c#編程指南(十) 平臺調用P-INVOKE徹底掌握, 字符串和指針

能夠說新手使用P-INVOKE最開始的頭疼就是C#和C++的字符串傳遞，由於這裏涉及到兩個問題。

第一：C#的string和C++的字符串首指針如何對應。

第二：字符串還有ANSI和UNICODE（寬字符串）之分。

 

本文分三部分闡述：

第一：字符串指針當輸入參數，

第二：字符串指針做爲返回值，

第三：字符串指針做爲輸入輸出參數。

 

C++部分的測試代碼很簡單這裏就所有貼出來了：

 

1 #include " stdafx.h "
2 #include " TestDll.h "
3 #include < stdio.h >
4 #include < string .h >
5 #include < tchar.h >
6
7
8   static char * _hello = " Hello,World!! " ;
9   static TCHAR * _helloW = TEXT( " Hello,World!! " );
10
11   void __stdcall PrintString( char * hello)
12 {
13 printf( " %s\n " ,hello);
14 }
15
16   void __stdcall PrintStringW(TCHAR * hello)
17 {
18 _tprintf(TEXT( " %s\n " ),hello);
19 }
20
21
22   char * __stdcall GetStringReturn()
23 {
24 return _hello;
25 }
26
27 TCHAR * __stdcall GetStringReturnW()
28 {
29 return _helloW;
30 }
31
32
33   void __stdcall GetStringParam( char * outHello, int len)
34 { // output "aaaaaaaa"
35   for ( int i = 0 ; i < len - 1 ;i ++ ) outHello[i] = ' a ' ;
36 outHello[len - 1 ] = ' \0 ' ;
37 }
38
39   void __stdcall GetStringParamW(TCHAR * outHello, int len)
40 { // output "aaaaaaaa" unicode version.
41   for ( int i = 0 ; i < len - 1 ;i ++ ) outHello[i] = TEXT( ' a ' );
42 outHello[len - 1 ] = TEXT( ' \0 ' );
43 }

 

 

 

下面看C#如何調用。

 

第一：字符串指針做爲輸入參數，能夠使用byte[] 和MarshalAs來解決。（注意四個P-INVOKE，兩個ANSI版本,和兩個UNICODE版本），推薦使用MarshalAs方法簡單明瞭。

 

1 [DllImport( " TestDll " , EntryPoint = " PrintString " )]
2 public static extern void PrintStringByBytes( byte [] hello);
3
4 [DllImport( " TestDll " , EntryPoint = " PrintString " )]
5 public static extern void PrintStringByMarshal([MarshalAs(UnmanagedType.LPStr)] string hello);
6
7 [DllImport( " TestDll " , EntryPoint = " PrintStringW " )]
8 public static extern void PrintStringByBytesW( byte [] hello);
9
10 [DllImport( " TestDll " , EntryPoint = " PrintStringW " )]
11 public static extern void PrintStringByMarshalW([MarshalAs(UnmanagedType.LPWStr)] string hello);
12
13
14 public void Run()
15 {
16 PrintStringByBytes(Encoding.ASCII.GetBytes( " use byte[] " ));
17 PrintStringByMarshal( " use MarshalAs " );
18 PrintStringByBytesW(Encoding.Unicode.GetBytes( " use byte[] " ));
19 PrintStringByMarshalW( " use MarshalAs " );
20 }

 

 

 

第二：字符串指針做爲返回值，和上面同樣也有兩種聲明方法，一樣也包含兩個版本。注意：Marshal.PtrToStringAnsi()函數的使用，把字符串指針轉變爲C#的string.推薦使用MarshalAs方法簡單明瞭。

 

1 [DllImport( " TestDll " , EntryPoint = " GetStringReturn " )]
2 public static extern IntPtr GetStringReturnByBytes();
3
4 [DllImport( " TestDll " , EntryPoint = " GetStringReturn " )]
5 [ return :MarshalAs(UnmanagedType.LPStr)]
6 public static extern string GetStringReturnByMarshal();
7
8 [DllImport( " TestDll " , EntryPoint = " GetStringReturnW " )]
9 public static extern IntPtr GetStringReturnByBytesW();
10
11 [DllImport( " TestDll " , EntryPoint = " GetStringReturnW " )]
12 [ return : MarshalAs(UnmanagedType.LPWStr)]
13 public static extern string GetStringReturnByMarshalW();
14
15
16 public void Run()
17 { // Marshal.PtrToStringAuto(GetStringReturnByBytes()); 自動判斷類型不錯。
18   Console.WriteLine(Marshal.PtrToStringAnsi(GetStringReturnByBytes()));
19 Console.WriteLine(GetStringReturnByMarshal());
20 Console.WriteLine(Marshal.PtrToStringUni(GetStringReturnByBytesW()));
21 Console.WriteLine(GetStringReturnByMarshalW());
22 }

 

 

 

第三：字符串指針做爲輸入輸出參數時，由於要求有固定的容量，因此這裏使用的是StringBuilder，你們仔細看了，固然也有byte[]版本。這個看你們喜歡那個版本就是用那個.

 

1 [DllImport( " TestDll " , EntryPoint = " GetStringParam " )]
2 public static extern void GetStringParamByBytes( byte [] outHello, int len);
3
4 [DllImport( " TestDll " , EntryPoint = " GetStringParam " )]
5 public static extern void GetStringParamByMarshal([Out, MarshalAs(UnmanagedType.LPStr)]StringBuilder outHello, int len);
6
7 [DllImport( " TestDll " , EntryPoint = " GetStringParamW " )]
8 public static extern void GetStringParamByBytesW( byte [] outHello, int len);
9
10 [DllImport( " TestDll " , EntryPoint = " GetStringParamW " )]
11 public static extern void GetStringParamByMarshalW([Out, MarshalAs(UnmanagedType.LPWStr)]StringBuilder outHello, int len);
12
13
14 public byte [] _outHello = new byte [ 10 ];
15 public byte [] _outHelloW = new byte [ 20 ];
16 public StringBuilder _builder = new StringBuilder( 10 ); // 很重要設定string的容量。
17  
18 public void Run()
19 {
20 //
21   GetStringParamByBytes(_outHello, _outHello.Length);
22 GetStringParamByMarshal(_builder, _builder.Capacity);
23 GetStringParamByBytesW(_outHelloW, _outHelloW.Length / 2 );
24 GetStringParamByMarshalW(_builder, _builder.Capacity);
25
26 //
27   Console.WriteLine(Encoding.ASCII.GetString(_outHello));
28 Console.WriteLine(_builder.ToString());
29 Console.WriteLine(Encoding.Unicode.GetString(_outHelloW));
30 Console.WriteLine(_builder.ToString());
31 }
32  

 

 

下載：代碼