How To Use Extensions For LINQ - C#

Introduction

Extension Methods are a new feature in C# 3.0, and they're simply user-made pre-defined functions. An Extension Method enables us to add methods to existing types without creating a new derived type, recompiling, or modifying the original types.

Extension Methods

• It enables us to add methods to existing types without creating a new derived type.
• It is a static method.
• It is defined in a static class.
• It uses “this” keyword to apply the extension to the particular type.
• We don’t want to compile the application. Visual Studio IntelliSense will find the extension method immediately.

Coding Part

1. I have created a Console application by selecting File->New Project->Console Application.
2. Then, created a Model class “Employee.cs” with member variables like below.
   
   1. public class Employee
   2. {
   3. public string Name { get; set; }
   4. public string MobileNo { get; set; }
   5. public string Salary { get; set; }
   7. public string String()
   8. {
   9. return string.Format("Name:{0} MobileNo:{1} Salary:{2}", Name, MobileNo, Salary);
   10. }
   11. }

3. Then, I added the values to employee list as below.
   
   1. static void AddEmployees()
   2. {
   3. employees.Add(new Employee() { Name = "Mushtaq", MobileNo = "9876543210", Salary = "20K" });
   4. employees.Add(new Employee() { Name = "Mohammed Mushtaq", MobileNo = "9876543211", Salary = "30K" });
   5. employees.Add(new Employee() { Name = "Mushtaq Ahmed", MobileNo = "9876543212", Salary = "20K" });
   6. employees.Add(new Employee() { Name = "Raj", MobileNo = "9876543213", Salary = "25K" });
   7. }

4. Now, we will get results from a list using Linq for “FirstOrDefault”, “LastOrDefault” or “IndexOf”.
   
   1. Console.WriteLine(employees.Where(x => x.Name.Contains("Mushtaq")).FirstOrDefault().String());
   2. Console.WriteLine(employees.Where(x => x.Name.Contains("Mushtaq")).LastOrDefault().String());
   3. Console.WriteLine(employees.IndexOf(employees.Where(x => x.Name.Contains("Mushtaq")).FirstOrDefault()));
   Result
   
   
   

5. Now, I have created a static class named as “LinqExtension.cs” and created extension methods for selecting “FirstOrDefault”, “LastOrDefault” and getting the index of the object from the list using LINQ.
   
   1. public static class LinqExtension
   2. {
   3. public static T WhereFirstOrDefault<T>(this List<T> list, Func<T, bool> predicate)
   4. {
   5. return list.Where(predicate).FirstOrDefault();
   6. }
   7. public static T WhereLastOrDefault<T>(this List<T> list, Func<T, bool> predicate)
   8. {
   9. return list.Where(predicate).LastOrDefault();
   10. }
   11. public static int IndexOf<T>(this List<T> list, Func<T, bool> predicate)
   12. {
   13. return list.IndexOf(list.WhereFirstOrDefault(predicate));
   14. }
   15. }