Question

以下.net到本机C代码不起作用，任何想法

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“这是来自C代码”但是没有，任何关于什么错误的想法？

仅供参考 - 我有另一个不使用ref类型字符串的测试函数，它可以正常工作

Answer 1

你的代码也在C方面错了。 __inout注释只告诉编译器你可以改变“c”参数指向的缓冲区。但是指针本身位于堆栈中，如果修改了“c”参数，则不会返回调用者。您的声明可能如下：

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;
   }
 }

和C＃方：

[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？基本上只需将CallingConvention = CallingConvention.Cdecl添加到DllImport语句中。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);

为什么这个Explicit P / Invoke不起作用？

2 个答案: