In this lesson, we're going to talk about enumerations – or rather “enums” as their known in C#. An enum is a special data type that can hold , at any moment, one of a limited set of possible values that you determine. To demonstrate this, imagine that we're creating an application that collects information about pet ownership. You will want to create a pet class, and capture the name of the pet, the age of the pet, and the type of pet.
Now, the pet type might be one of several different available options: a dog, a cat, a fish, an elephant, and so on. However we should know ahead of time that it is a limited set, and that we can’t declare just anything as a valid pet type:
It would be difficult to account for these various possibilities using a string data type for holding the PetType (in this case, you would probably create a property that checks for valid entries via a conditional statement in the “setter”). However, an enum – by definition – allows you to restrict your set of options only to values that are relevant. Here is how you create an enum:
And now you can change the data type, in the Pet class, from string to PetType:
And now, when you reference the Type property, you can only set it to one of the available options:
Perhaps more importantly, Intellisense recognizes that we have a limited set of options and stops us from using an invalid value at compile time:
You may be wondering what the real value is in limiting possibilities in your code. After all, somebody might have an uncommon pet such as a Zebra, or an Eel. By using an enum, you would just modify your code to accept those types of pets while retaining the added benefit of not allowing something seemingly innocuous – such as wanting a Velociraptor as a pet – to end up breaking your code.
Enumerations are used all over the place in the .NET Framework Class Library, primarily because they limit the possible values that you can pass into methods or set into properties. If the creators of the .NET Framework Class Library allowed you to pass any value you wanted to into a given method/property, there is a higher chance that an incorrect value will be provided and make for more debugging problems in your code.
Don't think of creating constraints inside of software development as necessarily a bad thing. In fact, it's often a great thing, in that it helps you to write more robust and error-free code than would otherwise be possible. The greatest challenge with writing code is simplifying everything, and enums can be an important part of that simplification process.
Create a new ASP.NET project for this lesson and call it “CS-ASP_053.” Add the following Server Controls and programmatic IDs to the Default.aspx file:
The second Server Control is a DropDownList. That can be accessed via the ToolBox:
Next, in Default.aspx.cs create a public enum that is incorporated via a property in a public Character class:
Now, in the Page_Load() method you can create a new Character and select its enum value:
This is a good place to give an example of why limiting user input is valuable. In software development, you want to do all you can to limit or eliminate dependency on open-ended data types like "magic strings", which are simply string input from users that could be any value. If we accepted a "magic string" instead of an enum for this type, it would be subject to all sorts of errors that won’t even be caught by the compiler and would require complete consistency on our part:
Another thing worth mentioning with the enum definition is that it can be nested within another class. In this case, it may actually make sense to considering that the class and enum seem to conceptually belong together:
Now, we need to modify our references because the CharacterType enum is accessed through the Character class first.
Let’s now demonstrate how we can use the DropDownList to select amongst the available values for the Character.CharacterType enum. We first have to populate the DropDownList with a set of default options. Click on the arrow beside the Server Control in the Design View, and select “Edit Items…”:
Then simply click the “Add” button and modify the displayed text and stored value for each item:
Back in the Button1_Click() method we (1) created a local variable to store the enum and then (2) returned to it a value chosen from the DropDownList Control. This value then (3) gets input through the static Enum.TryParse() method and, if it successfully parsed the string to an enum, it (4) gets stored into the hero.Type enum property:
You can also add a special message, to the bottom of the Button1_Click() method, depending on the enum value that is currently selected once the button is pressed:
Now the run the application, but first be sure to comment out the Page_Load() method as it will just interfere with the results:
Lesson 53 - Working with Enumerations