Learn to manage data collections using List<T> in C#

Artikkeli
03/07/2023

This introductory tutorial provides an introduction to the C# language and the basics of the List<T> class.

Prerequisites

The tutorial expects that you have a machine set up for local development. See Set up your local environment for installation instructions and an overview of application development in .NET.

If you prefer to run the code without having to set up a local environment, see the interactive-in-browser version of this tutorial.

A basic list example

Create a directory named list-tutorial. Make that the current directory and run dotnet new console.

Important

The C# templates for .NET 6 use top level statements. Your application may not match the code in this article, if you've already upgraded to the .NET 6. For more information see the article on New C# templates generate top level statements

The .NET 6 SDK also adds a set of implicit global using directives for projects that use the following SDKs:

Microsoft.NET.Sdk
Microsoft.NET.Sdk.Web
Microsoft.NET.Sdk.Worker

These implicit global using directives include the most common namespaces for the project type.

For more information, see the article on Implicit using directives

Open Program.cs in your favorite editor, and replace the existing code with the following:

List<string> names = ["<name>", "Ana", "Felipe"];
foreach (var name in names)
{
    Console.WriteLine($"Hello {name.ToUpper()}!");
}

Replace <name> with your name. Save Program.cs. Type dotnet run in your console window to try it.

You've created a list of strings, added three names to that list, and printed the names in all CAPS. You're using concepts that you've learned in earlier tutorials to loop through the list.

The code to display names makes use of the string interpolation feature. When you precede a string with the $ character, you can embed C# code in the string declaration. The actual string replaces that C# code with the value it generates. In this example, it replaces the {name.ToUpper()} with each name, converted to capital letters, because you called the ToUpper method.

Let's keep exploring.

Modify list contents

The collection you created uses the List<T> type. This type stores sequences of elements. You specify the type of the elements between the angle brackets.

One important aspect of this List<T> type is that it can grow or shrink, enabling you to add or remove elements. Add this code at the end of your program:

Console.WriteLine();
names.Add("Maria");
names.Add("Bill");
names.Remove("Ana");
foreach (var name in names)
{
    Console.WriteLine($"Hello {name.ToUpper()}!");
}

You've added two more names to the end of the list. You've also removed one as well. Save the file, and type dotnet run to try it.

The List<T> enables you to reference individual items by index as well. You place the index between [ and ] tokens following the list name. C# uses 0 for the first index. Add this code directly below the code you just added and try it:

Console.WriteLine($"My name is {names[0]}");
Console.WriteLine($"I've added {names[2]} and {names[3]} to the list");

You can't access an index beyond the end of the list. Remember that indices start at 0, so the largest valid index is one less than the number of items in the list. You can check how long the list is using the Count property. Add the following code at the end of your program:

Console.WriteLine($"The list has {names.Count} people in it");

Save the file, and type dotnet run again to see the results.

Search and sort lists

Our samples use relatively small lists, but your applications may often create lists with many more elements, sometimes numbering in the thousands. To find elements in these larger collections, you need to search the list for different items. The IndexOf method searches for an item and returns the index of the item. If the item isn't in the list, IndexOf returns -1. Add this code to the bottom of your program:

var index = names.IndexOf("Felipe");
if (index == -1)
{
    Console.WriteLine($"When an item is not found, IndexOf returns {index}");
}
else
{
    Console.WriteLine($"The name {names[index]} is at index {index}");
}

index = names.IndexOf("Not Found");
if (index == -1)
{
    Console.WriteLine($"When an item is not found, IndexOf returns {index}");
}
else
{
    Console.WriteLine($"The name {names[index]} is at index {index}");

}

The items in your list can be sorted as well. The Sort method sorts all the items in the list in their normal order (alphabetically for strings). Add this code to the bottom of your program:

names.Sort();
foreach (var name in names)
{
    Console.WriteLine($"Hello {name.ToUpper()}!");
}

Save the file and type dotnet run to try this latest version.

Before you start the next section, let's move the current code into a separate method. That makes it easier to start working with a new example. Place all the code you've written in a new method called WorkWithStrings(). Call that method at the top of your program. When you finish, your code should look like this:

WorkWithStrings();

void WorkWithStrings()
{
    List<string> names = ["<name>", "Ana", "Felipe"];
    foreach (var name in names)
    {
        Console.WriteLine($"Hello {name.ToUpper()}!");
    }

    Console.WriteLine();
    names.Add("Maria");
    names.Add("Bill");
    names.Remove("Ana");
    foreach (var name in names)
    {
        Console.WriteLine($"Hello {name.ToUpper()}!");
    }

    Console.WriteLine($"My name is {names[0]}");
    Console.WriteLine($"I've added {names[2]} and {names[3]} to the list");

    Console.WriteLine($"The list has {names.Count} people in it");

    var index = names.IndexOf("Felipe");
    if (index == -1)
    {
        Console.WriteLine($"When an item is not found, IndexOf returns {index}");
    }
    else
    {
        Console.WriteLine($"The name {names[index]} is at index {index}");
    }

    index = names.IndexOf("Not Found");
    if (index == -1)
    {
        Console.WriteLine($"When an item is not found, IndexOf returns {index}");
    }
    else
    {
        Console.WriteLine($"The name {names[index]} is at index {index}");

    }

    names.Sort();
    foreach (var name in names)
    {
        Console.WriteLine($"Hello {name.ToUpper()}!");
    }
}

Lists of other types

You've been using the string type in lists so far. Let's make a List<T> using a different type. Let's build a set of numbers.

Add the following to your program after you call WorkWithStrings():

List<int> fibonacciNumbers = [1, 1];

That creates a list of integers, and sets the first two integers to the value 1. These are the first two values of a Fibonacci Sequence, a sequence of numbers. Each next Fibonacci number is found by taking the sum of the previous two numbers. Add this code:

var previous = fibonacciNumbers[fibonacciNumbers.Count - 1];
var previous2 = fibonacciNumbers[fibonacciNumbers.Count - 2];

fibonacciNumbers.Add(previous + previous2);

foreach (var item in fibonacciNumbers)
{
    Console.WriteLine(item);
}

Save the file and type dotnet run to see the results.

Tip

To concentrate on just this section, you can comment out the code that calls WorkWithStrings();. Just put two / characters in front of the call like this: // WorkWithStrings();.

Challenge

See if you can put together some of the concepts from this and earlier lessons. Expand on what you've built so far with Fibonacci Numbers. Try to write the code to generate the first 20 numbers in the sequence. (As a hint, the 20th Fibonacci number is 6765.)

Complete challenge

You can see an example solution by looking at the finished sample code on GitHub.

With each iteration of the loop, you're taking the last two integers in the list, summing them, and adding that value to the list. The loop repeats until you've added 20 items to the list.

Congratulations, you've completed the list tutorial. You can continue with additional tutorials in your own development environment.

You can learn more about working with the List type in the .NET fundamentals article on collections. You'll also learn about many other collection types.

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