可以说新手使用P-INVOKE最开始的头疼就是C#和C++的字符串传递,因为这里涉及到两个问题。
第一:C#的string和C++的字符串首指针如何对应。
第二:字符串还有ANSI和UNICODE(宽字符串)之分。
本文分三部分阐述:
第一:字符串指针当输入参数,
第二:字符串指针作为返回值,
第三:字符串指针作为输入输出参数。
C++部分的测试代码很简单这里就全部贴出来了:
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#如何调用。
第一:字符串指针作为输入参数,可以使用byte[] 和MarshalAs来解决。(注意四个P-INVOKE,两个ANSI版本,和两个UNICODE版本),推荐使用MarshalAs方法简单明了。
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:字符串指针作为返回值,和上面一样也有两种声明方法,同样也包含两个版本。注意:Marshal.PtrToStringAnsi()函数的使用,把字符串指针转变为C#的string.推荐使用MarshalAs方法简单明了。
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()); 自动判断类型不错。18 Console.WriteLine(Marshal.PtrToStringAnsi(GetStringReturnByBytes()));19 Console.WriteLine(GetStringReturnByMarshal());20 Console.WriteLine(Marshal.PtrToStringUni(GetStringReturnByBytesW()));21 Console.WriteLine(GetStringReturnByMarshalW());22 }
第三:字符串指针作为输入输出参数时,因为要求有固定的容量,所以这里使用的是StringBuilder,大家仔细看了,当然也有byte[]版本。这个看大家喜欢那个版本就是用那个.
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); //很重要设定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
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