在C ++和C#ByRef之间传递字节数组会引发AccessViolationException [英] Passing byte array between C++ and C# ByRef raises AccessViolationException

问题描述

我试图创建一个Win32 DLL公开了一些在C#中调用的函数，如下所示

I am trying to create a Win32 DLL exposes some functions which are called in C# as follows

__declspec(dllexport) int GetData(unsigned char* *data, int* size)
{
    try
    {
        int tlen = 3;
        unsigned char* tchr = new unsigned char[5];
        tchr[0] = 'a';
        tchr[1] = 'b';
        tchr[2] = 'c';

        *size = tlen;
        *data = tchr;

        return 1;
    }
    catch (char *p)
    {
        return 0;
    }
}

在C#方面

[DllImport("MyDll.dll")]
static extern int GetData(ref byte[] data, ref int size);

static void Main()
{
    try
    {
        int hr = 0;
        byte[] gData = null;
        int gSize = 0;
        hr = GetData(ref gData, ref gSize);
        Console.WriteLine(gSize);
        for (int i = 0; i < gSize; i++)
            Console.WriteLine((char)gData[i]);
    }
    catch (Exception p)
    {
        Console.WriteLine(p.ToString());
    }
}

当我运行C#代码时，AccessViolationException发生在GetData函数上，这是C ++代码中异常的标志，但是，下面的C ++代码段可以正常工作而没有任何错误.

When I run C# code, AccessViolationException happens on GetData function which is a sign of exception in C++ code however, following C++ code snippet works fine without any error.

int _tmain(int argc, _TCHAR* argv[])
{
    unsigned char* data = NULL;
    int size = NULL;
    GetData(&data, &size);
    printf("%d", size);
    for (int i = 0; i < size; i++)
        printf("%c,", data[i]);
    return 0;
}

如果将C#main函数和C ++ _tmain函数进行比较，它们几乎是相似的，所以我可能在哪里出错?

If you compare C# main function and C++ _tmain, they are almost analoguous so where I may make a mistake?

推荐答案

您将返回对C ++ new的调用所分配的数组，并希望封送处理程序将其转换为C#字节[].那不会发生.

You are returning an array allocated by a call to C++ new and hoping that the marshaler will turn it into a C# byte[]. That won't happen.

您需要通过引用传递一个指针，然后用手将其编组.您的p/调用应如下所示:

You'll need to pass a pointer by reference and then marshal it by hand. Your p/invoke should look like this:

[DllImport("MyDll.dll")]
static extern int GetData(out IntPtr data, out int size);

当函数返回数据时，它将指向数组，您可以使用Marshal类读取内容.我想您会将其复制到新的字节数组.

When the function returns data will point to the array and you can read the contents using the Marshal class. I guess you would copy it to a new byte array.

var arr = new byte[size];
Marshal.Copy(data, arr, 0, size);

其他一些要点:

1. 调用约定不匹配.本机端是cdecl，托管端是stdcall.
2. 您需要导出一个解除分配器以删除本机函数返回的内存.考虑重新设计，其中调用方分配缓冲区.

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