C#中is,as,using关键字的使用说明

string str ="test"; 
object obj = str;
if(obj is string) {string str2 = (string)obj};

string str ="test"; 
object obj = str;
string str2 = obj as tring;
if(str2 !=null) {转换成功}

using (Test test =new Test()) { 各种操作;}
 calss Test :IDisposable {
   public void Dispose() {回收操作;}
 }

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.IO;
 
namespace test1
{
 public partial class Form9 : Form
 {
  public Form9()
  {
   InitializeComponent();
  }
 
  private void button1_Click(object sender, EventArgs e)
  {
   //转为object
   if (obj_rdb.Checked)
   {
    //使用using关键字，在代码块执行完成之后自动回收资源
    //由于FileStream已经实现了IDisposable接口，可以直接使用
    using (FileStream fileStream = new FileStream(@"d:\test.txt", System.IO.FileMode.Create))
    {
     object obj = fileStream as object; //直接使用as转换
     if (obj != null)
     {
      MessageBox.Show("FileStream转换为object成功", "提示信息");
     }
     else
     {
      MessageBox.Show("FileStream转换为object失败", "错误信息");
     }
    }
   }
   else
   {
    using (FileStream fileStream = new FileStream(@"d:\test.txt", System.IO.FileMode.Create))
    {
      //直接强制转换
     try
     {
      Stream stream = (Stream)fileStream;
      MessageBox.Show("FileStream转换为Stream成功", "提示信息");
     }catch(Exception ex)
     {
      MessageBox.Show(ex.Message, "错误信息");
     }
     
    }
   }
   
  }
 }
}

 public interface ITimeProvider
  {
    DateTime CurrentDate { get; }
    string CurrentYear { get; }
  }

 public class TimeProvider : ITimeProvider
  {
    public DateTime CurrentDate { get { return DateTime.Now; } }
    public string CurrentYear { get { return DateTime.Now.Year.ToString(); } }
  }

 public class Assembler
  {
    private static Dictionary<Type, Type> dictionary = new Dictionary<Type, Type>();
    public Assembler()
    {
      dictionary.Add(typeof(ITimeProvider), typeof(TimeProvider));
    }
    public object Create(Type type)
    {
      if (type == null || !dictionary.ContainsKey(type)) throw new NullReferenceException();
      Type targetType = dictionary[type];
      return Activator.CreateInstance(targetType);
    }
 
    public T Create<T>()
    {
      return (T)Create(typeof(T));
    }
  }

 public class Client
  {
    private ITimeProvider timeProvider;
    public Client(ITimeProvider timeProvider)
    {
      this.timeProvider = timeProvider;
    }
    public string GetYear()
    {
      return timeProvider.CurrentYear .ToString();
    }
    public string GetDatetime()
    {
      return timeProvider.CurrentDate.ToString();
    }
  }

   ITimeProvider timeProvider = (new Assembler()).Create<ITimeProvider>();
      Client clinet = new Client(timeProvider);
      Console.WriteLine(clinet.GetYear());
      Console.WriteLine(clinet.GetDatetime());