add fp3 tests

csharp-fp3.Tests/AccountTests.cs

@@ -0,0 +1,73 @@
+using CsharpFp3.Domain;
+
+namespace CsharpFp3.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_returns_OpeningBalanceMustBeNonNegative()
+    {
+        var result = AccountDomain.Open(Guid.NewGuid(), new Money(-1m));
+
+        Assert.IsType<AccountError.OpeningBalanceMustBeNonNegative>(result.Error);
+    }
+
+    [Fact]
+    public void Withdrawing_a_zero_amount_returns_AmountMustBePositive()
+    {
+        var account = AccountDomain.Open(Guid.NewGuid(), new Money(100m)).Value;
+
+        var result = AccountDomain.Withdraw(account, new Money(0m));
+
+        Assert.IsType<AccountError.AmountMustBePositive>(result.Error);
+    }
+
+    [Fact]
+    public void Withdrawing_a_negative_amount_returns_AmountMustBePositive()
+    {
+        var account = AccountDomain.Open(Guid.NewGuid(), new Money(100m)).Value;
+
+        var result = AccountDomain.Withdraw(account, new Money(-10m));
+
+        Assert.IsType<AccountError.AmountMustBePositive>(result.Error);
+    }
+
+    [Fact]
+    public void Withdrawing_more_than_the_balance_returns_InsufficientBalance()
+    {
+        var account = AccountDomain.Open(Guid.NewGuid(), new Money(100m)).Value;
+
+        var result = AccountDomain.Withdraw(account, new Money(101m));
+
+        Assert.IsType<AccountError.InsufficientBalance>(result.Error);
+    }
+
+    [Fact]
+    public void InsufficientBalance_error_reports_the_current_balance_and_attempted_amount()
+    {
+        var account = AccountDomain.Open(Guid.NewGuid(), new Money(100m)).Value;
+
+        var error = Assert.IsType<AccountError.InsufficientBalance>(
+            AccountDomain.Withdraw(account, new Money(150m)).Error
+        );
+
+        Assert.Equal(100m, error.Balance.Amount);
+        Assert.Equal(150m, error.Amount.Amount);
+    }
+
+    [Fact]
+    public void Successive_withdrawals_are_each_applied_to_the_running_balance()
+    {
+        var account = AccountDomain.Open(Guid.NewGuid(), new Money(300m)).Value;
+
+        var result1 = AccountDomain.Withdraw(account, new Money(100m));
+        var updatedAccount1 = result1.Value;
+
+        var result2 = AccountDomain.Withdraw(updatedAccount1, new Money(100m));
+        var updatedAccount2 = result2.Value;
+
+        Assert.Equal(100m, updatedAccount2.Balance.Amount);
+    }
+}