C# Extension Properties

C# just recently added extension properties. This new feature allows you to add new properties to exisisting types/classes. Even ones that you do not have control over. They behave similarly to the pre-existing extension methods feature.

Example

Let's start by making a new Person class with a few properties:

public class Person
{
    public required string FirstName { get; init; }
    public required string LastName { get; init; }
    public required DateOnly Birthday { get; init; }
}

Now, let's add a few additional properties to the Person class using the new extension property feature:

public static class PersonExtensions
{
    extension(Person person)
    {
        public bool CanPurchaseAlcohol => person.Birthday <= GetCurrentDate().AddYears(-21);
        public string FullName => $"{person.FirstName} {person.LastName}";
    }

    private static DateOnly GetCurrentDate() => DateOnly.FromDateTime(DateTime.Today);
}

Now you can use those extension properties elsewhere in your code like they are normal class properties:

Person p1 = new
{
    FirstName = "Ryan",
    LastName = "Rickgauer",
    Birthday = DateOnly.ParseExact("1996/03/08", "yyyy/MM/dd", CultureInfo.InvariantCulture),
};

Console.WriteLine(p1.FullName);             // Ryan Rickgauer
Console.WriteLine(p1.CanPurchaseAlcohol);   // true

Real World Example

The previous example shows how you can add these new properties to a class you created. However, that doesn't really help us out too much since we just could have added those 2 new properties to our Person class ourselves.

The true power of these extension properties comes from the fact that you can add new properties to existing classes that you don't control. For instance, here's a snippet from some code I use to extract data from an ActionExecutingContext class. This class is provided to us from C#, and I can't just add new properties to the class definition like I could before in my Person class:

public static class ActionExecutingContextUtility
{
    extension(ActionExecutingContext context)
    {
        public int ClientId => context.GetSessionClientId();
        public CollectionId CollectionId => context.GetActionItem<CollectionId>();
        public QuestionId QuestionIdBase => context.GetActionItem<QuestionId>();
        public QuestionForm QuestionForm => context.GetActionItem<QuestionForm>();
        public AnswerId AnswerId => context.GetActionItem<AnswerId>();
        public AnswerSelectionForm AnswerSelectionForm => context.GetActionItem<AnswerSelectionForm>();
        public AnswerTextForm AnswerTextForm => context.GetActionItem<AnswerTextForm>();
    }

    public static T GetActionItem<T>(this ActionExecutingContext context)
    {
        return (T)context.ActionArguments.Values.First(a => a is T)!;
    }
}

Now when I implement an IAsyncActionFilter on a controller/endpoint I can just call these new property extensions on the given ActionExecutingContext parameter:

public class GetCollectionFilter(GetCollectionAuth auth) : IAsyncActionFilter
{
    private readonly GetCollectionAuth _auth = auth;

    public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next)
    {
        var hasAuth = await _auth.HasPermissionAsync(new()
        {
            ClientId = context.ClientId,            // property extension
            CollectionId = context.CollectionId,    // property extension
        });

        await this.HandlePermissionsAuthResponse(context, next, hasAuth);
    }
}

C# Extension Properties

10/30/2026

Example

Real World Example

Further Reading