System.Data.SQLClient.SqlConnection在Open之后为什么需要及时Close？

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：/article/4765945.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）

1

internal 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）

1

private 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。。。）

1

internal 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。