Building a User Subscription Module in ASP.NET MVC with C# 14

Install-Package Microsoft.EntityFramework -Version 6.4.4
Install-Package Microsoft.AspNet.Identity.EntityFramework
Install-Package Stripe.net // Optional for real payment integration

public class SubscriptionPlan
{
    public int Id { get; set; }

    public string Name
    {
        get => field;
        set => field = value ?? throw new ArgumentNullException(nameof(value));
    }

    public decimal Price { get; set; }
    public string Description { get; set; }
    public int DurationInDays { get; set; }
}

public class UserSubscription
{
    public int Id { get; set; }
    public string UserId { get; set; }
    public int PlanId { get; set; }
    public DateTime StartDate { get; set; }
    public DateTime EndDate { get; set; }
    public bool IsActive => DateTime.UtcNow <= EndDate;

    public virtual ApplicationUser User { get; set; }
    public virtual SubscriptionPlan Plan { get; set; }
}

public class ApplicationUser : IdentityUser
{
    public virtual ICollection<UserSubscription> Subscriptions { get; set; }
}

public class ApplicationDbContext : IdentityDbContext<ApplicationUser>
{
    public DbSet<SubscriptionPlan> SubscriptionPlans { get; set; }
    public DbSet<UserSubscription> UserSubscriptions { get; set; }

    public ApplicationDbContext() : base("DefaultConnection", throwIfV1Schema: false) { }

    public static ApplicationDbContext Create() => new();
}

context.SubscriptionPlans.AddOrUpdate(p => p.Name,
    new SubscriptionPlan { Name = "Free", Price = 0, Description = "Basic access", DurationInDays = 30 },
    new SubscriptionPlan { Name = "Pro", Price = 9.99M, Description = "Pro features", DurationInDays = 30 },
    new SubscriptionPlan { Name = "Enterprise", Price = 29.99M, Description = "All features", DurationInDays = 90 }
);

Seeding is essential to provide default options without requiring manual input every time the database is recreated. This is useful not only in development but also in CI/CD pipelines for automated deployments. These plans can be adjusted or extended later to suit business needs.

This code ensures that users always have valid plans to choose from. It's especially useful when the app is first launched, ensuring a smooth experience from the very first interaction.

5. Controllers and Views

Here’s a controller that uses a primary constructor:

[Authorize]
public class SubscriptionController(ApplicationDbContext db) : Controller
{
    public ActionResult Index() => View(db.SubscriptionPlans.ToList());

    public ActionResult Subscribe(int id) => View(db.SubscriptionPlans.Find(id));

    [HttpPost]
    public ActionResult SubscribeConfirmed(int id)
    {
        var userId = User.Identity.GetUserId();
        var plan = db.SubscriptionPlans.Find(id);

        var subscription = new UserSubscription
        {
            UserId = userId,
            PlanId = plan.Id,
            StartDate = DateTime.UtcNow,
            EndDate = DateTime.UtcNow.AddDays(plan.DurationInDays)
        };

        db.UserSubscriptions.Add(subscription);
        db.SaveChanges();

        return RedirectToAction("MySubscription");
    }

    public ActionResult MySubscription()
    {
        var userId = User.Identity.GetUserId();
        var activeSub = db.UserSubscriptions
            .Include("Plan")
            .Where(s => s.UserId == userId && s.IsActive)
            .OrderByDescending(s => s.EndDate)
            .FirstOrDefault();

        return View(activeSub);
    }
}

This controller manages the user's interaction with subscription plans. It retrieves available plans, handles subscription actions, and displays the user's current subscription. Using C# 14 primary constructors simplifies dependency injection, especially when there are few dependencies.

The controller is tied to Identity for user management, ensuring that actions like subscribing are tied to the correct user context. This integration of business logic and authentication is central to secure and personalized application flows.

6. Access Control Based on Subscription

Create a custom authorization attribute:

public class SubscriptionAuthorizeAttribute : AuthorizeAttribute
{
    protected override bool AuthorizeCore(HttpContextBase httpContext)
    {
        var userId = httpContext.User.Identity.GetUserId();
        var db = new ApplicationDbContext();
        var sub = db.UserSubscriptions.FirstOrDefault(x => x.UserId == userId && x.EndDate >= DateTime.UtcNow);
        return sub != null;
    }
}

Use it like this:

[SubscriptionAuthorize]
public ActionResult PremiumContent()
{
    return View();
}

This custom attribute checks if the logged-in user has an active subscription. It’s a lightweight and reusable solution to protect controller actions or entire controllers. It works well for scenarios where only subscribers should access certain parts of the application.

By encapsulating this logic into an attribute, we keep our controller actions clean and focused on what they need to do, rather than how authorization should be enforced. This improves both readability and maintainability.

7. Optional: Mock Payment Service

Handling payments is a critical part of any subscription system, but during development or for MVP testing, it's useful to mock this functionality. A mock payment service allows us to simulate successful transactions without integrating a third-party provider like Stripe or PayPal.

Below is a simple mock PaymentService class. It returns true to simulate a successful payment transaction, regardless of input. This allows you to develop and test your system logic without incurring real charges or managing payment API credentials.

public class PaymentService
{
    public bool ChargeUser(string userId, decimal amount)
    {
        // Simulate a successful payment
        Console.WriteLine($"Charging user {userId} for ${amount}");
        return true;
    }
}

You can integrate this into your subscription flow by calling it before saving the subscription. For example:

var paymentService = new PaymentService();
if (paymentService.ChargeUser(userId, plan.Price))
{
    var subscription = new UserSubscription
    {
        UserId = userId,
        PlanId = plan.Id,
        StartDate = DateTime.UtcNow,
        EndDate = DateTime.UtcNow.AddDays(plan.DurationInDays)
    };

    db.UserSubscriptions.Add(subscription);
    db.SaveChanges();
}

Using a mock now allows you to fully test your end-to-end subscription logic while making it simple to switch to a real provider later. When you're ready for live payments, just replace it PaymentService with a real implementation that connects to your gateway of choice.

8. Extension Members in C# 14

Use extension members to clean check the premium status:

public static class SubscriptionExtensions
{
    extension(UserSubscription subscription)
    {
        public bool IsPremium => ["Pro", "Enterprise"].Contains(subscription.Plan?.Name);
    }
}

Then in your code:

if (userSubscription.IsPremium)
{
    // Show premium features
}

Extension members let you encapsulate logic related to a class without modifying the class itself. This is ideal for computed properties like determining plan tier. It keeps your domain model lean while making your code more expressive.

This feature is especially useful in scenarios where your business logic spans multiple types or you need utility-style operations across many parts of the app. You avoid cluttering your core model with too much logic.

9. Views

Here's a simple Index.cshtml:

@model IEnumerable<SubscriptionPlan>
<h2>Choose a Plan</h2>
@foreach (var plan in Model)
{
    <div>
        <h3>@plan.Name</h3>
        <p>@plan.Description</p>
        <p>Price: [email protected]</p>
        @Html.ActionLink("Subscribe", "Subscribe", new { id = plan.Id })
    </div>
}

This view lists all available subscription plans. Each plan includes a button that links to the subscribe confirmation page. The use of Razor syntax makes it easy to iterate through models and generate dynamic HTML.

Views like this help users quickly understand the value proposition of each plan. With Bootstrap or Tailwind CSS, you could further enhance the design for a polished, responsive experience.

Conclusion

We built a user subscription module using ASP.NET MVC and C# 14, leveraging modern language features for better performance and readability. We explored models, controllers, access control, extension methods, and mocked payment processing. This provides a solid foundation for a SaaS or subscription-based application.

Next steps might include adding webhook support, real payment integration (e.g., Square, Stripe), user dashboards, or analytics.