Coding Guidelines: Modeling Domain Objects in C#

Purpose: Practical guidelines for modeling rich domain objects in C# with side-by-side bad vs. better examples, rationale, and usage notes. Apply these patterns to design expressive, safe, and maintainable domain models.

Principles for Modeling Domain Objects

Model concepts, not storage shapes

Bad: Database/JSON-shaped class that ignores domain meaning

public sealed class OrderData
{
    public int Id { get; set; }
    public string CustomerName { get; set; } = "";
    public decimal Total { get; set; }
    public string Status { get; set; } = "";
}

Problems:

• CustomerName is a raw string instead of a concept.
• Total has no currency or rules.
• Status is stringly-typed.
• Type says nothing about allowed behaviors.

Better: Domain language and behavior embedded in types

public enum OrderStatus
{
    Draft,
    Submitted,
    Paid,
    Shipped,
    Cancelled
}

public sealed record PersonName(string Value)
{
    public PersonName(string value) : this()
    {
        if (string.IsNullOrWhiteSpace(value))
            throw new ArgumentException("Name must not be empty.", nameof(value));

        Value = value.Trim();
    }

    public string Value { get; } = Value;
}

public sealed record Money(decimal Amount, string Currency)
{
    public Money(decimal amount, string currency) : this()
    {
        if (amount < 0)
            throw new ArgumentOutOfRangeException(nameof(amount));

        if (string.IsNullOrWhiteSpace(currency))
            throw new ArgumentException("Currency is required.", nameof(currency));

        Amount = amount;
        Currency = currency;
    }

    public decimal Amount { get; } = Amount;
    public string Currency { get; } = Currency;
}

public sealed class Order
{
    public Order(int id, PersonName customerName, Money total)
    {
        Id = id;
        CustomerName = customerName;
        Total = total;
        Status = OrderStatus.Draft;
    }

    public int Id { get; }
    public PersonName CustomerName { get; }
    public Money Total { get; private set; }
    public OrderStatus Status { get; private set; }

    public void Submit()
    {
        if (Status != OrderStatus.Draft)
            throw new InvalidOperationException("Only draft orders can be submitted.");

        Status = OrderStatus.Submitted;
    }
}

Prefer rich domain objects over anemic DTO-style objects

Bad: Logic outside, invariants unprotected

public sealed class BankAccount
{
    public decimal Balance { get; set; }
}

public sealed class BankAccountService
{
    public void Withdraw(BankAccount account, decimal amount)
    {
        if (amount <= 0)
            throw new ArgumentOutOfRangeException(nameof(amount));

        if (account.Balance < amount)
            throw new InvalidOperationException("Insufficient funds.");

        account.Balance -= amount;
    }
}

Better: Entity owns behavior and guards its state

public sealed class BankAccount
{
    public BankAccount(decimal initialBalance)
    {
        if (initialBalance < 0)
            throw new ArgumentOutOfRangeException(nameof(initialBalance));

        Balance = initialBalance;
    }

    public decimal Balance { get; private set; }

    public void Deposit(decimal amount)
    {
        if (amount <= 0)
            throw new ArgumentOutOfRangeException(nameof(amount));

        Balance += amount;
    }

    public void Withdraw(decimal amount)
    {
        if (amount <= 0)
            throw new ArgumentOutOfRangeException(nameof(amount));

        if (Balance < amount)
            throw new InvalidOperationException("Insufficient funds.");

        Balance -= amount;
    }
}

Use value objects where equality is by value

Bad: Primitive obsession leads to leaky validation and accidental equality

public sealed class User
{
    public string Email { get; set; } = "";
}

Better: Encapsulate validation and normalization in a value object

public sealed record EmailAddress
{
    public EmailAddress(string value)
    {
        if (string.IsNullOrWhiteSpace(value))
            throw new ArgumentException("Email must not be empty.", nameof(value));

        string normalized = value.Trim().ToLowerInvariant();

        if (!normalized.Contains('@'))
            throw new ArgumentException("Email is invalid.", nameof(value));

        Value = normalized;
    }

    public string Value { get; }

    public override string ToString() => Value;
}

public sealed class User
{
    public User(EmailAddress emailAddress)
    {
        EmailAddress = emailAddress;
    }

    public EmailAddress EmailAddress { get; }
}

Usage

var first = new EmailAddress("Test@Example.com");
var second = new EmailAddress("test@example.com");

bool same = first == second; // true

Preserve invariants inside the type

Bad: Invalid values are allowed and must be checked everywhere else

public sealed class PercentageDiscount
{
    public decimal Percent { get; set; }
}

Better: Constructor enforces the valid range

public sealed record PercentageDiscount
{
    public PercentageDiscount(decimal percent)
    {
        if (percent < 0 || percent > 100)
            throw new ArgumentOutOfRangeException(nameof(percent), "Discount must be between 0 and 100.");

        Percent = percent;
    }

    public decimal Percent { get; }
}

Eliminate invalid states instead of checking for them everywhere

Bad: Boolean flags and nullables create contradictory combinations

public sealed class Payment
{
    public bool IsPaid { get; set; }
    public DateTime? PaidAtUtc { get; set; }
}

Better: Represent state explicitly with a closed set of possibilities

public abstract record PaymentStatus;

public sealed record Unpaid : PaymentStatus;

public sealed record Paid(DateTime PaidAtUtc) : PaymentStatus;

public sealed class Payment
{
    public Payment()
    {
        Status = new Unpaid();
    }

    public PaymentStatus Status { get; private set; }

    public void MarkPaid(DateTime paidAtUtc)
    {
        if (Status is Paid)
            throw new InvalidOperationException("Payment is already marked as paid.");

        Status = new Paid(paidAtUtc);
    }
}

Encapsulate mutation; prefer immutability for value-like concepts

Bad: Mutable bags of public setters introduce shared-state bugs

public sealed class Address
{
    public string Street { get; set; } = "";
    public string City { get; set; } = "";
    public string PostalCode { get; set; } = "";
}

Better: Immutable value object; controlled change via with-expressions

public sealed record Address
{
    public Address(string street, string city, string postalCode)
    {
        if (string.IsNullOrWhiteSpace(street))
            throw new ArgumentException("Street is required.", nameof(street));
        if (string.IsNullOrWhiteSpace(city))
            throw new ArgumentException("City is required.", nameof(city));
        if (string.IsNullOrWhiteSpace(postalCode))
            throw new ArgumentException("Postal code is required.", nameof(postalCode));

        Street = street.Trim();
        City = city.Trim();
        PostalCode = postalCode.Trim();
    }

    public string Street { get; }
    public string City { get; }
    public string PostalCode { get; }
}

Usage

Address oldAddress = new("Main Street 1", "Vienna", "1010");
Address newAddress = oldAddress with { PostalCode = "1020" };

For entities, allow mutation but keep it controlled through methods.

Treat collections and relationships as part of the model

Bad: Exposed mutable collections break aggregate rules

public sealed class Order
{
    public List<OrderLine> Lines { get; } = new();
}

public sealed class OrderLine
{
    public string ProductCode { get; set; } = "";
    public int Quantity { get; set; }
}

Better: Hide the list; expose intention-revealing operations

public sealed class Order
{
    private readonly List<OrderLine> orderLines = new();

    public IReadOnlyList<OrderLine> OrderLines => orderLines;

    public void AddLine(string productCode, int quantity)
    {
        if (string.IsNullOrWhiteSpace(productCode))
            throw new ArgumentException("Product code is required.", nameof(productCode));

        if (quantity <= 0)
            throw new ArgumentOutOfRangeException(nameof(quantity));

        OrderLine? existingLine = orderLines.SingleOrDefault(line => line.ProductCode == productCode);
        if (existingLine is not null)
        {
            existingLine.IncreaseQuantity(quantity);
            return;
        }

        orderLines.Add(new OrderLine(productCode, quantity));
    }
}

public sealed class OrderLine
{
    public OrderLine(string productCode, int quantity)
    {
        if (string.IsNullOrWhiteSpace(productCode))
            throw new ArgumentException("Product code is required.", nameof(productCode));
        if (quantity <= 0)
            throw new ArgumentOutOfRangeException(nameof(quantity));

        ProductCode = productCode;
        Quantity = quantity;
    }

    public string ProductCode { get; }
    public int Quantity { get; private set; }

    public void IncreaseQuantity(int amount)
    {
        if (amount <= 0)
            throw new ArgumentOutOfRangeException(nameof(amount));

        Quantity += amount;
    }
}

Push behavior to the place where the data has meaning

Bad: Procedural helpers operate on passive data structures

public sealed class Money
{
    public decimal Amount { get; set; }
    public string Currency { get; set; } = "";
}

public static class PricingFunctions
{
    public static Money Add(Money left, Money right)
    {
        if (left.Currency != right.Currency)
            throw new InvalidOperationException("Currencies must match.");

        return new Money
        {
            Amount = left.Amount + right.Amount,
            Currency = left.Currency
        };
    }
}

Better: The concept owns its operations and invariants

public sealed record Money
{
    public Money(decimal amount, string currency)
    {
        if (string.IsNullOrWhiteSpace(currency))
            throw new ArgumentException("Currency is required.", nameof(currency));

        Amount = amount;
        Currency = currency.Trim().ToUpperInvariant();
    }

    public decimal Amount { get; }
    public string Currency { get; }

    public Money Add(Money other)
    {
        if (other is null)
            throw new ArgumentNullException(nameof(other));

        if (Currency != other.Currency)
            throw new InvalidOperationException("Currencies must match.");

        return new Money(Amount + other.Amount, Currency);
    }
}

Usage

Money total = new Money(10m, "EUR").Add(new Money(5m, "EUR"));

Use factories/builders when construction has rules

Bad: Objects can be created in half-baked, inconsistent shapes

public sealed class Subscription
{
    public string Email { get; set; } = "";
    public DateTime StartDateUtc { get; set; }
    public DateTime EndDateUtc { get; set; }
    public bool AutoRenew { get; set; }
}

Better: Factory centralizes creation logic and enforces invariants

public sealed class Subscription
{
    private Subscription(EmailAddress emailAddress, DateTime startDateUtc, DateTime endDateUtc, bool autoRenew)
    {
        EmailAddress = emailAddress;
        StartDateUtc = startDateUtc;
        EndDateUtc = endDateUtc;
        AutoRenew = autoRenew;
    }

    public EmailAddress EmailAddress { get; }
    public DateTime StartDateUtc { get; }
    public DateTime EndDateUtc { get; }
    public bool AutoRenew { get; }

    public static Subscription CreateAnnual(EmailAddress emailAddress, DateTime startDateUtc, bool autoRenew)
    {
        DateTime endDateUtc = startDateUtc.AddYears(1);

        if (endDateUtc <= startDateUtc)
            throw new InvalidOperationException("End date must be after start date.");

        return new Subscription(emailAddress, startDateUtc, endDateUtc, autoRenew);
    }
}

Usage

Subscription subscription = Subscription.CreateAnnual(
    new EmailAddress("person@example.com"),
    DateTime.UtcNow,
    autoRenew: true);

Reduce primitive obsession

Bad: Everything is string/int/decimal with unspecified units and formats

public sealed class Shipment
{
    public string TrackingNumber { get; set; } = "";
    public string CountryCode { get; set; } = "";
    public decimal Weight { get; set; }
}

Better: Narrow domain types communicate meaning and constraints

public sealed record CountryCode
{
    public CountryCode(string value)
    {
        if (string.IsNullOrWhiteSpace(value) || value.Trim().Length != 2)
            throw new ArgumentException("Country code must be a 2-letter ISO code.", nameof(value));

        Value = value.Trim().ToUpperInvariant();
    }

    public string Value { get; }
}

public sealed record Kilograms
{
    public Kilograms(decimal value)
    {
        if (value <= 0)
            throw new ArgumentOutOfRangeException(nameof(value));

        Value = value;
    }

    public decimal Value { get; }
}

public sealed record TrackingNumber
{
    public TrackingNumber(string value)
    {
        if (string.IsNullOrWhiteSpace(value))
            throw new ArgumentException("Tracking number is required.", nameof(value));

        Value = value.Trim();
    }

    public string Value { get; }
}

public sealed class Shipment
{
    public Shipment(TrackingNumber trackingNumber, CountryCode destinationCountryCode, Kilograms weight)
    {
        TrackingNumber = trackingNumber;
        DestinationCountryCode = destinationCountryCode;
        Weight = weight;
    }

    public TrackingNumber TrackingNumber { get; }
    public CountryCode DestinationCountryCode { get; }
    public Kilograms Weight { get; }
}

Common Pitfalls vs. Good Practices

Litmus Test

Can I create a nonsensical instance of the type without cheating?

If yes, the model is usually too weak. Strengthen the type with invariants and behavior until invalid states are impossible to represent.

Adoption Checklist

• Replace primitive properties that carry meaning (email, money, country, weights) with value objects.
• Make value-like concepts immutable; expose with-expressions for changes.
• Hide collections; add intention-revealing methods that enforce rules.
• Push domain operations onto the owning type, not helper services.
• Guard all constructors/factories with input validation and invariant checks.

Sources / Further Reading

Mapping each principle to key sources:

• Model concepts, not storage shapes: Evans, Domain-Driven Design (2003); Vernon, Implementing DDD (2013); Fowler,

  Anemic Domain Model.

• Prefer rich domain objects over anemic DTOs: Fowler,

  Anemic Domain Model; Evans (2003); Vernon (2013).

• Use value objects, equality by value: Evans (2003); Vernon (2013); Wlaschin, Domain Modeling Made Functional (2018).
• Preserve invariants inside the type: Evans (2003); Vernon (2013); Robert C. Martin, Clean Code (2008) (guard clauses).
• Eliminate invalid states: Wlaschin (2018) — illegal states unrepresentable; Option vs null; Evans (2003).
• Encapsulate mutation; prefer immutability: Microsoft Docs — C# records and with-expressions; Evans (2003).
• Collections and relationships are part of the model: Evans (2003) — Aggregates; Vernon (2013) — consistency boundaries.
• Push behavior to where data has meaning: Evans (2003); Vernon (2013); Martin, Clean Code (2008).
• Use factories/builders when construction has rules: Evans (2003); Vernon (2013); Martin, Clean Code (2008).
• Reduce primitive obsession: Evans (2003) — Value Objects; Vernon (2013); Wlaschin (2018).