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，最后用
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。
（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：
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);
}
然后在倒数第一行设置断点，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。）