public class APP_DLL
    {
        [DllImport("ruihua.dll", CallingConvention = CallingConvention.Cdecl)] //引入dll，并设置字符集
        public static extern int test(byte[] src1, int w1, int h1, int channel1, byte[] src2, int w2, int h2, int channel2, string str);
    }

    public class APP_DLL
    {
        [DllImport("ruihua.dll", CallingConvention = CallingConvention.Cdecl)] //引入dll，并设置字符集
        public static extern int test(byte[] src1, int w1, int h1, int channel1, byte[] src2, int w2, int h2, int channel2, IntPtr str);
    }

　　IntPtr ptrIn = Marshal.StringToHGlobalAnsi(obj.ToString());

　　string retlust = Marshal.PtrToStringAnsi(ptrIn);