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Lists

A limitation of arrays is that their size is fixed (static)—once declared, the number of elements in an array cannot be changed at runtime. In contrast, a List is an ordered collection of elements that can grow or shrink as needed, making it dynamic.

Like arrays, each element in a List must be of the same data type, and you can access elements by their index. However, the flexibility to adjust the number of elements makes List a more versatile option in many cases.

Declaring a List

To use a List, you need to include the System.Collections.Generic namespace. When declaring a List, you must specify the data type of its elements using angle brackets. Here’s an example:

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using System.Collections.Generic; // Remember to include this
...
...
List<int> Marks = new List<int>();  // Initialize a list of integers
Marks.Add(78);                      // Add elements using Add()
Marks.Add(65);
Marks.Add(82);

foreach(int mark in Marks)           // Use foreach to iterate over the list
{                                    // No changes to the list are allowed during foreach
    Console.WriteLine(mark);         // Output each element
}

// or

for (int i = 0; i < Marks.Count; i++)  // Use a for loop to access elements by index
{
    if (Marks[i] > 75)
    {
        Console.WriteLine($"{Marks[i]} = Distinction");
    }
}

Modifying a List

Unlike arrays, a List allows elements to be added and removed dynamically. You can remove elements either by value or by their index:

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Marks.Remove(65);   // Removes the first occurrence of 65
Marks.RemoveAt(0);  // Removes the element at index 0

Checking if a List Contains an Element

You can check if a list contains a specific element using the Contains() method:

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if (Marks.Contains(54))  // Checks if 54 is in the list
{
    // Do something
}

To avoid duplicates, you can use this method before adding new elements:

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if (!Marks.Contains(54))
{
    Marks.Add(54);
}

Other Useful Methods

Here are some other common methods for working with lists:

  • Insert(index, value) - Insert an element at a specific position.
  • Clear() - Remove all elements from the list.
  • IndexOf(value) - Find the index of the first occurrence of an element.
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Marks.Insert(1, 90); // Inserts 90 at index 1
int index = Marks.IndexOf(82); // Gets the index of 82
Marks.Clear(); // Removes all elements from the list

Differences Between Arrays and Lists

  • Static vs Dynamic: Arrays have a fixed size, while lists can grow or shrink.
  • Length vs Count: Use .Length for arrays and .Count for lists to get the number of elements.
  • Indexing: Both arrays and lists are zero-indexed, meaning the first element is at index 0.

Efficiency Consideration

While lists are flexible, some operations (like removing elements from the middle) can be less efficient than arrays because elements need to be shifted to fill gaps.

Summary

List in C# offers a flexible, dynamic alternative to arrays, allowing you to modify the size of the collection at runtime. With methods like Add(), Remove(), and Contains(), lists are easy to work with and are suitable for situations where the number of elements is not known in advance or can change during program execution.