[Medium] Use case

UseCase.cs

public static class Specifications
{
    // create specification from expression, delegate is compiled on first usage
    public static readonly Specification<Department> ActiveDepartment = new(x => x.Active);

    // create specification from delegate, expression is decompiled from delegate on first usage
    public static readonly Specification<User> ActiveUser = new(
        default,
        x => x.Active
    );

    // negation
    public static readonly Specification<User> InactiveUser = !ActiveUser;
    
    // combination
    public static readonly Specification<User> SubscribedUser = ActiveUser && new Specification<User>(x => x.Subscription == Subscription.Subscribed);

    // create from expression and delegate because in DB collation is case insensitive
    public static readonly Specification<Department> CustomerServiceDepartment = new(
        x => x.Name == "Customer Service",
        x => string.Equals(x.Name?.TrimEnd(), "Customer Service", StringComparison.InvariantCultureIgnoreCase)
    );

    // reuse ActiveDepartment specification inside another specification
    public static readonly Specification<User> UserInCustomerServiceDepartment = new(x => x.Departments.Any(CustomerServiceDepartment && ActiveDepartment));
}

public class UserService 
{
    private readonly DbContext _context;
    
    public UserService(DbContext context) 
    {
        _context = context;
    }
    
    public Task<User> GetUser(int userId)
    {
        return _context.Set<User>()
            .Where(x => x.userId == userId)
            .Where(Specifications.ActiveUser)  // cool, we are using the same specification 
            .SingleOrDefaultAsync();
    }

    public Task<User> GetUserInDepartment(int userId, int departmentId)
    {
        return _context.Set<User>()
            .Where(x => x.userId == userId)
            .Where(Specifications.ActiveUser)  // cool, we are using the same specification 
            .Where(x => x.Departments.Any(z => z.Id == departmentId))
            .SingleOrDefaultAsync();
    }
}