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csharp-oop/Domain/Account.cs

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+namespace CsharpOop.Domain;
+
+/// Domain entity / aggregate root representing a bank account
+public sealed class Account : AggregateRoot<AccountId>
+{
+    public Money Balance { get; private set; }
+
+    // Often present to satisfy serializers / ORMs.
+    private Account()
+    {
+        Balance = new Money(0);
+    }
+
+    public Account(AccountId id, Money openingBalance)
+    {
+        if (openingBalance.Amount < 0)
+            throw new ArgumentOutOfRangeException(nameof(openingBalance));
+
+        Id = id;
+
+        Balance = openingBalance;
+    }
+
+    // Domain behaviour attached to the entity.
+    public void Withdraw(Money amount)
+    {
+        if (amount.Amount <= 0)
+            throw new InvalidOperationException("Withdrawal amount must be positive.");
+
+        if (Balance.Amount - amount.Amount < 0)
+            throw new InvalidOperationException("Balance cannot go below zero.");
+
+        Balance = Balance.Subtract(amount);
+    }
+}

csharp-oop/Domain/AccountId.cs

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+namespace CsharpOop.Domain;
+
+/// Value object used to wrap the aggregate identity
+public sealed record AccountId(Guid Value);

csharp-oop/Domain/AggregateRoot.cs

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+namespace CsharpOop.Domain;
+
+/// Conventional DDD base type for aggregates
+public abstract class AggregateRoot<TId>
+{
+    public TId Id { get; protected set; } = default!;
+}

csharp-oop/Domain/IAccountRepository.cs

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+namespace CsharpOop.Domain;
+
+/// Repository abstraction used to load and save accounts
+public interface IAccountRepository
+{
+    Account? GetById(AccountId id);
+    void Save(Account account);
+}

csharp-oop/Domain/Money.cs

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+namespace CsharpOop.Domain;
+
+/// Value object used to represent money and enforce simple invariants.
+/// Note that this implementation uses immutable patterns for the data
+/// by returning a new instance for each modification. This is an early
+/// recommendation for DDD with OO, but not necessarily the common practice
+/// in many real-world implementations.
+public sealed class Money
+{
+    // Potentially with a setter - see note above
+    public decimal Amount { get; }
+
+    public Money(decimal amount)
+    {
+        Amount = amount;
+    }
+
+    // In many existing DDD/OO codebases you may actually see the use
+    // of mutable value types.
+    //
+    // public void Add(Money other)
+    // {
+    //     this.Amount += other.Amount;
+    // }
+
+    // On the other hand, sometimes these helpers may be left out
+    // and operations encoded directly "from the outside":
+    // newBalance = new Money(oldBalance.Amount - charge.Amount)
+    //
+    public Money Add(Money other) => new(Amount + other.Amount);
+
+    public Money Subtract(Money other) => new(Amount - other.Amount);
+}