linq-Basic Query Operator Select/where statement

The basic query operations in LINQ are the same as those in SQL, and for similarities and differences, let's look at the following:

1) Select

Grammar:

<span style= "FONT-FAMILY:SIMHEI;FONT-SIZE:18PX;" > public static ienumerable<tresult> select<tsource,tresult> (this ienumerable<tsource>source, Func<tsource,tresult>selector) </span>

Description:

• The Select method itself is a generic collection extension method
  
• It acts on the ienumerable<tsource> type
• It only accepts one func<tsource,tresult> type parameter
• Func<tsource,tresult> is a generic delegate, located under the System namespace, in the system Core DLL, where selector is an extractor.
  

2) Where

Grammar:

<span style= "FONT-FAMILY:SIMHEI;FONT-SIZE:18PX;" >public static ienumerable<tsource> where<tsource> (This ienumerable<tsource>source,func< Tsource,bool>predicate) </span>

Description:

• The Where method is also a generic extension method
• It acts on the ienumerable<tsource> type as with select ()
• It accepts only one func<tsource,bool> generic delegate parameter
• The predicate here is a condition of judgment.
  

3) Examples:

First, let's write an extension class that provides the output method for ienumerablet<string>

<span style= "FONT-FAMILY:SIMHEI;FONT-SIZE:18PX;" >//Extension classes, as long as the static can be public    static class Extraclass    {        //for ienumerablet<string> to provide output method        public static void Print (this ienumerable<string> ie)        {            //Gets the interface that can be traversed            ienumerator<string> result = ie. GetEnumerator ();            Console.WriteLine ("\ n-------------------------------------\ n");            while (result. MoveNext ())            {                Console.WriteLine (result). current);            }            Console.WriteLine ("\ n---------------------------------------\ n");}    } </span>

write a generic collection and store several data for it:

<span style= "FONT-FAMILY:SIMHEI;FONT-SIZE:18PX;" >  private void Btnselect_click (object sender, EventArgs e)        {            //linq to Objects            //generic collection data persons            list<string> persons = new list<string> ();            Persons. Add ("Li si");            Persons. Add ("Meng Meng");            Persons. ADD ("Huo Huo");            Persons. Add ("Niu nan");            Persons. Add ("Hu Hu");            Persons. Add ("Tu zi");            Persons. ADD ("Huo er");            Persons. Add ("Huo Xu");</span>

This allows us to display the output according to the selection:

1. Output all elements in person:

<span style= "FONT-FAMILY:SIMHEI;FONT-SIZE:18PX;" >//Output all the elements in person var result=persons. Select (p = p);</span>

Show Results:

2, output person in the surname Huo (the following three statements all show the same effect)

<span style= "FONT-FAMILY:SIMHEI;FONT-SIZE:18PX;" >//Output person in the surname Huo (the following three statements must answer the same effect)    //var result=persons. Where (P=>p.startswith ("Huo"));   var result = persons. Select (p=>p). Where (p = p.startswith ("Huo"));  var result = persons. Where (P = = P.startswith ("Huo")). Select (p=>p);</span>

Show the effect:

Of course, you can also directly use where, using bool delegate parameters to execute,

<span style= "FONT-FAMILY:SIMHEI;FONT-SIZE:18PX;" >  var result=persons. Where (P=>judge (P));                   Result. Print ();        }        public bool Judge (string s)        {            if (S.startswith ("Huo"))            {                return true;            }            else            {                return false;            }        } </span>

Displays the same effect, but directly shows the use of the WHERE () statement

Examples are just a few common query statements, others in the continuing study!

linq-Basic Query Operator Select/where statement