Introduction
C# 14 introduces extension members, one of the most impactful changes since LINQ. With this feature, you can extend types you don’t own with properties, methods, indexers, and even static members.
In this article, you’ll learn:
What extension members are in C# 14
The difference between extension members and extension methods
How to implement extension properties and methods
The limitations and pitfalls you should know
Best practices and real-world use cases
1. What Are Extension Members in C# 14?
Extension members build on extension methods by allowing developers to add:
Instance members (properties, methods, indexers)
Static members (factories, constants, operators)
They are declared inside a static class
using the new extension
block syntax.
2. C# 14 Extension Members vs Extension Methods
Feature | Extension Methods | Extension Members |
---|
Methods | ✅ Yes | ✅ Yes |
Properties | ❌ No | ✅ Yes |
Indexers | ❌ No | ✅ Yes |
Static Members | ❌ No | ✅ Yes |
Fields (state) | ❌ No | ❌ No |
3. Examples of Extension Properties and Methods
Example: Extension Property + Method
public static class EnumerableExtensions
{
extension<T>(IEnumerable<T> source)
{
// Extension property
public bool IsEmpty => !source.Any();
// Extension method
public IEnumerable<T> GreaterThan(T value) where T : IComparable<T>
=> source.Where(x => x.CompareTo(value) > 0);
}
}
Usage:
var numbers = new List<int> { 1, 2, 3 };
Console.WriteLine(numbers.IsEmpty); // False
var result = numbers.GreaterThan(2); // [3]
4. Static Extension Members in C# 14
public static class ListExtensions
{
extension<T>(List<T>)
{
// Static extension property
public static List<T> Empty => new List<T>();
// Static extension method
public static List<T> Combine(List<T> a, List<T> b) => a.Concat(b).ToList();
}
}
Usage:
var emptyList = List<int>.Empty;
var combined = List<int>.Combine(new List<int> { 1 }, new List<int> { 2, 3 });
// [1, 2, 3]
5. Limitations of Extension Members
❌ No fields/backing state — you can’t add private fields.
❌ Not all generics convert cleanly — some extension methods can’t be rewritten.
⚠️ Tooling still catching up — IDEs may show inconsistent suggestions.
6. Best Practices for Using Extension Members
Use properties for calculated values (IsEmpty
, Last
).
Use methods when parameters are required (GreaterThan
).
Use static members for factory-like helpers (Empty
, Combine
).
Avoid clutter: don’t move business logic into extension members unnecessarily.
7. Conclusion: Are Extension Members the Future?
Extension members don’t replace extension methods — but they greatly expand what’s possible. For APIs, libraries, and framework development, they’re a game-changer. For everyday coding, use them where they improve clarity without overcomplicating code.
8. FAQ
Q1: Can extension members replace extension methods?
No. Extension methods are still valid. Extension members extend the concept, allowing properties and static members, but both coexist.
Q2: Can I add fields with extension members?
No. You can’t add new state — only behavior and derived properties.
Q3: Do extension members work with interfaces?
Yes. You can extend interfaces with new properties and methods.
Q4: Do I need .NET 10 to use extension members?
Yes, extension members are part of the C# 14 / .NET 10 release.
Q5: Will my old extension methods still work?
Yes. All existing extension methods remain valid and fully compatible.