调用C++动态链接库出现错误

解决方式：将托管 System.String 中的内容复制到非托管内存（Marshal.StringToHGlobalAnsi）

class HttpsSend
    {
        [DllImport("httpsSend.dll")]
        public static extern IntPtr markInvoice(IntPtr url, IntPtr input);

        public static string Send(string url, string content)
        {
            IntPtr urlPtr = Marshal.StringToHGlobalAnsi(url);
            IntPtr contentPtr = Marshal.StringToHGlobalAnsi(content);

            IntPtr outPutIntPtr = markInvoice(urlPtr, contentPtr);

            string outPut = Marshal.PtrToStringAnsi(outPutIntPtr);

            return outPut;
        }
    }