Filtering through a data set is one of the most basic operations a developer should know how to perform.
Filtering refers to the process of restricting the result set to contain only those elements that satisfy a specified condition. It is also known as selection.
To be practical, imagine trying to search and collect oranges from a basket of fruits. That is filtering.
In this article, we will see different ways to filter through a List<T> class.
Before we start filtering the List collection, we will create a public class called Employee that holds the employee details.
The Employee class is like a blueprint that holds the details for each employee. It has private fields for the name, id, and department of each employee.
The Employee class also has getter and setter methods to be able to set and get the value of each of the private field.
Here is the code that demonstrates what I was just explaining above:
public class Employee
{
private string _name;
private int _id;
private string _department;
public string Name
{
get { return _name; }
set { _name = value; }
}
public int Id
{
get { return _id; }
set { _id = value; }
}
public string Department
{
get { return _department; }
set { _department = value; }
}
}
Inside the Program class, let's create a method that will house a list using the Employee class already created above. Each employee will have an ID, a name, and a department.
public static void ListOperations()
{
List<Employee> employees = new List<Employee>()
{
new Employee(){ Id=1, Name="John Doe", Department="Software"},
new Employee(){ Id=2, Name="Angela Su", Department="Sales"},
new Employee(){ Id=3, Name="Frank Kelvin", Department="Marketing"},
new Employee(){ Id=4, Name="Joe Dustin", Department="Sales"},
new Employee(){ Id=5, Name="Glory GG", Department="Software"},
new Employee(){ Id=6, Name="Antonella Cruz", Department="Marketing"},
new Employee(){ Id=5, Name="Andrew Logan", Department="Software"},
new Employee(){ Id=6, Name="Billy Cruz", Department="Marketing"},
new Employee(){ Id=5, Name="Sally Jane", Department="Software"},
new Employee(){ Id=4, Name="Jon Snow", Department="Sales"},
};
}
The above code is the declaration of a list of employees in the ListOperations method. Our goal is to filter through the list to get the employees that work in the software department.
There are several ways to filter through a list in C#. Let's look at them now:
How to filter through a list using the iterative method
In this method, you loop through a list and search for the member of each iteration that passes the condition. Below is a code sample that uses the iterative method:
Console.WriteLine("Filtering through the Employee list using the " +
"Iterative method");
//goal: Filter through the list to get employees in the company’s software //department
foreach (var employee in employees)
{
if(employee.Department == "Software")
{
Console.WriteLine(employee.Name);
}
}
How to filter through a list using the LINQ query syntax (Where clause)
Language-Integrated Query (LINQ) is a powerful way to retrieve data from data sources in C#. This method filters the list collection and returns a new collection based on a given criterion.
Console.WriteLine("\nWhere clause - LINQ query syntax ");
var filteredResults = from employee in employees
where employee.Department == "Software"
select employee.Name;
//Looping through your filtered results
foreach(var result in filteredResults)
{
Console.WriteLine(result);
}
In the above code, the LINQ query syntax uses the where operator to filter the employees who are working in the Software department from the employees list. We then loop through the filtered results to print our results to the console.
How to filter through a list using the LINQ method syntax (Where clause)
Unlike the query syntax above, the LINQ extension method uses the lambda expression. The lambda expression is passed as a predicate. It is indeed a shortcut to the LINQ query syntax above.
Below is a code sample that uses the LINQ method syntax.
Console.WriteLine("\nWhere clause - LINQ method syntax ");
var filteredResultsTwo = employees.Where(employee => employee.Department == "Software");
//Looping through your filtered results
foreach(var employee in filteredResultsTwo)
{
Console.WriteLine(employee.Name);
}In the above code, the LINQ method syntax is used to filter the employees list and return a new list of employees who are working in the Software department. It uses a lambda expression as a predicate function. We then loop through the filtered results to print our results to the console.
The complete code for various methods of filtering a list is provided below:
namespace Linq
{
public class Employee
{
private string _name;
private int _id;
private string _department;
public string Name
{
get { return _name; }
set { _name = value; }
}
public int Id
{
get { return _id; }
set { _id = value; }
}
public string Department
{
get { return _department; }
set { _department = value; }
}
}
internal class Program
{
//GOAL: Our goal is to filter through the list to get the employees
//that work in the software department.
public static void ListOperations()
{
List<Employee> employees = new List<Employee>()
{
new Employee(){ Id=1, Name="John Doe", Department="Software"},
new Employee(){ Id=2, Name="Angela Su", Department="Sales"},
new Employee(){ Id=3, Name="Frank Kelvin", Department="Marketing"},
new Employee(){ Id=4, Name="Joe Dustin", Department="Sales"},
new Employee(){ Id=5, Name="Glory GG", Department="Software"},
new Employee(){ Id=6, Name="Antonella Cruz", Department="Marketing"},
new Employee(){ Id=5, Name="Andrew Logan", Department="Software"},
new Employee(){ Id=6, Name="Billy Cruz", Department="Marketing"},
new Employee(){ Id=5, Name="Sally Jane", Department="Software"},
new Employee(){ Id=4, Name="Jon Snow", Department="Sales"},
};
//1: Iterative method
Console.WriteLine("Filtering through the Employee list using the " +
"Iterative method");
foreach (var employee in employees)
{
if(employee.Department == "Software")
{
Console.WriteLine(employee.Name);
}
}
//2: LINQ query syntax (using the Where clause)
Console.WriteLine("\nWhere clause - LINQ query syntax ");
var filteredResults = from employee in employees
where employee.Department == "Software"
select employee.Name;
foreach(var result in filteredResults)
{
Console.WriteLine(result);
}
//3: LINQ method syntax (using the Where clause)
Console.WriteLine("\nWhere clause - LINQ method syntax ");
var filteredResultsTwo = employees.Where(employee => employee.Department == "Software");
foreach(var employee in filteredResultsTwo)
{
Console.WriteLine(employee.Name);
}
}
static void Main(string[] args)
{
ListOperations();
}
}
}
Conclusion
Filtering allows you to select only those elements that meet a particular condition. In this article, we showed how to filter a list in C# using various methods.
I hope this article has given you enough information to filter the List<T> collections easily.
Happy coding!