Why should you care?

Business rules like “email must be unique” belong here so Application and tests share one truth.

See it live — copy this example

Create a Web API project (dotnet new webapi), paste the code, then run dotnet run.

namespace User.Domain.Entities;

public class UserProfile
{
    public Guid Id { get; private set; }
    public string Name { get; private set; } = "";
    public Email Email { get; private set; } = null!;

    public static UserProfile Create(string name, string email)
    {
        if (string.IsNullOrWhiteSpace(name)) throw new ArgumentException("Name required");
        return new UserProfile { Id = Guid.NewGuid(), Name = name.Trim(), Email = Email.Create(email) };
    }
}

public interface IUserRepository
{
    Task<UserProfile?> GetByIdAsync(Guid id, CancellationToken ct = default);
    Task AddAsync(UserProfile user, CancellationToken ct = default);
}

Run Example »

Edit the code and click Run — like W3Schools Try it Yourself.

What happened?

• Create factory method validates invariants.
• IUserRepository is implemented in Infrastructure — Domain only declares the contract.

Try it yourself

1. Add UserProfile and Email value object.
2. Add IUserRepository interface.
3. Write one unit test for UserProfile.Create with invalid email.
4. Change a string or route in the example and save — watch Swagger or the RabbitMQ Management UI update.
5. Break the code on purpose (remove a semicolon), read the error message, then fix it.

Remember

Domain = entities + rules + interfaces. No EF or ASP.NET references. Factory methods enforce invariants.