add simplification steps

csharp-fp1/Domain/Account.cs

@@ -0,0 +1,36 @@
+namespace CsharpFp1.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.");
+
+        // Sometimes, validation is modelled with custom exceptions instead
+        if (Balance.Amount - amount.Amount < 0)
+            throw new InsufficientBalanceException(Balance, amount);
+
+        Balance = Balance.Subtract(amount);
+    }
+}