C＃使用语句的重点是什么？

发布于 2025-01-27 17:50:31 字数 286 浏览 2 评论 0原文

我不是在谈论对组件的引用，而是代码中的使用语句。

例如，这有什么区别：

using (DataReader dr = .... )
{
    ...stuff involving data reader...
}

这是什么：

{
    DataReader dr = ...

    ...stuff involving data reader...
}

当垃圾收集器出现时，数据标准肯定会被垃圾收集器清除吗？

原文

I'm not talking about references to assemblies, rather the using statement within the code.

For example what is the difference between this:

using (DataReader dr = .... )
{
    ...stuff involving data reader...
}

and this:

{
    DataReader dr = ...

    ...stuff involving data reader...
}

Surely the DataReader is cleaned up by the garbage collector when it goes out of scope anyway?

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迷爱 2025-02-03 17:50:32

使用语句的A 的点是，您使用该语句创建的对象隐含在块末尾处置。在您的第二个代码段中，数据读取器永远不会关闭。这是一种确保不再将一次性资源保留在要求的方法，即使抛出了例外，也将被释放。这是：

using (var obj = new SomeDisposableType())
{
    // use obj here.
}

在功能上等效：

var obj = new SomeDisposableType();

try
{
    // use obj here.
}
finally
{
    obj.Dispose();
}

您可以使用相同的范围对多个可一次性对象这样的范围：

using (var obj1 = new SomeDisposableType())
using (var obj2 = new SomeOtherDisposableType())
{
    // use obj1 and obj2 here.
}

如果您需要交织其他代码，例如

var table = new DataTable();

using (var connection = new SqlConnection("connection string here"))
using (var command = new SqlCommand("SQL query here", connection))
{
    connection.Open();

    using (var reader = command.ExecuteReader()
    {
        table.Load(reader);
    }
}

The point of a using statement is that the object you create with the statement is implicitly disposed at the end of the block. In your second code snippet, the data reader never gets closed. It's a way to ensure that disposable resources are not held onto any longer than required and will be released even if an exception is thrown. This:

using (var obj = new SomeDisposableType())
{
    // use obj here.
}

is functionally equivalent to this:

var obj = new SomeDisposableType();

try
{
    // use obj here.
}
finally
{
    obj.Dispose();
}

You can use the same scope for multiple disposable objects like so:

using (var obj1 = new SomeDisposableType())
using (var obj2 = new SomeOtherDisposableType())
{
    // use obj1 and obj2 here.
}

You only need to nest using blocks if you need to interleave other code, e.g.

var table = new DataTable();

using (var connection = new SqlConnection("connection string here"))
using (var command = new SqlCommand("SQL query here", connection))
{
    connection.Open();

    using (var reader = command.ExecuteReader()
    {
        table.Load(reader);
    }
}

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