c# - 将指针数组从 C++ 返回到 C#

Question

下面我有一段来自 C++ 的代码片段。我需要返回指针数组(指向 TempStruct)。

问题是在 C# 端我只得到一个元素。另一方面，我得到了 AV。

**C++**
extern "C" __declspec(dllexport) void GetResult(TempStruct** outPtr, long *size)
{       
    *outPtr = (TempStruct*)new TempStruct*[2];

     outPtr[0] = new TempStruct("sdf", 123);
     outPtr[1] = new TempStruct("abc", 456);

    *size = 2;      
}

**C#**    
[DllImport("test.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
public static extern void GetResult(out IntPtr outPtr, out int size);

IntPtr ptr = IntPtr.Zero;
int length;        
GetResult(out ptr, out length);

int size = Marshal.SizeOf(typeof(TempStruct));
TempStruct[] someData2 = new TempStruct[length];

for (int i = 0; i < length; i++)
{
   IntPtr wskptr = (IntPtr)(ptr.ToInt64() + (size * i));
   someData2[i] = (TempStruct)Marshal.PtrToStructure(wskptr, typeof(TempStruct));            
} 

Answer 1

你做错了。

您正在混合指针类型。

通过使用 new TempStruct()，您可以创建指向 TempStruct 的指针数组。我为您提供的示例创建了一个 TempStruct 数组。看到区别了吗？

现在... TempStruct** outPtr 应该是 TempStruct*** outPtr (因为你想返回 (*) 一个数组(*) 的指针 (*)... 或者 TempStruct**& 如果您愿意的话:-)

改变这一行

someData2[i] = (TempStruct)Marshal.PtrToStructure(Marshal.ReadIntPtr(wskptr), typeof(TempStruct));

因为你必须阅读单指针。

我希望你能删除各种 TempStruct 并使用

delete[] ptr;

用于删除结构数组的运算符。

完整示例:

C++:

struct TempStruct
{
    char* str;
    int num;

    // Note the strdup. You don't know the source of str.
    // For example if the source is "Foo", then you can't free it.
    // Using strdup solves this problem.
    TempStruct(const char *str, int num) 
        : str(strdup(str)), num(num)
    {
    }

    ~TempStruct()
    {
        free(str);
    }
};

extern "C"
{
    __declspec(dllexport) void GetResult(TempStruct ***outPtr, int *size)
    {
        *outPtr = new TempStruct*[2];

        (*outPtr)[0] = new TempStruct("sdf", 123);
        (*outPtr)[1] = new TempStruct("abc", 456);

        *size = 2;
    }

    __declspec(dllexport) void FreeSomeData(TempStruct **ptr, int size)
    {
        for (int i = 0; i < size; i++)
        {
            delete ptr[i];
        }

        delete[] ptr;
    }
}

C#:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1), Serializable]
internal struct TempStruct
{
    public string str;
    public int num;
}

[DllImport("NativeLibrary.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
static extern void GetResult(out IntPtr outPtr, out int numPtr);

[DllImport("NativeLibrary.dll", CallingConvention = CallingConvention.Cdecl)]
static extern void FreeSomeData(IntPtr ptr, int num);

// C++ will return its TempStruct array in ptr
IntPtr ptr;
int size;

GetResult(out ptr, out size);

TempStruct[] someData2 = new TempStruct[size];

for (int i = 0; i < size; i++)
{
    IntPtr ptr2 = Marshal.ReadIntPtr(ptr, i * IntPtr.Size);
    someData2[i] = (TempStruct)Marshal.PtrToStructure(ptr2, typeof(TempStruct));
}

// Important! We free the TempStruct allocated by C++. We let the
// C++ do it, because it knows how to do it.
FreeSomeData(ptr, size);

请注意，您不需要在 C# struct 上使用 [Serializable] 和 Pack=1

对于 C++ 更正确:

__declspec(dllexport) void GetResult(TempStruct **&outPtr, int &size)
{
    outPtr = new TempStruct*[2];

    outPtr[0] = new TempStruct("sdf", 123);
    outPtr[1] = new TempStruct("abc", 456);

    size = 2;
}

它更正确，因为 outPtr 和 size 都不能为 NULL。见 https://stackoverflow.com/a/620634/613130 。 C# 签名相同。