LInq 学习笔记

 学习网站http://www.tutorialsteacher.com/linq/linq-joining-operator-join

// Student collection

IList<Student> studentList = new List<Student>>() {

        newStudent() { StudentID = 1, StudentName= "John", Age = 13} ,

        newStudent() { StudentID = 2, StudentName= "Moin",  Age = 21 } ,

        newStudent() { StudentID = 3, StudentName= "Bill",  Age = 18 } ,

        newStudent() { StudentID = 4, StudentName= "Ram" , Age = 20} ,

        newStudent() { StudentID = 5, StudentName= "Ron" , Age = 15 }

    };

 

// LINQ Query Syntax to find out teenager students

var teenAgerStudent = from s in studentList

                     where s.Age > 12 && s.Age< 20

                     select s;

   var treestudent= studentList.where(s=>s.Age>20);

var teenAgerStudents = studentList.Where(s => s.Age > 12 && s.Age < 20)

                                  .ToList<Student>();

 

s =>

{

   int youngAge = 18;

    Console.WriteLine("Lambda expression with multiple statements in the body");

    return s.Age >= youngAge;

}

1 where

IList<Student> studentList = newList<Student>() { 

        newStudent() { StudentID = 1, StudentName = "John", Age = 13} ,

        newStudent() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,

        newStudent() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,

        newStudent() { StudentID = 4, StudentName = "Ram" , Age = 20} ,

        newStudent() { StudentID = 5, StudentName = "Ron" , Age = 15 } 

    };

var filteredResult = from s in studentList

                    where s.Age > 12 && s.Age < 20

                    select s.StudentName;

---------------------------------------------------

public static void Main()

{

    var filteredResult = from s in studentList

                         where isTeenAger(s)

                         select s;

}

public static bool IsTeenAger(Student stud)

{

    return stud.Age > 12 && stud.Age < 20;  

}

--------------------------------------------------------

IList<Student> studentList = newList<Student>() { 

        newStudent() { StudentID = 1, StudentName = "John", Age = 18 } ,

        newStudent() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,

        newStudent() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,

        newStudent() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,

        newStudent() { StudentID = 5, StudentName = "Ron" , Age = 19 } 

    };

var filteredResult = studentList.Where((s, i) => { 

            if(i % 2 ==  0) // if it is even element

                returntrue;

        returnfalse;

    });

foreach (var std in filteredResult)

        Console.WriteLine(std.StudentName);

----------------------------------------------------------------------------

var filteredResult = from s in studentList

                   where s.Age > 12

                   where s.Age < 20

                   select s;

 

var filteredResult = studentList.Where(s => s.Age > 12).Where(s=> s.Age < 20);

OfType

IList mixedList = newArrayList();

mixedList.Add(0);

mixedList.Add("One");

mixedList.Add("Two");

mixedList.Add(3);

mixedList.Add(new Student() { StudentID = 1, StudentName = "Bill" });

var stringResult = from s in mixedList.OfType<string>()// result: One Two

                   select s;

var intResult = from s in mixedList.OfType<int>()//Result 0 3

                select s;

OrderBy

IList<Student> studentList = newList<Student>() { 

    newStudent() { StudentID = 1, StudentName = "John", Age = 18 } ,

    newStudent() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,

    newStudent() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,

    newStudent() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,

    newStudent() { StudentID = 5, StudentName = "Ron" , Age = 19 } 

};

var orderByResult = from s in studentList

                   orderby s.StudentName 

                   select s;

var orderByDescendingResult = from s in studentList

                   orderby s.StudentName descending

                   select s;

IList<Student> studentList = newList<Student>() { 

    newStudent() { StudentID = 1, StudentName = "John", Age = 18 } ,

    newStudent() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,

    newStudent() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,

    newStudent() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,

    newStudent() { StudentID = 5, StudentName = "Ron" , Age = 19 } 

};

var studentsInAscOrder = studentList.OrderBy(s => s.StudentName);

IList<Student> studentList = newList<Student>() { 

    newStudent() { StudentID = 1, StudentName = "John", Age = 18 } ,

    newStudent() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,

    newStudent() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,

    newStudent() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,

    newStudent() { StudentID = 5, StudentName = "Ron" , Age = 19 } 

};

var studentsInDescOrder = studentList.OrderByDescending(s => s.StudentName);

IList<Student> studentList = newList<Student>() { 

    newStudent() { StudentID = 1, StudentName = "John", Age = 18 } ,

    newStudent() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,

    newStudent() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,

    newStudent() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,

    newStudent() { StudentID = 5, StudentName = "Ron" , Age = 19 }, 

    newStudent() { StudentID = 6, StudentName = "Ram" , Age = 18 }

};

var orderByResult = from s in studentList

                   orderby s.StudentName, s.Age 

                   select new { s.StudentName, s.Age };

ThenBy  MUti order

IList<Student> studentList = newList<Student>() { 

    newStudent() { StudentID = 1, StudentName = "John", Age = 18 } ,

    newStudent() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,

    newStudent() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,

    newStudent() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,

    newStudent() { StudentID = 5, StudentName = "Ron" , Age = 19 }, 

    newStudent() { StudentID = 6, StudentName = "Ram" , Age = 18 }

};

var thenByResult = studentList.OrderBy(s => s.StudentName).ThenBy(s => s.Age);

var thenByDescResult = studentList.OrderBy(s => s.StudentName).ThenByDescending(s => s.Age);

GroupBy & ToLookup

IList<Student> studentList = new List<Student>() {

        new Student() { StudentID = 1, StudentName= "John", Age = 18 } ,

        new Student() { StudentID = 2, StudentName= "Steve",  Age = 21 } ,

        new Student() { StudentID = 3, StudentName= "Bill",  Age = 18 } ,

        new Student() { StudentID = 4, StudentName= "Ram" , Age = 20 } ,

        new Student() { StudentID = 5, StudentName= "Abram" , Age = 21 }

    };

 

var groupedResult = from s in studentList

                   group s by s.Age;

 

//iterate each group       

foreach (var ageGroup in groupedResult)

{

    Console.WriteLine("Age Group: {0}", ageGroup .Key); //Each grouphas a key

            

    foreach(Student s in ageGroup) // Each grouphas inner collection

        Console.WriteLine("Student Name: {0}", s.StudentName);

--------------------------------------------

var groupedResult = studentList.GroupBy(s =>s.Age);

 

ToLookup

ToLookup is the same as GroupBy; the only difference is GroupBy execution is deferred, whereas ToLookup execution is immediate. Also, ToLookup is only applicable in Method syntax. ToLookup is not supported in the query syntax.

Example: ToLookup in method syntax C#

IList<Student> studentList = new List<Student>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
        new Student() { StudentID = 2, StudentName = "Steve",  Age = 21 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
        new Student() { StudentID = 5, StudentName = "Abram" , Age = 21 } 
    };

var lookupResult = studentList.ToLookup(s => s.age);

foreach (var group in lookupResult)
{
    Console.WriteLine("Age Group: {0}", group.Key);  //Each group has a key 
             
    foreach(Student s in group)  //Each group has a inner collection  
        Console.WriteLine("Student Name: {0}", s.StudentName);
}

Join

The result like the opration inner jion in sql

Join in Method Syntax:

Join() method overloads:

public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this IEnumerable<TOuter> outer, 
                                                        IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, 
                                                        Func<TInner, TKey> innerKeySelector, 
                                                        Func<TOuter, TInner, TResult> resultSelector);

public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this IEnumerable<TOuter> outer, 
                                                        IEnumerable<TInner> inner, 
                                                        Func<TOuter, TKey> outerKeySelector,
                                                        Func<TInner, TKey> innerKeySelector, 
                                                        Func<TOuter, TInner, TResult> resultSelector,
                                                        IEqualityComparer<TKey> comparer);

As you can see in the first overload method takes five input parameters (except the first 'this' parameter): 1) outer 2) inner 3) outerKeySelector 4) innerKeySelector 5) resultSelector.

Let's take a simple example. The following example joins two string collection and return new collection that includes matching strings in both the collection.

Example: Join operator C#

IList<string> strList1 = new List<string>() { 
    "One", 
    "Two", 
    "Three", 
    "Four"
};

IList<string> strList2 = new List<string>() { 
    "One", 
    "Two", 
    "Five", 
    "Six"
};

var innerJoin = strList1.Join(strList2,
                      str1 => str1, 
                      str2 => str2, 
                      (str1, str2) => str1);

The following example demonstrates LINQ Join query.

Example: Join Query C#

IList<Student> studentList = new List<Student>() { 
    new Student() { StudentID = 1, StudentName = "John", StandardID =1 },
    new Student() { StudentID = 2, StudentName = "Moin", StandardID =1 },
    new Student() { StudentID = 3, StudentName = "Bill", StandardID =2 },
    new Student() { StudentID = 4, StudentName = "Ram" , StandardID =2 },
    new Student() { StudentID = 5, StudentName = "Ron"  } 
};

IList<Standard> standardList = new List<Standard>() { 
    new Standard(){ StandardID = 1, StandardName="Standard 1"},
    new Standard(){ StandardID = 2, StandardName="Standard 2"},
    new Standard(){ StandardID = 3, StandardName="Standard 3"}
};

var innerJoin = studentList.Join(// outer sequence 
                      standardList,  // inner sequence 
                      student => student.StandardID,    // outerKeySelector
                      standard => standard.StandardID,  // innerKeySelector
                      (student, standard) => new  // result selector
                                    {
                                        StudentName = student.StudentName,
                                        StandardName = standard.StandardName
                                    });

The following example of Join operator in query syntax returns a collection of elements from studentList and standardList if their Student.StandardID and Standard.StandardID is match.

Example: Join operator in query syntax C#

IList<Student> studentList = new List<Student>() { 
    new Student() { StudentID = 1, StudentName = "John", Age = 13, StandardID =1 },
    new Student() { StudentID = 2, StudentName = "Moin",  Age = 21, StandardID =1 },
    new Student() { StudentID = 3, StudentName = "Bill",  Age = 18, StandardID =2 },
    new Student() { StudentID = 4, StudentName = "Ram" , Age = 20, StandardID =2 },
    new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 } 
};

IList<Standard> standardList = new List<Standard>() { 
    new Standard(){ StandardID = 1, StandardName="Standard 1"},
    new Standard(){ StandardID = 2, StandardName="Standard 2"},
    new Standard(){ StandardID = 3, StandardName="Standard 3"}
};

var innerJoin = from s in studentList // outer sequence
                      join st in standardList //inner sequence 
                      on s.StandardID equals st.StandardID // key selector 
                      select new { // result selector 
                                    StudentName = s.StudentName, 
                                    StandardName = st.StandardName 
                                };