我們都知道，垃圾回收可以分為Dispose和Finalize兩類，關於這兩者的區別已經太多了 ，一個是正常的垃圾回收GC所調用的方法，另外一個是終結器Finalizer，所調用的方法，在 Effective C#一書中，有著明確的建議是說使用IDispose接口來代替Finalize。原因是因為 Finalize終結會增加垃圾回收對象的代數，從而影響垃圾回收。
有了上述的原因，我們現在只來看使用IDispose接口的類。
在.NET中，絕大多數的類都是運行在托管的環境下，所以都由GC來負責回收，那麼我們就 不需要實現IDispose接口，而是由GC來自動負責。可是有一些類使用的是非托管資源，那麼 這個時候，我們就應該去實現IDispose接口，說個比較常用的SqlConnection之類。
寫段常用的連接SQL語句的模型：
string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings ["Study1ConnectionString1"].ConnectionString;
SqlConnection thisConnection = new SqlConnection(connectionString);
thisConnection.Open();
SqlCommand thisCommand = new SqlCommand();
thisCommand.Connection = thisConnection;
thisCommand.CommandText = "select * from [User]";
thisCommand.ExecuteNonQuery();
thisConnection.Close();
其實，作為非托管資源，為了防止我們忘記調用Close，一般都實現了Finalize，因此， 即使我們沒有Close掉，也會由終結器將這塊內存回收。但是，就增加了這塊垃圾的代數。
假設說我們寫了這樣的代碼：
string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings ["Study1ConnectionString1"].ConnectionString;
SqlConnection thisConnection = new SqlConnection(connectionString);
thisConnection.Open();
SqlCommand thisCommand = new SqlCommand();
thisCommand.Connection = thisConnection;
thisCommand.CommandText = "select * form [User]"; //SQL語句錯誤
thisCommand.ExecuteNonQuery();
thisConnection.Close();
這樣的話，我們打開的SqlConnection就沒有關閉，只能等待Finalize去關閉了。
這是非常不好的做法。於是，我們可以想到異常處理：
SqlConnection thisConnection = null;
try
{
string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings ["Study1ConnectionString1"].ConnectionString;
thisConnection = new SqlConnection(connectionString);
thisConnection.Open();
SqlCommand thisCommand = new SqlCommand();
thisCommand.Connection = thisConnection;
thisCommand.CommandText = "select * form [User]";
thisCommand.ExecuteNonQuery();
}
finally
{
if (thisConnection != null)
{
thisConnection.Close();
}
}
這樣做就不錯了，但是代碼看起來有些丑陋，可是使用using就讓代碼優雅了很多，這也 是C#比JAVA棒很多的地方，呵呵！
string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings ["Study1ConnectionString1"].ConnectionString;
using (SqlConnection thisConnection = new SqlConnection())
{
thisConnection.Open();
SqlCommand thisCommand = new SqlCommand();
thisCommand.Connection = thisConnection;
thisCommand.CommandText = "select * form [User]";
thisCommand.ExecuteNonQuery();
}
代碼量是不是小了很多呢？優雅了許多呢！
其實，在IL的位置，代碼仍然是一樣的，他同樣把代碼給編譯成了try-finally的處理形 式！
接下來，再來看下我們常用的使用數據庫的方式：
string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings ["Study1ConnectionString1"].ConnectionString;
SqlConnection thisConnection = new SqlConnection(connectionString);
thisConnection.Open();
SqlCommand thisCommand = new SqlCommand();
thisCommand.Connection = thisConnection;
thisCommand.CommandText = "select * from [User]";
SqlDataReader thisReader = thisCommand.ExecuteReader();
thisReader.Close();
thisConnection.Close();
還是上面的問題，我們考慮用using語句來將之代碼重構：
string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings ["Study1ConnectionString1"].ConnectionString;
using (SqlConnection thisConnection = new SqlConnection (connectionString))
{
thisConnection.Open();
SqlCommand thisCommand = new SqlCommand();
thisCommand.Connection = thisConnection;
thisCommand.CommandText = "select * from [User]";
using (SqlDataReader reader = thisCommand.ExecuteReader())
{
while (reader.Read())
{
//操作
}
}
}
我先把這段代碼翻譯成我們熟悉的try-finally的處理形式：
SqlConnection thisConnection = null;
try
{
string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings ["Study1ConnectionString1"].ConnectionString;
thisConnection = new SqlConnection(connectionString);
thisConnection.Open();
SqlCommand thisCommand = new SqlCommand();
thisCommand.Connection = thisConnection;
thisCommand.CommandText = "select * from [User]";
SqlDataReader reader = null;
try
{
reader = thisCommand.ExecuteReader();
while (reader.Read())
{
//操作
}
}
finally
{
reader.Close();
}
}
finally
{
thisConnection.Close();
}
更丑陋的代碼吧！所以有個原則是：盡量避免using語句的嵌套。
怎麼樣解決呢？很容易，自己寫我們的try-finally吧！
SqlConnection thisConnection = null;
SqlDataReader reader = null;
try
{
string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings ["Study1ConnectionString1"].ConnectionString;
thisConnection = new SqlConnection(connectionString);
thisConnection.Open();
SqlCommand thisCommand = new SqlCommand();
thisCommand.Connection = thisConnection;
thisCommand.CommandText = "select * from [User]";
reader = thisCommand.ExecuteReader();
while (reader.Read())
{
//操作
}
}
finally
{
if (thisConnection != null)
{
thisConnection.Close();
}
if (reader != null)
{
reader.Close();
}
}
這樣就好了！
關於using 的這節我就寫到這，最後對全文做個總結，其實就是一句話：盡量使用using 來進行非托管資源的資源回收。
