C-Sharp .Net

When to Use `IList` and When to Use `List` in C#

If you’re venturing into C# programming, you might stumble upon a common dilemma: Should I use IList or List? Understanding when to use each of these can greatly enhance your coding practices, making your applications more flexible and maintainable.

Understanding the Basics

To navigate this question, first, let’s clarify the difference:

IList<T>: This is an interface that allows you to define a contract for a list without tying yourself to a specific implementation. It offers more flexibility.
List<T>: This is a concrete class that implements the IList interface, providing a specific behavior and set of features.

Now that we understand the basics, let’s explore the decision-making process in a more organized manner.

Guidelines for Usage

Here are two essential rules that can guide your decision on when to use IList or List:

1. Accept the Most Basic Type that Will Work

When designing your functions or methods that accept a collection, prefer using the most basic interface that fulfills your needs. This means:

Use IList<T>, ICollection<T>, or IEnumerable<T> rather than specific implementations like List<T>.
This approach keeps your code flexible. Imagine if you later decide to switch out a List<T> for a different collection type, such as a Stack<T>—if you use an interface, you’re already prepared for that switch.

Example:

public void ProcessItems(IEnumerable<MyType> items)
{
    foreach (var item in items)
    {
        // Process each item
    }
}

In the above example, using IEnumerable<MyType> allows you to pass any collection of MyType, maintaining flexibility.

2. Return the Richest Type Your User Will Need

Conversely, when you are returning a collection from a function, you want to provide the most comprehensive set of functionalities to the caller. Here’s what to consider:

If your internal data structure is a List<T>, it’s often best to return it as a List<T> as well.
This way, users can utilize the specific methods available in List<T> without needing to cast or convert.

Example:

public List<MyType> GetItems()
{
    List<MyType> items = new List<MyType>();
    // Populate the list
    return new List<MyType>(items); // Returning as List<MyType>
}

This practice ensures that the user has immediate access to all methods associated with List<T> without extra hassle.

Conclusion

Deciding whether to utilize IList or List may seem daunting at first, but by following these straightforward guidelines, you can make informed decisions that enhance your code’s flexibility and usability.

Remember: For inputs, lean on the interfaces—IList<T>, ICollection<T>, IEnumerable<T>.
For outputs, opt for the concrete types like List<T> to maximize available functionality.