C# Indexer

Introduction

What is an Indexer?

The difference between Properties and Indexers

When to use Indexer?

• If you need a collection of its instances.
  1. using System;
  2. using System.Linq;
  4. namespace CSharpIndexFeatureExplore {
  5. public enum Title {
  6. MR = 0,
  7. MS = 1
  8. }
  10. public class Customer {
  11. public Customer () : this (5) { }
  13. public Customer (int lengthOfCustomers) {
  14. this._customers = new Customer[lengthOfCustomers];
  15. }
  16. public Title Title { get; set; }
  17. public string TitleInString {
  18. get {
  19. string title = Title.ToString ();
  21. return new string (title.Select ((ch, index) => (index == 0) ? ch : Char.ToLower (ch)).ToArray ());
  22. }
  23. }
  24. public string FirstName { get; set; }
  26. public string LastName { get; set; }
  28. public int CustomersLength { get { return this._customers.Length; } }
  30. private readonly Customer[] _customers;
  31. public Customer this [int index] {
  32. get { return this._customers[index]; }
  33. set { this._customers[index] = value; }
  34. }
  35. public override string ToString () => $"{this.TitleInString}. {this.LastName}, {this.FirstName}";
  36. }
  37. }

  Now let us try to make a unit-test so could see how indexers work.
  1. using System;
  2. using CSharpIndexFeatureExplore;
  3. using Microsoft.VisualStudio.TestTools.UnitTesting;
  5. namespace UnitTestProject1 {
  7. [TestClass]
  8. public class IndexFeatureInstanceList {
  10. private Customer _customer;
  12. [TestInitialize ()]
  13. public void TestInit () {
  14. this._customer = new Customer (5) { };
  16. this.Initialize_Customer_Collection ();
  17. }
  19. private void Initialize_Customer_Collection () {
  20. this._customer[0] = new Customer { Title = Title.MR, FirstName = "Red", LastName = "Ford" };
  21. this._customer[1] = new Customer { Title = Title.MS, FirstName = "Anne", LastName = "Green" };
  22. this._customer[2] = new Customer { Title = Title.MR, FirstName = "Jack", LastName = "Robinsons" };
  23. this._customer[3] = new Customer { Title = Title.MS, FirstName = "Michelle", LastName = "Miguelito" };
  24. this._customer[4] = new Customer { Title = Title.MS, FirstName = "Trix", LastName = "Delacruz" };
  25. }
  27. [TestMethod]
  28. [Priority (1)]
  29. public void Test_Create_Instances_Of_Customer_Class () {
  31. Assert.IsNotNull (_customer);
  32. Assert.IsTrue (_customer.CustomersLength > 0);
  34. }
  36. [TestMethod]
  37. [Priority (2)]
  38. public void Test_Loop_Throught_Customer_Instances () {
  39. int length = this._customer.CustomersLength;
  41. for (int i = 0; i < length; i++) {
  42. Console.WriteLine (this._customer[i].ToString ());
  43. }
  44. }
  45. }
  46. }
• If you need a collection of values directly related to your instance.
1. using System;
2. using System.Collections.Generic;
3. using System.Linq;
4. using System.Runtime.CompilerServices;
6. namespace CSharpIndexFeatureExplore {
7. public enum FoodType {
8. JUNK = 0,
9. GO = 1,
10. GROW = 2
11. }
13. public class Food {
14. private Dictionary<FoodType, string[]> foodCollection;
16. public Food () {
17. foodCollection = new Dictionary<FoodType, string[]> { { FoodType.JUNK, new string[] { "Potato Chips", "Sweet Potato Chips" } },
18. { FoodType.GO, new string[] { "Sweet Potatoes", "Rice", "Pasta", "Bread" } },
19. { FoodType.GROW, new string[] { "Chicken", "Eggs", "Fish", "Meat" } }
20. };
21. }
23. public int FruitsLength { get; set; }
25. [IndexerName ("FoodPyramid")]
26. public string[] this [FoodType index, string foodType = ""] {
27. get {
28. return foodCollection[index];
29. }
31. set {
32. if (!Enum.GetNames (typeof (FoodType)).Any (f => f == foodType)) {
33. throw new ApplicationException ("Food type not available");
34. }
36. foodCollection[index] = value;
37. }
38. }
39. }
40. }
Before we go to the unit-test. Let us try to see if the "IndexerName" attribute would work with other programming language. In this scenario, we have used the VB programming language.
1. Imports CSharpIndexFeatureExplore
3. Module Module1
5. Sub Main()
6. Dim fruits As New Food()
7. Dim myCollectionOfFruits = fruits.FoodPyramid(1)
9. For Each item As String In myCollectionOfFruits
10. Console.WriteLine(item)
11. Next
13. End Sub
15. End Module
Again, let us try to make a unit-test so could see how indexers work in this scenario.
1. using System;
2. using System.Collections.Generic;
3. using CSharpIndexFeatureExplore;
4. using Microsoft.VisualStudio.TestTools.UnitTesting;
6. namespace UnitTestProject1 {
8. [TestClass]
9. public class IndexFeaturesRepresentsList {
11. private Food _food;
13. [TestInitialize ()]
14. public void Init () {
15. this._food = new Food ();
16. }
18. [TestMethod]
19. public void Test_Get_FoodPyramid () {
20. var foodTypes = (FoodType[]) Enum.GetValues (typeof (FoodType));
22. foreach (var foodType in foodTypes) {
23. var results = this._food[foodType, foodType.ToString ()];
25. Assert.IsNotNull (results);
27. Assert.IsTrue (results.Length > 0);
29. foreach (var food in results) {
30. Console.WriteLine ($"{foodType.ToString()} => {food} ");
31. }
32. }
33. }
35. [TestMethod]
36. [ExpectedException (typeof (KeyNotFoundException))]
37. public void Test_Get_FoodPyramid_Not_Exists () {
38. var food = this._food[(FoodType) 3];
39. }
41. [TestMethod]
42. public void Test_Get_FoodPyramid_Not_Exists_Handle () {
43. Exception expectedException = null;
45. try {
46. var food = this._food[(FoodType) 4];
47. } catch (Exception ex) {
48. expectedException = ex;
49. }
51. Assert.IsNotNull (expectedException);
52. }
54. }
55. }

Framework Common Library Which Uses Indexers