为什么这个显式 P/Invoke 不起作用？

Question

以下 .net 到本机 C 代码不起作用，有什么想法

extern "C" {
   TRADITIONALDLL_API int TestStrRef( __inout char* c)    {
   int rc = strlen(c);
   std::cout << "the input to TestStrRef is: >>" << c << "<<" ;
   c = "This is from the C code ";
   return rc;
   }
 }

 [DllImport("MyDll.dll", SetLastError = true)]
 static extern int TestStrRef([MarshalAs(UnmanagedType.LPStr)] ref string s);
 String abc = "InOut string";
 TestStrRef(ref abc);

此时 Console.WriteLine(abc) 应该打印“This is from the C code”但没有，对问题有什么想法吗？

仅供参考 - 我有另一个不使用引用类型字符串的测试函数，它工作得很好

Answer 1

你的C端代码也错了。 __inout 注释只是告诉编译器您可以更改“c”参数指向的缓冲区。但是指针本身位于堆栈中，如果您修改了“c”参数，则指针本身不会返回给调用者。
您的声明可能如下所示：

extern "C" {
   TRADITIONALDLL_API int TestStrRef( __inout char** c)    {
   int rc = strlen(*c);
   std::cout << "the input to TestStrRef is: >>" << *c << "<<" ;
   *c = "This is from the C code ";
   return rc;
   }
 }

C# 方面：

[DllImport("MyDll.dll", SetLastError = true)]
static extern int TestStrRef(ref IntPtr c);

{
    String abc = "InOut string";
    IntPtr ptrOrig = Marshal.StringToHGlobalAnsi(abc)        
    IntPtr ptr = ptrOrig; // Because IntPtr is structure, ptr contains copy of ptrOrig
    int len = TestStrRef(ref ptr);
    Marshal.FreeHGlobal(ptrOrig); // You need to free memory located to abc' native copy
    string newAbc = Marshal.PtrToStringAnsi(ptr); 
    // You cannot free memory pointed by ptr, because it pointed to literal string located in dll code.
}

Answer 2

这对你有用吗？基本上只需添加 CallingConvention = CallingConvention.Cdecl到 DllImport 语句。您可能还想指定 CharSet（例如：CharSet:=CharSet.Unicode）

 [DllImport("MyDll.dll", CallingConvention = CallingConvention.Cdecl, SetLastError = true)]
 static extern int TestStrRef([MarshalAs(UnmanagedType.LPStr)] ref string s);