c# 如何动态调用如下dll？

import的方法调用dll和动态调用有什么区别？
原来静态调用：
 #region struc declare        const int MaxSensorCountPerChannel = 30;
        const int MaxFreq = 150;
        const int MaxChannelNum = 11;
        [StructLayout(LayoutKind.Sequential)]
        private struct SensorList
        {
            public int nSensorCount;
            [MarshalAs(UnmanagedType.ByValArray, SizeConst = MaxSensorCountPerChannel)]
            public float[] fWaveLengthVec;
            [MarshalAs(UnmanagedType.ByValArray, SizeConst = MaxSensorCountPerChannel)]
            public float[] fPowerDBVec;
        }
        [StructLayout(LayoutKind.Sequential)]
        private struct PerChannel
        {
            public int nChNum;
            public int nFreq;
            [MarshalAs(UnmanagedType.ByValArray, SizeConst = MaxFreq)]
            public SensorList[] tSensorListVec;
        }
        #endregion        #region import dll
        [DllImport("ReadData.dll")]
        private static extern int SEN_Init();        [DllImport("ReadData.dll")]
        private static extern void SEN_Close();        [DllImport("ReadData.dll")]
        private static extern void SEN_ReadDSPData(IntPtr pp);        [DllImport("ReadData.dll")]
        private static extern bool SEN_IsConnected();        [DllImport("ReadData.dll")]
        private static extern int SEN_SendParaData();如何改成动态调用？