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This article mainly introduces the example analysis of LINQ query syntax, the article is very detailed, has a certain reference value, interested friends must read it!

Clocking 3.0 LINQ query syntax

First, let's take a look at a very simple example of a LINQ query that queries an int array of numbers less than 5 and arranges them in size order:

Class Program {static void Main (string [] args) {int [] arr = new int [] {8,5,89,3,56,4,1,58}; var m = from n in arr where n < 5 orderby n select n; foreach (var n in m) {Console.WriteLine (n);} Console.ReadLine ();}}

The above code is familiar to us except the LINQ query syntax, while the LINQ query syntax is very familiar with the SQL query syntax, except for the sequence.

Q: why does the LINQ query syntax start with the from keyword instead of the select keyword? At the beginning of select, this way of writing is closer to and easier to understand with SQL?

A: to put it simply, the select keyword is placed at the back for the Intelisence function of IDE.

It is not uncommon for programming languages to write LINQ query syntax at the beginning of select. If you have used the 2005 version of VB9 CTP, the LINQ query syntax of VB9 is the select keyword first, but the select keyword comes first, which is very big when doing Intelligent perception (Intelisence). After the tradeoff of the Microsoft IDE group, it is determined to put the from keyword first.

For example: if you look at the http://blog.joycode.com/saucer/archive/2005/09/16/63513.aspx blog, at that time the query syntax of VB9 LINQ was still at the top of the select parameter. But then the VB9 beta was changed to do the same thing as C #, with the from keyword at the top.

Suppose you want to write code like this: Select p.Name, p.Age From p In persons Where xxx, the code is entered character by character. Before we wrote about p in persons, the type of p could not be speculated, so we wrote Select p. Properties such as Name do not pop up with smart prompts. In this way, you need to write the sentence From first, and then come back to write Select. After much deliberation, the Microsoft IDE group decided that it would be better to write Select later. So the writing method in the programming language is determined to be written in this way.

Let's look at a slightly more complex LINQ query:

In the language strings we have listed, we want to list them according to the length of the characters. The implementation code is as follows:

Static void Main (string [] args)

{

String [] languages = {"Java", "C#", "C++", "Delphi", "VB.net"

"VC.net", "C++ Builder", "Kylix", "Perl", "Python"}

Var query = from item in languages

Orderby item

Group item by item.Length into lengthGroups

Orderby lengthGroups.Key descending

Select lengthGroups

Foreach (var item in query)

{

Console.WriteLine ("strings of length", item.Key)

Foreach (var val in item)

{

Console.WriteLine (val)

}

}

Console.ReadLine ()

}

The into keyword indicates that the result of the previous query is treated as the generator of the subsequent query, which is used with group by.

Group by in LINQ should not be confused with Group by in SQL. Because SQL is a two-dimensional structure, some logic of Group by is constrained by two-dimensional structure, so it can not be as flexible as Group by in LINQ.

The above is all the contents of the article "sample Analysis of LINQ query Grammar". Thank you for reading! Hope to share the content to help you, more related knowledge, welcome to follow the industry information channel!

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