对于ADO.NET连接池，大家不会陌生。不过多次用过ADO.NET连接池的各位.NET程序员，却不一定深入了解它，具体原因是：
ADO.NET中提供了连接池的功能，多数开发人员很少设置它，因为它是默认的。
界面设置如下图：
关闭连接池也很简单，在连接字符串如下：
Data Source=(local);Initial Catalog=AdventureWorks;Integrated Security=SSPI;Pooling=False;
但连接池的本质是什么样的呢？
用Reflector，打开System.Data.SqlClient.SqlConnection的ConnectionString属性的设置值的方法，如下：
- private void ConnectionString_Set(string value)
- {
- DbConnectionOptions userConnectionOptions = null;
- DbConnectionPoolGroup group = this.ConnectionFactory.GetConnectionPoolGroup(value, null, ref userConnectionOptions);
- DbConnectionInternal innerConnection = this.InnerConnection;
- bool allowSetConnectionString = innerConnection.AllowSetConnectionString;
- if (allowSetConnectionString)
- {
- allowSetConnectionString= this.SetInnerConnectionFrom(DbConnectionClosedBusy.SingletonInstance, innerConnection);
- if (allowSetConnectionString)
- {
- this._userConnectionOptions = userConnectionOptions;
- this._poolGroup = group;
- this._innerConnection = DbConnectionClosedNeverOpened.SingletonInstance;
- }
- }
- if (!allowSetConnectionString)
- {
- throw ADP.OpenConnectionPropertySet("ConnectionString", innerConnection.State);
- }
- if (Bid.TraceOn)
- {
- string str = (userConnectionOptions != null) ? userConnectionOptions.UsersConnectionStringForTrace() : "";
- Bid.Trace("<prov.DbConnectionHelper.ConnectionString_Set|API> %d#, '%ls'\n", this.ObjectID, str);
- }
- }
再连接 到红色的GetConnectionPoolGroup方法，如下代码
- internal DbConnectionPoolGroup GetConnectionPoolGroup(string connectionString, DbConnectionPoolGroupOptions poolOptions,
- ref DbConnectionOptions userConnectionOptions)
- {
- DbConnectionPoolGroup group;
- if (ADP.IsEmpty(connectionString))
- {
- return null;
- }
- if (!this._connectionPoolGroups.TryGetValue(connectionString, out group) || (group.IsDisabled && (group.PoolGroupOptions != null)))
- {
- DbConnectionOptions options = this.CreateConnectionOptions(connectionString, userConnectionOptions);
- if (options == null)
- {
- throw ADP.InternalConnectionError(ADP.ConnectionError.ConnectionOptionsMissing);
- }
- string str = connectionString;
- if (userConnectionOptions == null)
- {
- userConnectionOptions = options;
- str = options.Expand();
- if (str != connectionString)
- {
- return this.GetConnectionPoolGroup(str, null, ref userConnectionOptions);
- }
- }
- if ((poolOptions == null) && ADP.IsWindowsNT)
- {
- if (group != null)
- {
- poolOptions = group.PoolGroupOptions;
- }
- else
- {
- poolOptions = this.CreateConnectionPoolGroupOptions(options);
- }
- }
- DbConnectionPoolGroup group2 = new DbConnectionPoolGroup(options, poolOptions) {
- ProviderInfo = this.CreateConnectionPoolGroupProviderInfo(options)
- };
- lock (this)
- {
- Dictionary<string, DbConnectionPoolGroup> dictionary = this._connectionPoolGroups;
- if (!dictionary.TryGetValue(str, out group))
- {
- Dictionary<string, DbConnectionPoolGroup> dictionary2 = new Dictionary<string, DbConnectionPoolGroup>(1 + dictionary.Count);
- foreach (KeyValuePair<string, DbConnectionPoolGroup> pair in dictionary)
- {
- dictionary2.Add(pair.Key, pair.Value);
- }
- dictionary2.Add(str, group2); this.PerformanceCounters.NumberOfActiveConnectionPoolGroups.Increment();
- group = group2;
- this._connectionPoolGroups = dictionary2;
- }
- return group;
- }
- }
- if (userConnectionOptions == null)
- {
- userConnectionOptions = group.ConnectionOptions;
- }
- return group;
- }
TryGetValue是判断是否存在连接字符串为connectionString的连接，存在返回到group，不存在就调用CreateConnectionOptions创建一个DbConnectionOptions，***用
- lock (this)
- {
- Dictionary<string, DbConnectionPoolGroup> dictionary = this._connectionPoolGroups;
- if (!dictionary.TryGetValue(str, out group))
- {
- Dictionary<string, DbConnectionPoolGroup> dictionary2 = new Dictionary<string, DbConnectionPoolGroup>(1 + dictionary.Count);
- foreach (KeyValuePair<string, DbConnectionPoolGroup> pair in dictionary)
- {
- dictionary2.Add(pair.Key, pair.Value);
- }
- dictionary2.Add(str, group2); this.PerformanceCounters.NumberOfActiveConnectionPoolGroups.Increment();
- group = group2;
- this._connectionPoolGroups = dictionary2;
- }
- return group;
- }
这段代码放到连接池中，在这里，可能显示的看到，ado.NET的连接池实质上是一个Dictionary<string, DbConnectionPoolGroup>泛型集合。
所谓的连接池，就是一个与连接对象Connection相关的集合，这不只是简单的集合，而是有一定的机制在内部。我们做开发时，可能建立Connection连接对象，关闭连接对象，有时候还调用Dispose来释放连接。下次再用时，便重新实例化一个连接。但在池中的连接不随连接对象的Close或Dispose而释放。如果下次重新建立连接，连接字符串与前一次完全一模一样，则连接池就会把上次可用的连接对象赋给连接去用。如果两个连接字符串有一点不一样，即使在某一个地方多一个空格，连接池也不会以为是相同的连接，这点微软可能在内部只直接去比较两个字符串了，而不是比较连接数据库字符串的键值互相匹配。
连接池的好处就是保留连接对象，防止下次重头再来实例化一个连接对象。
- string constr1 = "Data Source=(local);Initial Catalog=AdventureWorks;Integrated Security=SSPI;";
- string constr2 = "Data Source=(local);Initial Catalog=Pubs;Integrated Security=SSPI;";
- string AssMark = "System.Data,Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089";
- Assembly ass = Assembly.Load(AssMark);
- Type SqlConType = null;
- foreach (Type conType in ass.GetExportedTypes())
- {
- Console.WriteLine(conType .ToString ());
- if ("System.Data.SqlClient.SqlConnection" == conType.ToString())
- {
- SqlConType = conType;
- }
- }
- if (SqlConType != null)
- {
- Type[] types1 = new Type[0];
- ConstructorInfo constructorInfoObj1 = SqlConType.GetConstructor(
- BindingFlags.Instance | BindingFlags.Public, null,
- CallingConventions.HasThis, types1, null);
- SqlConnection con1 = (SqlConnection)constructorInfoObj1.Invoke(null);
- con1.ConnectionString = constr1;
- SqlConnection con2 = (SqlConnection)constructorInfoObj1.Invoke(null);
- con2.ConnectionString = constr2;
- PropertyInfo PI = SqlConType.GetProperty("PoolGroup", BindingFlags.Instance | BindingFlags.NonPublic);
- object poolGroup1 = PI.GetValue(con1, null);
- object poolGroup2 = PI.GetValue(con2, null);
- }
（说明：可能找到结果后觉得非常简单，但怎么找到结果的，却是费了很大劲，几乎是5个小时，所以相把找到结果的过程简单说一下：
一开始用Reflector发现SqlConnection中有一个PoolGroup的属性，于是就想在运行时候比较两个SqlConnection对象的这个属性，但由于这个属性是的访问修饰符是internal的，不能直接访问，只有用反射，代码（是经过优化的）如下：
然后在倒数***行设置断点，为比较poolGroup1和poolGroup2的不同，结果发现，当连接字符串一样时，这两个对象的_objectID相同，字符串有一点不同就会不同，这点说明连接池中是用字符串本身比较的，而不是字符串中键值对进行比较。同还发现当con1和con2的ConnectionString不赋值时这两个对象都是null，由此说明关键是ConnectionString赋值上，所以才开始用Reflector查看这个属性的赋值方法，才有上面的代码。
作者简介：
桂素伟，微软MVP，51CTO专家堂成员。微软中文技术论坛Visual C#、IT职业规划谈两个版块版主，微软WebCast讲师。
原文标题：ado.net连接池
链接：http://www.cnblogs.com/axzxs2001/archive/2010/02/25/1673100.html
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