[StructLayout(LayoutKind.Explicit, Size=16, CharSet=CharSet.Ansi)]
public class MySystemTime 
{
   [FieldOffset(0)]public ushort wYear; 
   [FieldOffset(2)]public ushort wMonth;
   [FieldOffset(4)]public ushort wDayOfWeek; 
   [FieldOffset(6)]public ushort wDay; 
   [FieldOffset(8)]public ushort wHour; 
   [FieldOffset(10)]public ushort wMinute; 
   [FieldOffset(12)]public ushort wSecond; 
   [FieldOffset(14)]public ushor wMilliseconds; 
}

[C#] 
//Applied to a parameter.
  public void M1 ([MarshalAs(UnmanagedType.LPWStr)]String msg);
//Applied to a field within a class.
  class MsgText {
    [MarshalAs(UnmanagedType.LPWStr)] Public String msg;
  }
//Applied to a return value.
[return: MarshalAs(UnmanagedType.LPWStr)]
public String GetMessage();

using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;

namespace DllImportTest
{
    /**//// <summary>
    /// 定义一个用于接收非托管API方法返回值的类
    /// StructLayout定义了该类的各个成员在物理上的排列方式
    /// </summary>
    [StructLayout(LayoutKind.Explicit, Size = 16, CharSet = CharSet.Ansi)]
    public class MySystemTime
    {
        [FieldOffset(0)]
        public ushort wYear;
        [FieldOffset(2)]
        public ushort wMonth;
        [FieldOffset(4)]
        public ushort wDayOfWeek;
        [FieldOffset(6)]
        public ushort wDay;
        [FieldOffset(8)]
        public ushort wHour;
        [FieldOffset(10)]
        public ushort wMinute;
        [FieldOffset(12)]
        public ushort wSecond;
        [FieldOffset(14)]
        public ushort wMilliseconds;
    }
    /**//// <summary>
    /// 用LibWrapper的静态方法来调用非托管API方法"GetSystemTime"
    /// </summary>
    class LibWrapper
    {
        [DllImport("kernel32.dll", EntryPoint = "GetSystemTime")]
        //如果定义的方法名称与要进行封装的非托管API方法不同则需要在DLLImport中指定入口点.
        public static extern void gettime([MarshalAs(UnmanagedType.LPStruct)]MySystemTime st);
    }

    class TestApplication
    {
        public static void Main()
        {
            try
            {
                MySystemTime sysTime = new MySystemTime();
                //LibWrapper.GetSystemTime(sysTime);
                LibWrapper.gettime(sysTime);
                Console.WriteLine("The System time is {0}/{1}/{2} {3}:{4}:{5}", sysTime.wDay,
                   sysTime.wMonth, sysTime.wYear, sysTime.wHour, sysTime.wMinute, sysTime.wSecond);
            }
            catch (TypeLoadException e)
            {
                Console.WriteLine("TypeLoadException : " + e.Message);
            }
            catch (Exception e)
            {
                Console.WriteLine("Exception : " + e.Message);
            }
        }
    }

}