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.NET,你忘记了么?(二)——使用using清理非托管资源

原创 Linux操作系统 作者:iDotNetSpace 时间:2009-04-06 13:38:24 0 删除 编辑

我们都知道,垃圾回收可以分为Dispose和Finalize两类,关于这两者的区别已经太多了,一个是正常的垃圾回收GC所调用的方法,另外一个是终结器Finalizer,所调用的方法,在Effective C#一书中,有着明确的建议是说使用IDispose接口来代替Finalize。原因是因为Finalize终结会增加垃圾回收对象的代数,从而影响垃圾回收。

有了上述的原因,我们现在只来看使用IDispose接口的类。

在.NET中,绝大多数的类都是运行在托管的环境下,所以都由GC来负责回收,那么我们就不需要实现IDispose接口,而是由GC来自动负责。可是有一些类使用的是非托管资源,那么这个时候,我们就应该去实现IDispose接口,说个比较常用的SqlConnection之类。

写段常用的连接SQL语句的模型:

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();

 

其实,作为非托管资源,为了防止我们忘记调用Close,一般都实现了Finalize,因此,即使我们没有Close掉,也会由终结器将这块内存回收。但是,就增加了这块垃圾的代数。

假设说我们写了这样的代码:

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语句错误 thisCommand.ExecuteNonQuery(); thisConnection.Close();

 

这样的话,我们打开的SqlConnection就没有关闭,只能等待Finalize去关闭了。

这是非常不好的做法。于是,我们可以想到异常处理:

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(); } }

 

这样做就不错了,但是代码看起来有些丑陋,可是使用using就让代码优雅了很多,这也是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(); }

 

代码量是不是小了很多呢?优雅了许多呢!

其实,在IL的位置,代码仍然是一样的,他同样把代码给编译成了try-finally的处理形式!

接下来,再来看下我们常用的使用数据库的方式:

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();

 

还是上面的问题,我们考虑用using语句来将之代码重构:

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 } } }

我先把这段代码翻译成我们熟悉的try-finally的处理形式:

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(); }

更丑陋的代码吧!所以有个原则是:尽量避免using语句的嵌套。

怎么样解决呢?很容易,自己写我们的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(); } }

这样就好了!

关于using 的这节我就写到这,最后对全文做个总结,其实就是一句话:尽量使用using来进行非托管资源的资源回收。

来自 “ ITPUB博客 ” ,链接:http://blog.itpub.net/12639172/viewspace-580343/,如需转载,请注明出处,否则将追究法律责任。

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