般來說C++代碼復用到C#平台上就是比較簡單的事情了。