add test project

csharp-fp2.Tests/AccountTests.cs

@@ -0,0 +1,51 @@
+using CsharpFp2.Domain;
+
+namespace CsharpFp2.Tests;
+
+/// Tests covering domain-layer invariants on the Account aggregate directly,
+/// independent of the application layer.
+public class AccountTests
+{
+    [Fact]
+    public void Opening_an_account_with_a_negative_balance_throws()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() =>
+            new Account(new AccountId(Guid.NewGuid()), new Money(-1m))
+        );
+    }
+
+    [Fact]
+    public void Withdrawing_a_zero_amount_throws()
+    {
+        var account = new Account(new AccountId(Guid.NewGuid()), new Money(100m));
+
+        Assert.Throws<InvalidOperationException>(() => account.Withdraw(new Money(0m)));
+    }
+
+    [Fact]
+    public void Withdrawing_a_negative_amount_throws()
+    {
+        var account = new Account(new AccountId(Guid.NewGuid()), new Money(100m));
+
+        Assert.Throws<InvalidOperationException>(() => account.Withdraw(new Money(-10m)));
+    }
+
+    [Fact]
+    public void Withdrawing_more_than_the_balance_throws()
+    {
+        var account = new Account(new AccountId(Guid.NewGuid()), new Money(100m));
+
+        Assert.Throws<InsufficientBalanceException>(() => account.Withdraw(new Money(101m)));
+    }
+
+    [Fact]
+    public void Successive_withdrawals_are_each_applied_to_the_running_balance()
+    {
+        var account = new Account(new AccountId(Guid.NewGuid()), new Money(300m));
+
+        account.Withdraw(new Money(100m));
+        account.Withdraw(new Money(100m));
+
+        Assert.Equal(100m, account.Balance.Amount);
+    }
+}