將資料庫中位元據以非同步方式寫入磁碟

方式一：一次擷取，非同步寫入
/// <summary>
  /// 緩衝區大小
  /// </summary>
  public const int numPixels = 512 * 512;
 /// <summary>
  /// 將資料檔案寫入磁碟
  /// </summary>
  /// <param name="strSql"></param>
  /// <returns></returns>
  public static bool  MakeFileWithWriteListByAdapter(string strSql,out string strErr)
  {
   if(File.Exists(ConfigProxy.GetValueByKey("ListFile")))File.Delete(ConfigProxy.GetValueByKey("ListFile"));
   DataTable objTable;    
   if(!OleDataBaseProxy.ExecuteSql(strSql,out objTable,out strErr))return false;
   string outputPath = ConfigProxy.GetValueByKey("OutputPath");
   if(objTable.Rows.Count < 1) return false;  
   string strDirectory = outputPath + "//";
   if(!Directory.Exists(strDirectory)) Directory.CreateDirectory(strDirectory);
   for(int i = 0;i< objTable.Rows.Count; i ++)
   {
    
    string fileName = objTable.Rows[i]["附件名稱"].ToString();
    //記錄輸出資料行表
    LogProxy.WriteList(strDirectory + fileName);
    //擷取檔案資料
    byte [] ImageContent = (byte[])objTable.Rows[i]["附件內容"];    
    AutoResetEvent manualEvent = new AutoResetEvent(false);   
    FileStream fStream =
     new FileStream(strDirectory + fileName,FileMode.Create,
     FileAccess.ReadWrite, FileShare.None, 4096, true);   
    IAsyncResult asyncResult = fStream.BeginWrite(
     ImageContent, 0, ImageContent.Length,
     new AsyncCallback(EndWriteCallback),
     new State(fStream, manualEvent));   
    manualEvent.WaitOne(5000, false);
    fStream.Close();
   }
   strErr = "";
   return true;
  } 
class State
  {
   public FileStream fStream;
   public AutoResetEvent autoEvent;

   public State(FileStream fStream, AutoResetEvent autoEvent)
   {
    this.fStream   = fStream;
    this.autoEvent = autoEvent;
   }
  }
  static void EndWriteCallback(IAsyncResult asyncResult)
  {

   State stateInfo = (State)asyncResult.AsyncState;
   int workerThreads;
   int portThreads;
   try
   {
    ThreadPool.GetAvailableThreads(out workerThreads,
     out portThreads);   
    stateInfo.fStream.EndWrite(asyncResult);    
    Thread.Sleep(1500);
   }
   finally
   {    
    stateInfo.autoEvent.Set();
   }
  }

方式二：聯機讀取，非同步寫入

/// <summary>
  /// 緩衝區大小
  /// </summary>
  public const int numPixels = 512 * 512;
/// <summary>
  /// 將資料檔案寫入磁碟
  /// </summary>
  /// <param name="strSql"></param>
  /// <returns></returns>
  public static bool  MakeFileWithWriteListByReader(string strSql,out string strErr)
  { 
   if(File.Exists(ConfigProxy.GetValueByKey("ListFile")))File.Delete(ConfigProxy.GetValueByKey("ListFile"));
   string outputPath = ConfigProxy.GetValueByKey("OutputPath");
   string strDirectory = outputPath + "//";
   if(!Directory.Exists(strDirectory)) Directory.CreateDirectory(strDirectory);
   System.Data.OleDb.OleDbCommand cmd = new OleDbCommand();
   OleDbConnection Cnn = new OleDbConnection(ConfigProxy.GetValueByKey("OleConnectionString"));
   cmd.Connection = Cnn;
   cmd.CommandText = strSql;   
   //開啟串連
   try
   {
    Cnn.Open();
   }
   catch(Exception Err)
   {
    strErr = Err.Message;
    return false;
   }
   byte[] pixels = new byte[numPixels];  
   OleDbDataReader reader = cmd.ExecuteReader();
   byte[]ImageContent;
   //逐條處理
   while(reader.Read())
   {
    string fileName = reader.GetString(1);
    //記錄輸出資料行表
    LogProxy.WriteList(strDirectory + fileName);
    //擷取檔案資料
    ImageContent = new byte[Convert.ToInt64(reader.GetString(7))];
    reader.GetBytes(6,0,ImageContent,0,Convert.ToInt32(reader.GetString(7))); 
    AutoResetEvent manualEvent = new AutoResetEvent(false);   
    FileStream fStream =
     new FileStream(strDirectory + fileName,FileMode.Create,
     FileAccess.ReadWrite, FileShare.None, 4096, true);   
    IAsyncResult asyncResult = fStream.BeginWrite(
     ImageContent, 0, ImageContent.Length,
     new AsyncCallback(EndWriteCallback),
     new State(fStream, manualEvent));   
    manualEvent.WaitOne(5000, false);
    fStream.Close();
   }
   reader.Close();
   //關閉串連
   if(Cnn.State == System.Data.ConnectionState.Open)
   {
    Cnn.Close();
   } 
   strErr = "";
   //釋放資源
   Cnn.Dispose();
   cmd.Dispose();
   GC.Collect();
   return true; 
  }
  class State
  {
   public FileStream fStream;
   public AutoResetEvent autoEvent;

   public State(FileStream fStream, AutoResetEvent autoEvent)
   {
    this.fStream   = fStream;
    this.autoEvent = autoEvent;
   }
  }
  static void EndWriteCallback(IAsyncResult asyncResult)
  {

   State stateInfo = (State)asyncResult.AsyncState;
   int workerThreads;
   int portThreads;
   try
   {
    ThreadPool.GetAvailableThreads(out workerThreads,
     out portThreads);   
    stateInfo.fStream.EndWrite(asyncResult);    
    Thread.Sleep(1500);
   }
   finally
   {    
    stateInfo.autoEvent.Set();
   }
  }

 兩種方式的比較：
 
方式一：適合於資料庫負載較大，位元據大小已知的情況；
方式二：適合於資料庫負載較小，位元據大小未知的情況；

其中：兩種方式的非同步機制都是相同的，沒有任何區別；非同步機制的優點在於充分發揮了作業系統的優點
注意：在需要對效能進行同比測試的上下文中不能採用非同步機制而必須盡量採用同步機制，以提高真實性