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。