📓 3.0.0.15 Lists

If we want to create a collection where we can add or remove items, we need to use a list. For example, we might want to create a groceryList which could have any number of items in it. A list is a collection, usually of a single data type (like string or int). Unlike an array, a list can dynamically change in length.

Lists

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Creating Lists

Let’s open up the REPL and create a new list:

> List<string> groceryList = new List<string> {};

There's several steps to creating a C# list. From left to right:

1. Declare a List. We use the keyword List to create a List object. Notice this is capitalized, unlike data types like string or int.

2. Declare the data type the List will contain. It's best practice to declare the data type in angle brackets after List as we do with List<string> in the example above.

3. Give it a variable name. In the example above, we gave our List the name groceryList.

4. Create a new instance of List using its constructor. Finally, we create our List using its constructor with the new keyword: new List<string>. The example above states groceryList will be a new List containing strings.

5. Add data. Note that we use empty curly brackets at the end of the constructor to initialize an empty list without any data. If we try to run this without those curly brackets, we'll see: error CS1526: A new expression requires an argument list or (), [], or {} after type.

Note that there are several ways to initialize lists, and you might see them used interchangeably. We could use curly brackets with starting values inside like this:

List<string> groceryList2 = new List<string> {"eggs", "milk"};

Or, you might see parens used () instead of curly brackets {} to initialize an empty list:

List<string> groceryList3 = new List<string> ();

And there's still a few other ways to initialize a list. Dot Net Perls has helpful documentation on this topic with some more examples.

Adding Content to Lists

Once we've created a List, we can call the Add() method to add items to the List:

> List<string> groceryList = new List<string> {};
> groceryList
List<string>(0) { }
> groceryList.Add("spaghetti");
> groceryList.Add("tomatoes");
> groceryList.Add("basil");
> groceryList.Add("meatballs");
> groceryList
List<string>(4) { "spaghetti", "tomatoes", "basil", "meatballs" }

• In the code above, we create a new List of strings.

• Then we call Add() four times to add four separate strings: "spaghetti", "tomatoes", "basil", and "meatballs".

• We call groceryList to see its included items before and after we're done adding items.

Accessing Content in Lists

Items in a List have a unique index just like with arrays. We can access an item at a specific index using square brackets:

> groceryList[1]
"tomatoes"
> groceryList[3]
"meatballs"

We can also redefine/overwrite items at specific indexes using similar notation:

> groceryList[1] = "CANDY!";
> groceryList
{ "spaghetti", "CANDY!", "basil", "meatballs" }

Removing Content from Lists

If we check our pantry and realize we already have basil, we can remove it from our grocery list. To remove an item from a List, we'll use the built-in Remove() method:

> groceryList.Remove("basil");
> groceryList
{ "spaghetti", "CANDY!", "meatballs" }

When we call Remove() on an item in a List, C# will return true if the item is found and removed. If the argument is not present in a List, C# will return false.

The List<T> Class

We can learn more about lists by reviewing the information in the List<T> class. There are many helpful methods and properties! For example, check out this instance method called Insert() that lets us insert a new element at a specified index:

> List<string> groceryList2 = new List<string> { "spaghetti", "tomatoes", "basil", "meatballs" };
> groceryList2.Insert(0, "flowers")
1