LINQ study notes

本文介绍了LINQ的基本概念及语法,并深入探讨了高级查询操作,包括排序、数学方法、选择特定成员变量等技巧。

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LINQ study notes

1. LINQ concept:

LINQ stands for language integrated Query. You can retrieval datafrom various sources (SQL database, list, object, xml, array) by just using LINQ,instead of different languages.

 

2. Basic grammar:

  <code>

Var queryResult =

  From nin names

  Wheren.StartWith(“S”)

  orderbyn descending

  Selectn;

</code>

var keyword: to declare a variable without figuring out the type of it.The compiler will infer the type for it based on the result type.

In this example, ‘names’ is the data source for this query. The datasource must implement theIEnumerableinterface.

Deferred Execution: The query result variable will only save the plan of retrievaluntil the result is visited.

Lambda operator ( => ):read as goes to or such that.

 

3. AdvancedOperation keywords:

  Order by:

  Orderbyn            or    names.OrderBy(n=>n)

  Orderbyn descending   or   names.OrderByDescending(n=>n)

  Orderbyobj.a1, obj.a2, obj.a3   or   objs.Orderby(…).ThenbyDescending(…).Thenby(…).select(…)

  Mathmethod:

queryResult.Count(),qR.Max(), qR.Min(), qR.Average(), qR.Sum(n=>(long)n)

  While retrieval from object and hope to only returnparticular member variables of object:

  ……..select object.id=1          or   objs.select(obj => obj.id = 1   

 ……. Selectnew {obj.a1, obj.a2, obj.a3 }    or   objs.select(obj => new { obj.a1, obj.a2, obj.a3}

  Distinct:

  Objs.select(obj=>……).distinct();

  Anyand All: to check whether the particular record or all records meet therequirement and return a Boolean result:

  bool

  boolanyUSA = objs.Any( obj => obj.country == “USA”)

bool allUSA = objs.All ( obj=>obj.country == “USA”);

  Groupby:

  Group objby obj.Region into cg

Grouping therecords by Region and save the group name as cg.

 TOP:

  Foreach( var item in objs.Take(5)){……}

Do operation ontop five records. Remember to order the records first, if it is necessary.

  Foreach(var item in objs.Skip(5)){…..}

Ignore the top five records while doing operation on the result group.

 First and FirstOrDefault:

qs.First( obj=>obj.Region == “Africa”);

qs.FirstOrDefault(obj=>obj.Region == “Africa”);

      These twostatement both return the first record in the records set. I believe theFirstOrDefault is better in most of cases, as you don’t need to worry about theerror processing if the result is null;


4. Invoke procedure using LINQ to SQL

    Just simplely use the precedure name as a function will make it. For example:

    List<obj> = dbContext.StoredProcedure1();


   



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