C#调用C++ Dll内存指针报内存错误

C++代码如下：
 extern "C"   __declspec(dllexport) void _cdecl Test(char* path, int& width, int& height, int& step,uchar*& data)
{
   
    Mat thresholdImg, img = imread(path, IMREAD_GRAYSCALE);

    uchar* imgdata = new  uchar[img.rows*img.cols*3];

    imgdata = img.data;
    width = img.size().width;
    height = img.size().height;
    step = img.step;

    data = imgdata;

}

C# 代码如下：
  public static extern void CopyMemory(IntPtr dest, IntPtr src, uint count);


        [DllImport("MyDll.dll", EntryPoint = "Test",CallingConvention = CallingConvention.Cdecl)]

 
        private static  extern void Test(string path, ref int width, ref int height, ref int step,ref IntPtr imgdata);


        private void button1_Click(object sender, EventArgs e)
        {

            int width = new int();
            int height = new int();
            int step = new int();
            IntPtr imgData = new IntPtr();
         
            string imgPath = @"E:\OpenCV\WindowsFormsApp1\WindowsFormsApp1\bin\x64\Debug\picture.bmp";
            
            Test(imgPath, ref width, ref height, ref step, ref imgData);//调用C++ dll
            
            Bitmap bmp = new Bitmap(width, height, step, System.Drawing.Imaging.PixelFormat.Format8bppIndexed, imgData);//绘制图片
            bmp.Save(@"E:\OpenCV\WindowsFormsApp1\WindowsFormsApp1\bin\x64\Debug\test.bmp");//保存图片

        }


但是C#在" bmp.Save(@"E:\OpenCV\WindowsFormsApp1\WindowsFormsApp1\bin\x64\Debug\test.bmp");//保存图片" 的地方会报"尝试读取受保护的内存"
请问各位大神，有没有什么好的方法解决？