LINQ Examples in C#

0
1
0

The article demonstrates different illustrative examples to show use case of the LINQ in c#. As an illustration, they are done in the .NET interactive notebook installed in Visual Studio Code.

To find out how to install and use the examples provided here, follow the GitHub link. Take note that main positive feature of the interactive notebooks is their readiness to try/use immediately.

LINQ (Language Integrated Query) is a query syntax built into the C# programming language that allows you to access different data sources using the same syntax.

Linq Logo from Microsoft

Create Test Data for LINQ Examples

Run this code cell to import all the necessary libraries required for the examples provided in the notebook.

using System.Threading;
using System.Text;
using System.Text.RegularExpressions;
using System.Collections.Generic;
using System.Linq;

The test data is created for examples provided in this notebook.

class Person
{
  public string FirstName {get; set;}
  public string LastName {get; set;}
  public string Gender {get; set;}

  public string[] Skills {get; set; }

  public Person(string firstName, string lastName, string gender, int age, string[] skills = null)
  {
    FirstName = firstName;
    LastName = lastName;
    Gender = gender;
    Age = age;
    Skills = skills ?? new string[]{};
  }

  public string FullName
  {
    get { return FirstName+" "+LastName; }
  }
  
  public int Age {get; set;}
}

var persons = new List<Person>()
{
  new Person("John", "Doe", "male", 35, new string[] {"c#", "php", "javascript"}),
  new Person("Ann", "Peterson", "female", 30, new string[] {"photoshop", "painting", "video"}),
  new Person("Corey", "Barker", "male", 28, new string[] {"ArcGis", "QGIS", "javascript"}),
  new Person("Debra", "Varner", "female", 47, new string[] {"python", "php", "ai"}),
  new Person("Eric", "Davis", "male", 64, new string[] {"java", "c#", "ruby"}),
  new Person("Eileen", "Murillo", "female", 19, new string[] {"QA", "net", "video"}),
  new Person("Jason", "Wilson", "male", 38, new string[] {"c#", "node", "python"})
};

Action<Person> PrintPersonDetails = s => Console.WriteLine(
  $"First name: {s.FirstName}, "+ 
  $"Last name: {s.LastName}, " +
  $"Gender: {s.Gender}, " +
  $"Age: {s.Age}, "+
  "Skils: "+ String.Join(',',s.Skills)
);

Action<Person[]> PrintPersonArray = prs => { foreach (var p in prs) PrintPersonDetails(p); };

Some Illustrative LINQ Examples in C#

Example of LINQ Filtering, Ordering, and Selecting Data

The code below shows how to filter order data and select from it the specified properties.

var youngFullNames = persons.Where(p => p.Age < 36)
    .OrderByDescending(p => (p.FirstName, p.LastName))
    .Select(p => p.FullName);

Console.WriteLine(youngFullNames.ToArray()[1]);

LINQ Example about Filtering With Index

How to get the data with the index in a list? See the solution below.

var from2to4 = persons
              .Where((p, index) => (index>=2 && index<=4))
              .Select((p, index) => p);

PrintPersonArray(from2to4.ToArray());

Using Partitioning Operators in LINQ

Example with Take Method

The Take method first N elements from a  sequence.

var first4 = persons.Take(4);

PrintPersonArray(first4.ToArray());

Example with Skip Method

The skip method gets elements after first N element:

var after4 = persons.Skip(4);

PrintPersonArray(after4.ToArray());

Example with TakeWhile Method

Returns elements from a specified sequence while a specified condition is true.

Note: the algorithm works until first discovering false, and the rest of the elements will be not checked for the condition.

var from3to5 = persons.TakeWhile((p, index) =>index>5);

PrintPersonArray(from3to5.ToArray());

Example with SkipWhile Method

The elements are skipped in a sequence while a specified condition is true.

Note: Examine the result and take note that after the first false condition, the algorithm does not skip the rest of the elements with a true condition.

var firstJava = persons.SkipWhile(p=>!p.Skills.Contains("java"));

PrintPersonArray(firstJava.ToArray());

Example of LINQ Filtering Object Array by Type

The code below demonstrates how to get items of a specific type from an object array.

object[] objectArray = {
  "star", 
  persons[1], 
  "lake", 
  154.75, 
  false, 
  new int[] {1,5,7},
  persons[4],
  true
};

var onlyPersons = objectArray.OfType<Person>();

PrintPersonArray(onlyPersons.ToArray());

Example of LINQ Compound Filtering with Complex Objects

If you have a list of complex objects and one property is enumerable, then you can use compound filtering in order to filter the list by that property.

var selectedPesrsons = 
  from p in persons 
  from skill in p.Skills 
  where skill=="c#" 
  select p;

PrintPersonArray(selectedPesrsons.ToArray());

Example of LINQ Grouping

The example code below shows how to group query results based on a key value.

var genders = from p in persons
group p by p.Gender into g
orderby g.Key descending
where g.Count()>0 
select new {Gender = g.Key, Count = g.Count()};

foreach (var g in genders)
  Console.WriteLine($"Gender: {g.Gender}, Count: {g.Count}");

And this example demonstrates how to make grouping with the nested objects.

var genders = from p in persons
group p by p.Gender into g
orderby g.Key descending
where g.Count()>0 
select new {
  Gender = g.Key, 
  Count = g.Count(),
  fullNames = from r1 in g
  select r1.FullName
};

foreach (var g in genders)
{
  Console.WriteLine($"Gender: {g.Gender}, Count: {g.Count}");
  foreach (var fn1 in g.fullNames) 
    Console.WriteLine($"\t Full name: {fn1}");
}

How to Use Methods as Selectors and Filters in LINQ

The following example demonstrates how to create a selector by declaring and using a Func<t,TResult> delegate and assigning it a lambda expression.

Func<Person, string> selector = p => String.Join(',', p.Skills);

var skills = persons.Select(selector);

foreach (var skill in skills) 
  Console.WriteLine($"Skills: {skill}");

The code below shows how to filter data using a method.

Func<Person, string, Boolean> isGender =  ((person, gender)=>
{
    return person.Gender == gender;
});

var males = persons.Where(p=>isGender(p, "male"));

PrintPersonArray(males.ToArray());

Using JOIN Clause in LINQ Query

In the following example, we show how to make two sequences from one and then make a third by joining a common element.

var csharpSkilled = persons
                    .Where(p=>p.Skills.Contains("c#"))
                    .Select(p=> new {Name = p.FullName, Skills = p.Skills});

var personAges = persons.Select(p=> new {Name = p.FullName, Age = p.Age});

var csharpSkilledAges = 
(
  from s1 in csharpSkilled
  join a1 in personAges on s1.Name equals a1.Name
  select new {s1.Name, a1.Age, s1.Skills}
);

foreach(var cs1 in csharpSkilledAges) 
  Console.WriteLine($"Name: {cs1.Name}, Age: {cs1.Age}, Skills: {String.Join(',',cs1.Skills)}");

LINQ allows you to perform a JOIN by comparing objects with anonymous types created on the fly:

Take note of the variable numberSkils, it shows how you can specify variables in a LINQ query.

var csharpSkilled = persons
                    .Where(p=>p.Skills.Contains("c#"))
                    .Select(p=> new {FirstName = p.FirstName, LastName = p.LastName, Skills = p.Skills});

var personAges = persons.Select(p=> new {FirstName = p.FirstName, LastName = p.LastName, Age = p.Age});

var csharpSkilledAges = 
(
  from s1 in csharpSkilled
  let numberSkils = s1.Skills.Count()
  join a1 in personAges on new {s1.FirstName, s1.LastName} equals new {a1.FirstName, a1.LastName}
  select new {s1.FirstName, s1.LastName, a1.Age, s1.Skills, numberSkils}
);

foreach(var cs1 in csharpSkilledAges) 
  Console.WriteLine(
    $"First name: {cs1.FirstName}, "+
    $"Last name: {cs1.LastName}, "+
    $"Age: {cs1.Age}, "+
    $"Skills: {String.Join(',',cs1.Skills)}, "+
    $"numberSkils: {cs1.numberSkils}"
  );

Type Casting in LINQ Query

In the previous example, we demonstrated how to select anonymous type objects from an existing object sequence on the fly.

But consuming the anonymous types during coding is difficult. The following code shows how to cast an anonymous type to a known type:

var anonymoisTypeSequence = persons
                    .Where(p=>p.Skills.Contains("c#"))
                    .Select(
                      p=> new {
                        FirstName = p.FirstName, 
                        LastName = p.LastName, 
                        Gender = p.Gender,
                        Age = p.Age,
                        Skills = p.Skills
                      }
                    ).ToArray();

var knownTypeSequence = Array.ConvertAll<dynamic, Person>(anonymoisTypeSequence, new Converter<dynamic, Person>(
  an=> new Person(
        an.FirstName.ToString(), 
        an.LastName.ToString(),
        an.Gender.ToString(), 
        Convert.ToInt32(an.Age), 
        an.Skills as string[]
      )
));

PrintPersonArray(knownTypeSequence);

Was this helpful?

1 / 0

Leave a Reply 0

Your email address will not be published. Required fields are marked *