为什么要关闭数据库连接，可以不关闭吗？
首先要说明的是连接数是有限制的： 代码如下： for ( int i = 0; i 10000; i++){ SqlConnection conn = new SqlConnection( @Data Source=.\SQLEXPRESS; AttachDbFilename= E:\DB\NORTHWND.mdf ; Integrated Security=True;Connect Timeout=30;User Instance
首先要说明的是连接数是有限制的：
代码如下：
<span>for</span> (<span>int</span> i = 0; i new SqlConnection(<span>@"Data Source=.\SQLEXPRESS;
AttachDbFilename="</span><span>"E:\DB\NORTHWND.mdf"</span><span>";
Integrated Security=True;Connect Timeout=30;User Instance=True"</span>);
conn.Open();
Console.WriteLine(<span>"打开了{0}个连接"</span>, i);
}
运行结果如下：
过一会就会提示打开连接超时了：
可以看到数据库连接时有限制的，如果连接不关闭，而且使用的人比较多，那么系统很快就down掉了。
但是有时候由于某些原因应用程序可能只是几个人使用，所以就有人设计了：
在应用程序启动的时候打开数据库连接，在应用程序关闭的时候关闭数据库连接
那么使用这种方式有什么问题呢？
首先假设有一张表Nums，表定义如下：
Main代码如下：
SqlConnection conn = <span>new</span> SqlConnection(<span>@"Data Source=.\SQLEXPRESS;
AttachDbFilename="</span><span>"E:\DB\NORTHWND.mdf"</span><span>";
Integrated Security=True;Connect Timeout=30;User Instance=True"</span>);
conn.Open();
Parallel.For(1, 9999, (id) =>
{
ExecuteCommand(conn, id);
});
就是从1到9999开始执行ExecuteCommand
ExecuteCommand代码如下：
<span>private</span> <span>static</span> <span>void</span> ExecuteCommand(SqlConnection conn, <span>int</span> id)
{
Console.WriteLine(<span>"正在执行."</span> + id);
Thread.Sleep(100);
SqlCommand cmd = <span>new</span> SqlCommand(
<span>string</span>.Format(<span>"Insert into Nums values('{0}') "</span>, id), conn);
cmd.ExecuteNonQuery();
}
运行：
可以看到ExecuteNonQuery方法抛出了异常，原因是连接处于关闭状态。
可是我们的连接一直都是open着的啊，并没有调用close，dispose之类的方法啊。
于是在ExecuteCommand前面增加判断条件：
<span>if</span> (conn.State != System.Data.ConnectionState.Open)
conn.Open();
再次运行：
可以看到还是会出现连接已关闭的问题。你知道什么原因吗？
这里是由于多线程环境引起的。所以需要加锁。
<pre class="brush:php;toolbar:false"><span>private</span> <span>static</span> <span>object</span> syncObj = <span>new</span> <span>object</span>();
<span>private</span> <span>static</span> <span>void</span> ExecuteCommand(SqlConnection conn, <span>int</span> id)
{
<span>lock</span> (syncObj)
{
<span>if</span> (conn.State != System.Data.ConnectionState.Open)
conn.Open();
Console.WriteLine("<span>正在执行..</span>" + id);
Thread.Sleep(100);
SqlCommand cmd = <span>new</span> SqlCommand(
<span>string</span>.Format("<span>Insert into Nums values('{0}') </span>", id), conn);
cmd.ExecuteNonQuery();
}
}
再次运行：可以发现基本没问题了.
修改Parallel.For的最大值上限，要测试下是否可以长期执行了。
<pre class="brush:php;toolbar:false">Parallel.For(1, Int32.MaxValue, (id) =>
{
ExecuteCommand(conn, id);
});
一天测试下来，没出现任何问题。
结论：对于某些只有几个人使用的应用程序，可以不关闭数据库连接，但是在写代码的时候最好要加上连接是否打开的判断。
你有什么好的看法呢，欢迎留下!
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