C# C++ 字符串傳遞

可以說新手使用P-INVOKE最開始的頭疼就是C#和C++的字符串傳遞，因為這里涉及到兩個問題。

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：字符串指針作為返回值，

第三：字符串指針作為輸入輸出參數。

 

C++部分的測試代碼很簡單這里就全部貼出來了：

 

1#include "stdafx.h"
2#include "TestDll.h"
3#include <stdio.h>
4#include <string.h>
5#include <tchar.h>
6
7
8 staticchar* _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{
24return _hello;
25}
26
27TCHAR * __stdcall GetStringReturnW()
28{
29return _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：字符串指針作為輸入參數，可以使用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方法簡單明了。

 

1 [DllImport("TestDll", EntryPoint ="PrintString")]
2publicstaticexternvoid PrintStringByBytes(byte[] hello);
3
4 [DllImport("TestDll", EntryPoint ="PrintString")]
5publicstaticexternvoid PrintStringByMarshal([MarshalAs(UnmanagedType.LPStr)]string hello);
6
7 [DllImport("TestDll", EntryPoint ="PrintStringW")]
8publicstaticexternvoid PrintStringByBytesW(byte[] hello);
9
10 [DllImport("TestDll", EntryPoint ="PrintStringW")]
11publicstaticexternvoid PrintStringByMarshalW([MarshalAs(UnmanagedType.LPWStr)]string hello);
12
13
14publicvoid 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：字符串指針作為返回值，和上面一樣也有兩種聲明方法，同樣也包含兩個版本。注意：Marshal.PtrToStringAnsi()函數的使用，把字符串指針轉變為C#的string.推薦使用MarshalAs方法簡單明了。

 

1 [DllImport("TestDll", EntryPoint ="GetStringReturn")]
2publicstaticextern IntPtr GetStringReturnByBytes();
3
4 [DllImport("TestDll", EntryPoint ="GetStringReturn")]
5 [return:MarshalAs(UnmanagedType.LPStr)]
6publicstaticexternstring GetStringReturnByMarshal();
7
8 [DllImport("TestDll", EntryPoint ="GetStringReturnW")]
9publicstaticextern IntPtr GetStringReturnByBytesW();
10
11 [DllImport("TestDll", EntryPoint ="GetStringReturnW")]
12 [return: MarshalAs(UnmanagedType.LPWStr)]
13publicstaticexternstring GetStringReturnByMarshalW();
14
15
16publicvoid 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：字符串指針作為輸入輸出參數時，因為要求有固定的容量，所以這里使用的是StringBuilder，大家仔細看了，當然也有byte[]版本。這個看大家喜歡那個版本就是用那個.

 

1 [DllImport("TestDll", EntryPoint ="GetStringParam")]
2publicstaticexternvoid GetStringParamByBytes(byte[] outHello, int len);
3
4 [DllImport("TestDll", EntryPoint ="GetStringParam")]
5publicstaticexternvoid GetStringParamByMarshal([Out, MarshalAs(UnmanagedType.LPStr)]StringBuilder outHello, int len);
6
7 [DllImport("TestDll", EntryPoint ="GetStringParamW")]
8publicstaticexternvoid GetStringParamByBytesW(byte[] outHello, int len);
9
10 [DllImport("TestDll", EntryPoint ="GetStringParamW")]
11publicstaticexternvoid GetStringParamByMarshalW([Out, MarshalAs(UnmanagedType.LPWStr)]StringBuilder outHello, int len);
12
13
14publicbyte[] _outHello =newbyte[10];
15publicbyte[] _outHelloW =newbyte[20];
16public StringBuilder _builder =new StringBuilder(10); //很重要設定string的容量。
17 
18publicvoid 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}