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LINQ in Detail

LINQ in Detail

https://www.dotnettricks.com/learn/linq/sql-joins-with-csharp-linq

 Single() vs First()
Single will throw an exception if it finds more than one record matching the criteria. First will always select the first record from the list. If the query returns just 1 record, you can go with First().
Both will throw an InvalidOperationException exception if the collection is empty. Alternatively you can use SingleOrDefault(). This won't throw an exception if the list is empty

 _____________________________________________________________________________
Offtype: Use OfType operator to filter the above collection based on each element's type

ex: 
IList mixedList = new ArrayList();
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>()
                   select s;

________________________________________________________________

Orderby: A sorting operator arranges the elements of the collection in ascending or descending order. OrderBy sorts the values of a collection in ascending or descending order. It sorts the collection in ascending order by default because ascendingkeyword is optional here. Use descending keyword to sort collection in descending order.

IList<Student> studentList = new List<Student>() { 
    new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
    new Student() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,
    new Student() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,
    new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
    new Student() { 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;

_______________________________________________________________

 THENBY and THEN
Multiple sorting in method syntax is supported by using ThenBy and ThenByDescending extension methods.

 
IList<Student> studentList = new List<Student>() { 
    new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
    new Student() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,
    new Student() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,
    new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
    new Student() { StudentID = 5, StudentName = "Ron" , Age = 19 }, 
    new Student() { 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);


thenByDescResult would contain following elements. Ram with age 20 comes before Ram with age 18 because it has used ThenByDescending.


  ____________________________________________________________

 GROUPBY

 

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;





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

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

 AgeGroup: 18
StudentName: John
StudentName: Bill
AgeGroup: 21
StudentName: Steve
StudentName: Abram
AgeGroup: 20
StudentName: Ram
__________________________________________________________ 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

 
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



let's understand join metohod using following Student and Standard class where Student class includes StandardID that matches with StandardID of Standard class.






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
                                    });






Join operator in query syntax C#








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 
                                };






Join multiple tables method syntax


var categorizedProducts = product
    .Join(productcategory, p => p.Id, pc => pc.ProdId, (p, pc) => new { p, pc })
    .Join(category, ppc => ppc.pc.CatId, c => c.Id, (ppc, c) => new { ppc, c })
    .Select(m => new { 
        ProdId = m.ppc.p.Id, // or m.ppc.pc.ProdId
        CatId = m.c.CatId
        // other assignments
    });

______________________________________________________________________________










ALL 

The All operator evalutes each elements in the given collection on a specified condition and returns True if all the elements satisfy a condition.






IList<Student> studentList = new List<Student>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
        new Student() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
        new Student() { StudentID = 5, StudentName = "Ron" , Age = 19 } 
    };

// checks whether all the students are teenagers    
bool areAllStudentsTeenAger = studentList.All(s => s.Age > 12 && s.Age < 20);



ANY

Any checks whether any element satisfy given condition or not? In the following example, Any operation is used to check whether any student is teen age or not.






bool isAnyStudentTeenAger = studentList.Any(s => s.age > 12 && s.age < 20);





ALL vs ANY







All() is used to detemine if all elements satisfy a condition. Any() tells you whether there are any elements in the sequence at all:


var numbers = new[]{2,4,6,8,9};
var all = numbers.All(n => n % 2 == 0); // returns false - not all numbers are even
var any = numbers.Any(); // returns true - there sequence contains at least one element

__________________________________________________________



AGGREGATE:



The following example demonstrates Aggregate method that returns comma seperated elements of the string list.






IList<String> strList = new List<String>() { "One", "Two", "Three", "Four", "Five"};

var commaSeperatedString = strList.Aggregate((s1, s2) => s1 + ", " + s2);



___________________________________________________________



AVERAGE



Average extension method calculates the average of the numeric items in the collection. Average method returns nullable or non-nullable decimal, double or float value






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

var avgAge = studentList.Average(s => s.Age);

Console.WriteLine("Average Age of Student: {0}", avgAge);








__________________________________________________________________________







COUNT







The Count operator returns the number of elements in the collection or number of elements that have satisfied the given condition.








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

var numOfStudents = studentList.Count();

Console.WriteLine("Number of Students: {0}", numOfStudents);









__________________________________________________________________________







SINGLE OR SINGLEORDEFAULT











Single() returns the only element from a collection, or the only element that satisfies the specified condition. If a given collection includes no elements or more than one elements then Single() throws InvalidOperationException.







The SingleOrDefault() method does the same thing as Single() method. The only difference is that it returns default value of the data type of a collection if a collection is empty, includes more than one element or finds no element or more than one element for the specified condition.









IList<int> oneElementList = new List<int>() { 7 };
IList<int> intList = new List<int>() { 7, 10, 21, 30, 45, 50, 87 };
IList<string> strList = new List<string>() { null, "Two", "Three", "Four", "Five" };
IList<string> emptyList = new List<string>();

//following throws error because list contains more than one element which is less than 100
Console.WriteLine("Element less than 100 in intList: {0}", intList.Single(i => i < 100));

//following throws error because list contains more than one element which is less than 100
Console.WriteLine("Element less than 100 in intList: {0}", 
                                    intList.SingleOrDefault(i => i < 100));

//following throws error because list contains more than one elements
Console.WriteLine("The only Element in intList: {0}", intList.Single());

//following throws error because list contains more than one elements
Console.WriteLine("The only Element in intList: {0}", intList.SingleOrDefault());

//following throws error because list does not contains any element
Console.WriteLine("The only Element in emptyList: {0}", emptyList.Single());










__________________________________________________________________________







SequenceEqual 





The SequenceEqual method checks whether the number of elements, value of each element and order of elements in two collections are equal or not.










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

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

bool isEqual = strList1.SequenceEqual(strList2); // returns true
Console.WriteLine(isEqual);
















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

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

bool isEqual = strList1.SequenceEqual(strList2); // returns false
Console.WriteLine(isEqual);









__________________________________________________________________________





TakeWhile 









String colection = { "one", "two", "three", "four", "five" };

var strings = collection.TakeWhile(n => n.Length < 4);











Linq2.gif











Skip



int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

var firstNumbersLessThan6 = numbers.Skip(3); 







output of this example will be 3, 9, 8, 6, 7, 2 and 0.




































































































































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