问题标签 [linq]

Overview

One of the things I've asked a lot about on this site is LINQ. The questions I've asked have been wide and varied and often don't have much context behind them. So in an attempt to consolidate the knowledge I've acquired on Linq I'm posting this question with a view to maintaining and updating it with additional information as I continue to learn about LINQ.

I also hope that it will prove to be a useful resource for other people wanting to learn about LINQ.

What is LINQ?

From MSDN:

The LINQ Project is a codename for a set of extensions to the .NET Framework that encompass language-integrated query, set, and transform operations. It extends C# and Visual Basic with native language syntax for queries and provides class libraries to take advantage of these capabilities.

What this means is that LINQ provides a standard way to query a variety of datasources using a common syntax.

What flavours of LINQ are there?

Currently there are a few different LINQ providers provided by Microsoft:

• Linq to Objects which allows you to execute queries on any IEnumerable object.
• Linq to SQL which allows you to execute queries against a database in an object oriented manner.
• Linq to XML which allows you to query, load, validate, serialize and manipulate XML documents.
• Linq to Entities as suggested by Andrei
• Linq to Dataset

There are quite a few others, many of which are listed here.

What are the benefits?

• Standardized way to query multiple datasources
• Compile time safety of queries
• Optimized way to perform set based operations on in memory objects
• Ability to debug queries

So what can I do with LINQ?

Chook provides a way to output CSV files
Jeff shows how to remove duplicates from an array
Bob gets a distinct ordered list from a datatable
Marxidad shows how to sort an array
Dana gets help implementing a Quick Sort Using Linq

Where to start?

A summary of links from GateKiller's question are below:
Scott Guthrie provides an intro to Linq on his blog
An overview of LINQ on MSDN

ChrisAnnODell suggests checking out:

• Hooked on Linq
• 101 Linq Samples
• LinqPad

What do I need to use LINQ?

Linq is currently available in VB.Net 9.0 and C# 3.0 so you'll need Visual Studio 2008 or greater to get the full benefits. (You could always write your code in notepad and compile using MSBuild)

There is also a tool called LinqBridge which will allow you to run Linq like queries in C# 2.0.

Tips and tricks using LINQ

This question has some tricky ways to use LINQ

我有一个调用接口IStep，可以进行一些计算（参见“名词王国中的执行”）。在运行时，我想按类名选择适当的实现。

我有一个 System.Generic.Collections.List(Of MyCustomClass) 类型的对象。

给定整数变量 pagesize 和 pagenumber，如何仅查询 MyCustomClass 对象的任何单页？

我似乎遗漏了一些关于 LINQ 的东西。对我来说，它看起来像是将一些我最不喜欢的 SQL 元素转移到 C# 语言中，并将它们用于其他事情。

我的意思是，我可以看到在数据库以外的事物上使用类似 SQL 的语句的好处。但是，如果我想编写 SQL，那么为什么不直接编写 SQL 并将其排除在 C# 之外呢？我在这里想念什么？

我在尝试基于 NHibernate 测试以下 IRepository 时遇到了问题：

地狱如何以IQueryable<T>返回给定集合以交换某些表达式的方式模拟返回。我觉得我对IQueryable<T>……有些误解

Reading an article called "Increase LINQ Query Performance" in July's MSDN magazine, the author states that using an Imports in VB providing a path to schema in the current project will turn IntelliSense on for XElement. In the code provided, he uses statements like xelement.@name to retreive attributes values and so on.

I did not try this out myself in VB but I would like to use that in C#. This really looks like LINQ to XSD.

Is there any equivalent in C#? It seems that it is not possible to use a namespace inside C# code, there is no using equivalent to this Import statement.

作为一个没有在实际项目中使用过这两种技术的人，我想知道是否有人知道这两种技术如何相互补充以及它们的功能有多少重叠？

我在获取使用 Paging 并使用 LinqDataSource 作为数据源的 Gridview 中显示的项目的总行数时遇到问题。

我尝试了几种方法：

每次返回 -1。

只给我当前页面的计数，而不是总数。

还有其他建议吗？

在不枚举集合中的所有项目的情况下获取 IEnumerable 集合中的项目计数的最佳/最简单方法是什么？

可以使用 LINQ 或 Lambda 吗？

看到一篇关于 C# 中隐藏功能的帖子，但没有很多人写过 linq/lambdas 示例，所以......我想知道......

你见过/写过的 C# LINQ 和/或 Lambdas/匿名委托的最酷（最优雅的）用法是什么？

如果它也已投入生产，将获得奖励！