.NET，你忘记了么？（二）——使用using清理非托管资源

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。