System.Data.SQLClient.SqlConnection在Open之后为什么需

     如果System.Data.SQLClient.SqlConnection（下面简称dbconn）不及时Close，那么造成的后果可以在这里看到：http://www.cnblogs.com/juqiang/archive/2005/12/10/294269.HTML
  
  具体的原因，我们来看代码（用reflector反编译.net framework 2.0的dll）
  
  dbconn有一个方法叫做：Open，部分代码如下：
   1 try
   2 {
   3 statistics = SqlStatistics.StartTimer(this.Statistics);
   4 this.InnerConnection.OpenConnection(this, this.ConnectionFactory);
   5 SqlInternalConnectionSmi innerConnection = this.InnerConnection as SqlInternalConnectionSmi;
   6 if (innerConnection != null)
   7 {
   8 innerConnection.AutomaticEnlistment();
   9 }
  10注意上面的第4行，this.InnerConnection.OpenConnection(this, this.ConnectionFactory);这里的this.ConnectionFactory是一个DbConnectionFactory类型，定义在class SqlConnection中
  static SqlConnection()
  {
   EventInfoMessage = new object();
   _connectionFactory = SqlConnectionFactory.SingletonInstance;
   ExecutePermission = CreateExecutePermission();
  }在看第一段代码的第4行，这里用InnerConnection来进行OpenConnection，那么InnerConnection的类型，我们可以从下面看到：
  public SqlConnection()
  {
   this.ObjectID = Interlocked.Increment(ref _objectTypeCount);
   GC.SuppressFinalize(this);
   this._innerConnection = DbConnectionClosedNeverOpened.SingletonInstance;
  }它是一个DbConnectionClosedNeverOpened类型（具体做啥的，我们不管），它的基类是：DbConnectionClose。ok，确定好上述两个类型后，我们就看一下OpenConnection的代码：（class DbConnectionClosed）
   1 try
   2 {
   3 connectionFactory.PermissionDemand(outerConnection);
   4 to = connectionFactory.GetConnection(outerConnection);
   5 }
   6 catch
   7 {
   8 connectionFactory.SetInnerConnectionTo(outerConnection, this);
   9 throw;
  10 }看一下第4行，哈，这里绕到了我们上面的conn factory，我们看它里面的GetConnection怎么写的？（class DbConnectionFactory）
   1internal DbConnectionInternal GetConnection(DbConnection owningConnection)
   2{
   3 DbConnectionInternal connection;
   4 DbConnectionPoolGroup connectionPoolGroup = this.GetConnectionPoolGroup(owningConnection);
   5 DbConnectionPool connectionPool = this.GetConnectionPool(owningConnection, connectionPoolGroup);
   6 if (connectionPool == null)
   7 {
   8 connectionPoolGroup = this.GetConnectionPoolGroup(owningConnection);
   9 connection = this.CreateNonPooledConnection(owningConnection, connectionPoolGroup);
  10 this.PerformanceCounters.NumberOfNonPooledConnections.Increment();
  11 return connection;
  12 }
  13 connection = connectionPool.GetConnection(owningConnection);
  14 if (connection == null)
  15 {
  16 Bid.Trace("<prov.DbConnectionFactory.GetConnection|RES|CPOOL> %d#, GetConnection failed because a pool timeout occurred.\n", this.ObjectID);
  17 throw ADP.PooledOpenTimeout();
  18 }
  19 return connection;
  20}注意第5行里面的GetConnectionPool的调用，继续看它内部的代码：（class DbConnectionFactory）
   1private DbConnectionPool GetConnectionPool(DbConnection owningObject, DbConnectionPoolGroup connectionPoolGroup)
   2{
   3 if (connectionPoolGroup.IsDisabled && (connectionPoolGroup.PoolGroupOptions != null))
   4 {
   5 Bid.Trace("<prov.DbConnectionFactory.GetConnectionPool|RES|INFO|CPOOL> %d#, DisabledPoolGroup=%d#\n", this.ObjectID, connectionPoolGroup.ObjectID);
   6 DbConnectionPoolGroupOptions poolGroupOptions = connectionPoolGroup.PoolGroupOptions;
   7 DbConnectionOptions connectionOptions = connectionPoolGroup.ConnectionOptions;
   8 string connectionString = connectionOptions.UsersConnectionString(false);
   9 connectionPoolGroup = this.GetConnectionPoolGroup(connectionString, poolGroupOptions, ref connectionOptions);
  10 this.SetConnectionPoolGroup(owningObject, connectionPoolGroup);
  11 }
  12 return connectionPoolGroup.GetConnectionPool(this);
  13}注意第9行，我们go on...（Class DbConnectionFactory）
   1 if ((poolOptions == null) && ADP.IsWindowsNT)
   2 {
   3 if (group != null)
   4 {
   5 poolOptions = group.PoolGroupOptions;
   6 }
   7 else
   8 {
   9 poolOptions = this.CreateConnectionPoolGroupOptions(options);
  10 }
  11 }
  12依然是第9行（枯燥的数字。。。），继续往里面看：（Class DbConnectionFactory）
  protected override DbConnectionPoolGroupOptions CreateConnectionPoolGroupOptions(DbConnectionOptions connectionOptions)
  {
   SqlConnectionString str = (SqlConnectionString) connectionOptions;
   if (str.ContextConnection || !str.Pooling)
   {
   return null;
   }
   int connectTimeout = str.ConnectTimeout;
   if ((0 < connectTimeout) && (connectTimeout < 0x20c49b))
   {
   connectTimeout *= 0x3e8;
   }
   else if (connectTimeout >= 0x20c49b)
   {
   connectTimeout = 0x7fffffff;
   }
   return new DbConnectionPoolGroupOptions(str.IntegratedSecurity, str.MinPoolSize, str.MaxPoolSize, connectTimeout, str.LoadBalanceTimeout, str.Enlist, false);
  }看最后一行，调用了一个new操作，注意里面的参数！有MaxPoolSize，MinPoolSize等属性。而这些重要属性是从str，即SqlConnectionString的一个property（Class DbConnectionPool，快到终点了，坚持一下。。。）
   1internal SqlConnectionString(string connectionString) : base(connectionString, GetParseSynonyms(), false)
   2{
   3 bool inProc = InOutOfProcHelper.InProc;
   4 this._integratedSecurity = base.ConvertValueToIntegratedSecurity();
   5 this._async = base.ConvertValueToBoolean("asynchronous processing", false);
   6 this._connectionReset = base.ConvertValueToBoolean("connection reset", true);
   7 this._contextConnection = base.ConvertValueToBoolean("context connection", false);
   8 this._encrypt = base.ConvertValueToBoolean("encrypt", false);
   9 this._enlist = base.ConvertValueToBoolean("enlist", ADP.IsWindowsNT);
   10 this._mars = base.ConvertValueToBoolean("multipleactiveresultsets", false);
   11 this._persistSecurityInfo = base.ConvertValueToBoolean("persist security info", false);
   12 this._pooling = base.ConvertValueToBoolean("pooling", true);
   13 this._replication = base.ConvertValueToBoolean("replication", false);
   14 this._userInstance = base.ConvertValueToBoolean("user instance", false);
   15 this._connectTimeout = base.ConvertValueToInt32("connect timeout", 15);
   16 this._loadBalanceTimeout = base.ConvertValueToInt32("load balance timeout", 0);
   17 this._maxPoolSize = base.ConvertValueToInt32("max pool size", 100);
   18 this._minPoolSize = base.ConvertValueToInt32("min pool size", 0);
   19 this._packetSize = base.ConvertValueToInt32("packet size", 0x1f40);
  ===============================其他代码省略========================================
  181 }
  182}这段代码很长，但是我们注意上面的第17行，这里有一个默认值100，就是说，如果你没有在connection string中指定max pool size，那么该值就是100。
  那么，如果我们代码中对于dbconn进行了Open，没有进行Close，conn pool就会一直增长，一直涨到100，涨到这个Max Pool Size。原因我们看下面：（Class DbConnectionPool）
   1 case 2:
   2 Bid.PoolerTrace("<prov.DbConnectionPool.GetConnection|RES|CPOOL> %d#, Creating new connection.\n", this.ObjectID);
   3 try
   4 {
   5 fromTransactedPool = this.UserCreateRequest(owningObject);
   6 }
   7 catch
   8 {
   9 if (fromTransactedPool == null)
  10 {
  11 Interlocked.Decrement(ref this._waitCount);
  12 }
  13 throw;
  14 }
  15注意上面的第5行，UserCreateRequest，看这个方法：
  private DbConnectionInternal UserCreateRequest(DbConnection owningObject)
  {
   DbConnectionInternal internal2 = null;
   if (this.ErrorOccurred)
   {
   throw this._resError;
   }
   if ((this.Count >= this.MaxPoolSize) && (this.MaxPoolSize != 0))
   {
   return internal2;
   }
   if (((this.Count & 1) != 1) && this.ReclaimEmancipatedObjects())
   {
   return internal2;
   }
   return this.CreateObject(owningObject);
  }上面的代码很清楚的表明，如果count>=MaxPoolsize，会返回一个internal2，而这个field是一个Null！从其他的地方，我们也能找到类似的判断。
  
  上面写了这么多，结论就是，如果dbconn在Open之后，没有Close，那么会造成conn个数上涨，到100之后就会停下来。第101个链接的请求，是无法创建成功的。这样，db和web server很可能都是CPU很低，如0%，但是客户端的响应时间就是很长，造成性能下降。