Cards

public sealed class User : Entity
{
    public string Name { get; private set; }
    public string Email { get; private set; }
    public int? Age { get; private set; }
 
    public static Result<User> Create(string name, string email, int? age)
    {
        var nameResult = Validate.String(name, nameof(Name))
            .Required()
            .MinLength(3)
            .MaxLength(100)
            .Build();
 
        var emailResult = Validate.String(email, nameof(Email))
            .Required()
            .Email()
            .Build();
 
        var ageResult = age is null
            ? Result<int?>.Ok(null)
            : Validate.Number(age.Value, nameof(Age))
                .Between(0, 150)
                .Build()
                .Map(v => (int?)v);
 
        return Result.Combine(nameResult, emailResult, ageResult)
            .Map(() => new User
            {
                Name = nameResult.Value!,
                Email = emailResult.Value!,
                Age = ageResult.Value
            });
    }
}

public static class CreateUser
{
    public record Input(
        [Required, MinLength(3), MaxLength(100)]
        string Name,
 
        [Required, EmailAddress]
        string Email,
 
        [Range(0, 150)]
        int? Age
    );
 
    public record Output(UserDto User);
}

public class UserConfiguration : IEntityTypeConfiguration<User>
{
    public void Configure(EntityTypeBuilder<User> builder)
    {
        builder.ToTable("users");
 
        builder.HasKey(e => e.Id);
 
        builder.Property(e => e.Name)
            .IsRequired()
            .HasMaxLength(100);
 
        builder.Property(e => e.Email)
            .IsRequired()
            .HasMaxLength(255);
 
        builder.Property(e => e.Age);
 
        builder.HasIndex(e => e.Email)
            .IsUnique();
    }
}