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;

        }

    }