📓 Objects Within Objects Setup
When we think about our own individual To Do Lists, there are usually different types of tasks on that list. For instance, there are to dos that we need to complete for Epicodus, like our C# homework. There are chores at home such as washing dishes or mowing the lawn. There are likely other miscellaneous tasks, too, such as writing someone a birthday card.
Let's update our To Do List application to allow users to organize their tasks by type. Over the next several lessons, we'll create a Category
class. Each Category
object will represent and store different categories of To Do List Item
s such as "Work", "Home", and "School." This setup is commonly referred to as objects within objects.
Creating a Parent Class
Let's start by creating a Category
class in a new ToDoList/Models/Category.cs
file. It will also need a corresponding test file at ToDoList.Tests/ModelTests/CategoryTests.cs
. The updated project structure looks like this:
ToDoList.Solution
├── ToDoList
│ ├── Controllers
│ │ ├── HomeController.cs
│ │ └── ItemsController.cs
│ ├── Models
│ │ ├── Category.cs
│ │ └── Item.cs
│ ├── Program.cs
│ ├── Properties
│ │ └── launchSettings.json
│ ├── ToDoList.csproj
│ └── Views
│ ├── Home
│ │ └── Index.cshtml
│ └── Items
│ ├── DeleteAll.cshtml
│ ├── Index.cshtml
│ ├── New.cshtml
│ └── Show.cshtml
└── ToDoList.Tests
├── ModelTests
│ ├── CategoryTests.cs
│ └── ItemTests.cs
└── ToDoList.Tests.csproj
In our new file, we'll do the following: declare a namespace and class, import the System.Collections.Generic
library to use List
s, and declare properties.
using System.Collections.Generic;
namespace ToDoList.Models
{
public class Category
{
private static List<Category> _instances = new List<Category> {};
public string Name { get; set; }
public int Id { get; }
public List<Item> Items { get; set; }
}
}
_instances
will contain a staticList
of allCategory
objects, similar to the_instances
property we're currently using in theItem
class.Name
will contain a name for theCategory
of items.Id
will contain a unique ID number that will be assigned in the constructor, similar to theId
we implemented in theItem
class.Items
will contain aList
of allItem
objects that belong to thatCategory
. For instance, if we had aCategory
with aName
of "chores," this list would contain a series ofItem
objects withDescription
s like "mop the floor", "scrub the shower", or "do the dishes."Note that we're using an auto-implemented property with
Items
and declaring the data type as aList
ofItem
s.
It's time to add a constructor. We'll start with a test first. Let's configure our new test file with standard boilerplate code:
using Microsoft.VisualStudio.TestTools.UnitTesting;
using ToDoList.Models;
using System.Collections.Generic;
using System;
namespace ToDoList.Tests
{
[TestClass]
public class CategoryTests
{
}
}
Next, we'll add a test to confirm our constructor can successfully create Category
objects:
using Microsoft.VisualStudio.TestTools.UnitTesting;
using ToDoList.Models;
using System.Collections.Generic;
using System;
namespace ToDoList.Tests
{
[TestClass]
public class CategoryTests
{
[TestMethod]
public void CategoryConstructor_CreatesInstanceOfCategory_Category()
{
Category newCategory = new Category("test category");
Assert.AreEqual(typeof(Category), newCategory.GetType());
}
}
}
Let's continue with our logic and add the constructor so that we run this test and see it pass:
using System.Collections.Generic;
namespace ToDoList.Models
{
public class Category
{
private static List<Category> _instances = new List<Category> {};
public string Name { get; set; }
public int Id { get; }
public List<Item> Items { get; set; }
public Category(string categoryName)
{
Name = categoryName;
_instances.Add(this);
Id = _instances.Count;
Items = new List<Item>{};
}
}
}
The constructor only accepts an argument for
categoryName
, which is assigned to theName
property. All other properties are assigned automatically in the body of the constructor.We add each
Category
to the static_instances
list in the constructor when it's created.We assign an
Id
number equal to the number ofCategory
s in_instances
.We create a new empty
List
to eventually containItem
objects that belong to thisCategory
.
Next we'll add several methods and their corresponding tests. All of this should be review, so we'll go quickly.
First, let's test that a Category
can successfully retrieve its name. We'll add a test and watch it pass thanks to our constructor code:
...
[TestMethod]
public void GetName_ReturnsName_String()
{
//Arrange
string name = "Test Category";
Category newCategory = new Category(name);
//Act
string result = newCategory.Name;
//Assert
Assert.AreEqual(name, result);
}
...
Next, we'll test that we can retrieve Category
IDs:
...
[TestMethod]
public void GetId_ReturnsCategoryId_Int()
{
//Arrange
string name = "Test Category";
Category newCategory = new Category(name);
//Act
int result = newCategory.Id;
//Assert
Assert.AreEqual(1, result);
}
...
However, if we run our tests, they don't pass. We get a failure message:
Error Message: Assert.AreEqual failed. Expected:<1>. Actual:<3>. Stack Trace: at ToDoList.Tests.CategoryTests.GetId_ReturnsCategoryId_Int() in ToDoList.Solution/ToDoList.Tests/ModelTests/CategoryTests.cs:line 44
It says we expected to receive 1
but got an ID of 3
instead. This is because we assign each Category
Id
by running Id = _instances.Count;
in the constructor. The third test is receiving a Category
with an Id
of 3
because sample Category
s created in previous tests remain in the static _instances
list.
We can fix this by disposing of all Category
s between tests with a teardown method similar to the one we implemented in our Triangle
tests in the last course section. We'll update the top of our CategoryTests
class like this:
...
namespace ToDoList.Tests
{
[TestClass]
public class CategoryTests : IDisposable
{
public void Dispose()
{
Category.ClearAll();
}
...
...
...
Next, we'll define this ClearAll()
method:
...
public static void ClearAll()
{
_instances.Clear();
}
...
After these changes, all tests should pass.
Moving on, we know we'll also need functionality to retrieve all Category
objects to display in our app. Let's add that next. We'll start with a test:
...
[TestMethod]
public void GetAll_ReturnsAllCategoryObjects_CategoryList()
{
//Arrange
string name01 = "Work";
string name02 = "School";
Category newCategory1 = new Category(name01);
Category newCategory2 = new Category(name02);
List<Category> newList = new List<Category> { newCategory1, newCategory2 };
//Act
List<Category> result = Category.GetAll();
//Assert
CollectionAssert.AreEqual(newList, result);
}
...
Verify that it fails (it should throw a compiler error, since the method is not defined yet) and then add the static method to make it pass:
...
public static List<Category> GetAll()
{
return _instances;
}
...
We also know we'll want a Find()
method to locate and display specific Category
objects. First, a test:
...
[TestMethod]
public void Find_ReturnsCorrectCategory_Category()
{
//Arrange
string name01 = "Work";
string name02 = "School";
Category newCategory1 = new Category(name01);
Category newCategory2 = new Category(name02);
//Act
Category result = Category.Find(2);
//Assert
Assert.AreEqual(newCategory2, result);
}
...
Here's the logic to pass this test:
...
public static Category Find(int searchId)
{
return _instances[searchId-1];
}
...
Notice this method is quite similar to our Item
class Find()
method. It accepts an ID as an argument and then locates the Category
in the static _instances
array that matches.
After following along with all steps in this lesson, all of our tests will pass and the new Category
class will look like this:
using System.Collections.Generic;
namespace ToDoList.Models
{
public class Category
{
private static List<Category> _instances = new List<Category> {};
public string Name { get; set; }
public int Id { get; }
public List<Item> Items { get; set; }
public Category(string categoryName)
{
Name = categoryName;
_instances.Add(this);
Id = _instances.Count;
Items = new List<Item>{};
}
public static void ClearAll()
{
_instances.Clear();
}
public static List<Category> GetAll()
{
return _instances;
}
public static Category Find(int searchId)
{
return _instances[searchId-1];
}
}
}
The corresponding test file looks like this:
using Microsoft.VisualStudio.TestTools.UnitTesting;
using ToDoList.Models;
using System.Collections.Generic;
using System;
namespace ToDoList.Tests
{
[TestClass]
public class CategoryTests : IDisposable
{
public void Dispose()
{
Category.ClearAll();
}
[TestMethod]
public void CategoryConstructor_CreatesInstanceOfCategory_Category()
{
Category newCategory = new Category("test category");
Assert.AreEqual(typeof(Category), newCategory.GetType());
}
[TestMethod]
public void GetName_ReturnsName_String()
{
//Arrange
string name = "Test Category";
Category newCategory = new Category(name);
//Act
string result = newCategory.Name;
//Assert
Assert.AreEqual(name, result);
}
[TestMethod]
public void GetId_ReturnsCategoryId_Int()
{
//Arrange
string name = "Test Category";
Category newCategory = new Category(name);
//Act
int result = newCategory.Id;
//Assert
Assert.AreEqual(1, result);
}
[TestMethod]
public void GetAll_ReturnsAllCategoryObjects_CategoryList()
{
//Arrange
string name01 = "Work";
string name02 = "School";
Category newCategory1 = new Category(name01);
Category newCategory2 = new Category(name02);
List<Category> newList = new List<Category> { newCategory1, newCategory2 };
//Act
List<Category> result = Category.GetAll();
//Assert
CollectionAssert.AreEqual(newList, result);
}
[TestMethod]
public void Find_ReturnsCorrectCategory_Category()
{
//Arrange
string name01 = "Work";
string name02 = "School";
Category newCategory1 = new Category(name01);
Category newCategory2 = new Category(name02);
//Act
Category result = Category.Find(2);
//Assert
Assert.AreEqual(newCategory2, result);
}
}
}
We've set up our new Category
class with basic functionality that's thoroughly tested. In the next lesson, we'll build it out further so we can save Item
objects within Category
objects.
Repository Reference
Follow the link below to view how a sample version of the project should look at this point. Note that this is a link to a specific branch in the repository.
Example GitHub Repo for To Do List: 7_objects_within_objects_setup