add tests

csharp-oop/Domain/Account.cs

@@ -27,8 +27,9 @@ public sealed class Account : AggregateRoot<AccountId>
         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 InvalidOperationException("Balance cannot go below zero.");
+            throw new InsufficientBalanceException(Balance, amount);
 
         Balance = Balance.Subtract(amount);
     }

csharp-oop/Domain/InsufficientBalanceException.cs

@@ -0,0 +1,15 @@
+namespace CsharpOop.Domain;
+
+/// Custom domain exception thrown when a withdrawal would cause the balance to go below zero
+public sealed class InsufficientBalanceException : InvalidOperationException
+{
+    public Money CurrentBalance { get; }
+    public Money RequestedAmount { get; }
+
+    public InsufficientBalanceException(Money currentBalance, Money requestedAmount)
+        : base($"Insufficient balance. Current: {currentBalance.Amount:0.00}, Requested: {requestedAmount.Amount:0.00}")
+    {
+        CurrentBalance = currentBalance;
+        RequestedAmount = requestedAmount;
+    }
+}