0Pricing
C# Academy · Lesson

Arrays, slicing, List<T>, Dictionary<TKey,TValue>

Create and use arrays, copy a slice, and work with List and Dictionary for dynamic items and key-based lookups.

Arrays, slicing, List<T>, Dictionary<TKey,TValue> is a free C# Academy lesson on CoddyKit — lesson 1 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Lesson overview

Goal: Pick the right container.

  • Array: fixed size, fast indexing
  • List<T>: grows/shrinks
  • Dictionary<TKey,TValue>: fast key lookup
  • Simple array slice via copy

Array basics

Arrays are fixed-size and indexable. Use Length and zero-based indices.

using System;

public class Program
{
  public static void Main(string[] args)
  {
    int[] nums = new int[] { 10, 20, 30 };
    Console.WriteLine("Length = " + nums.Length);
    Console.WriteLine("First = " + nums[0]);
    Console.WriteLine("Last = " + nums[nums.Length - 1]);

    // Update by index
    nums[1] = 25;
    Console.WriteLine("Updated middle = " + nums[1]);
  }
}

Array slice via copy

No range operator in C# 6. Make a small helper with Array.Copy to copy a slice.

using System;

public class Program
{
  // Copy a slice [start, start+count) into a new array
  public static int[] Slice(int[] src, int start, int count)
  {
    int[] dst = new int[count];
    Array.Copy(src, start, dst, 0, count);
    return dst;
  }

  public static void Main(string[] args)
  {
    int[] data = new int[] { 5, 6, 7, 8, 9 };
    int[] middle = Slice(data, 1, 3); // {6,7,8}
    Console.WriteLine("Slice len=" + middle.Length + " first=" + middle[0] + " last=" + middle[middle.Length - 1]);
  }
}

List<T> basics

List<T> resizes automatically. Use Add/Insert/Remove and Count.

using System;
using System.Collections.Generic;

public class Program
{
  public static void Main(string[] args)
  {
    List<string> items = new List<string>();
    items.Add("apple");
    items.Add("banana");
    items.Insert(1, "kiwi"); // apple, kiwi, banana
    Console.WriteLine("Count = " + items.Count);
    Console.WriteLine("Second = " + items[1]);

    items.Remove("apple");
    Console.WriteLine("After remove, first = " + items[0]);
  }
}

Dictionary basics

Dictionary stores key-value pairs. Use ContainsKey/TryGetValue for safe lookups.

using System;
using System.Collections.Generic;

public class Program
{
  public static void Main(string[] args)
  {
    Dictionary<string, int> ages = new Dictionary<string, int>();
    ages["Ada"] = 28;           // add
    ages["Alan"] = 35;          // add
    ages["Alan"] = 36;          // update

    int value;
    if (ages.TryGetValue("Ada", out value))
    {
      Console.WriteLine("Ada age = " + value);
    }

    Console.WriteLine("Has Grace? " + ages.ContainsKey("Grace"));
  }
}

Choosing collections

Choose:

  • Array for fixed-size, numeric indexing
  • List<T> when size changes
  • Dictionary<K,V> when you have unique keys
  • Copy slices with Array.Copy helper

Keyed lookup choice

Quick check: Which collection is best when you need fast lookups by a unique key?

Recap

Recap: Arrays are fixed-size; copy slices with Array.Copy. Use List<T> for dynamic lists and Dictionary<K,V> for fast key lookups.

Frequently asked questions

Is the “Arrays, slicing, List<T>, Dictionary<TKey,TValue>” lesson free?

Yes — the full text of “Arrays, slicing, List<T>, Dictionary<TKey,TValue>” is free to read here on the web, and the C# Academy course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO.

What will I learn in “Arrays, slicing, List<T>, Dictionary<TKey,TValue>”?

Create and use arrays, copy a slice, and work with List and Dictionary for dynamic items and key-based lookups. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C# Academy?

No prior experience is required. C# Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 3, so you can start here or from the beginning and move at your own pace.

How long does the “Arrays, slicing, List<T>, Dictionary<TKey,TValue>” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C# Academy lesson?

Yes. Every C# Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Arrays, slicing, List , Dictionary
  2. foreach patterns; collection initializers
  3. LINQ preview: Where, Select, OrderBy, Take/Skip; deferred execution
← Back to C# Academy