add tests

.gitignore

@@ -55,4 +55,5 @@ nunit-*.xml
 
 .idea
 .vscode
+*.user
 

csharp-oop.Tests/AccountTests.cs

@@ -0,0 +1,50 @@
+using CsharpOop.Domain;
+
+namespace CsharpOop.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);
+    }
+}

csharp-oop.Tests/WithdrawMoneyHandlerTests.cs

@@ -0,0 +1,78 @@
+using CsharpOop.Applications;
+using CsharpOop.Contracts;
+using CsharpOop.Domain;
+using CsharpOop.Infrastructure;
+
+namespace CsharpOop.Tests;
+
+/// Tests covering the application-layer use case surface:
+/// what the WithdrawMoney feature does from the caller's perspective.
+public class WithdrawMoneyHandlerTests
+{
+    private static (WithdrawMoneyHandler handler, InMemoryAccountRepository repository) BuildHandler()
+    {
+        var repository = new InMemoryAccountRepository();
+        var handler = new WithdrawMoneyHandler(repository);
+        return (handler, repository);
+    }
+
+    [Fact]
+    public void Withdrawing_from_an_account_reduces_its_balance_by_the_withdrawn_amount()
+    {
+        var (handler, repository) = BuildHandler();
+        var accountId = Guid.NewGuid();
+        repository.Save(new Account(new AccountId(accountId), new Money(200m)));
+
+        handler.Handle(new WithdrawMoneyCommand { AccountId = accountId, Amount = 75m });
+
+        var account = repository.GetById(new AccountId(accountId))!;
+        Assert.Equal(125m, account.Balance.Amount);
+    }
+
+    [Fact]
+    public void Withdrawing_the_entire_balance_leaves_the_account_at_zero()
+    {
+        var (handler, repository) = BuildHandler();
+        var accountId = Guid.NewGuid();
+        repository.Save(new Account(new AccountId(accountId), new Money(100m)));
+
+        handler.Handle(new WithdrawMoneyCommand { AccountId = accountId, Amount = 100m });
+
+        var account = repository.GetById(new AccountId(accountId))!;
+        Assert.Equal(0m, account.Balance.Amount);
+    }
+
+    [Fact]
+    public void Withdrawing_from_a_non_existent_account_throws()
+    {
+        var (handler, _) = BuildHandler();
+        var command = new WithdrawMoneyCommand { AccountId = Guid.NewGuid(), Amount = 50m };
+
+        Assert.Throws<InvalidOperationException>(() => handler.Handle(command));
+    }
+
+    [Fact]
+    public void Withdrawing_more_than_the_available_balance_throws()
+    {
+        var (handler, repository) = BuildHandler();
+        var accountId = Guid.NewGuid();
+        repository.Save(new Account(new AccountId(accountId), new Money(50m)));
+
+        Assert.Throws<InsufficientBalanceException>(() =>
+            handler.Handle(new WithdrawMoneyCommand { AccountId = accountId, Amount = 100m }));
+    }
+
+    [Fact]
+    public void After_a_failed_withdrawal_the_balance_is_unchanged()
+    {
+        var (handler, repository) = BuildHandler();
+        var accountId = Guid.NewGuid();
+        repository.Save(new Account(new AccountId(accountId), new Money(50m)));
+
+        try { handler.Handle(new WithdrawMoneyCommand { AccountId = accountId, Amount = 999m }); }
+        catch (InsufficientBalanceException) { }
+
+        var account = repository.GetById(new AccountId(accountId))!;
+        Assert.Equal(50m, account.Balance.Amount);
+    }
+}

csharp-oop.Tests/csharp-oop.Tests.csproj

@@ -0,0 +1,26 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <RootNamespace>csharp_oop.Tests</RootNamespace>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\csharp-oop\csharp-oop.csproj" />
+  </ItemGroup>
+
+</Project>

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;
+    }
+}

csharp.sln

@@ -0,0 +1,54 @@
+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.0.31903.59
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "OOP", "OOP", "{CEC0EC27-0CEC-90C6-CABA-E58AB278E4DA}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "csharp-oop", "csharp-oop\csharp-oop.csproj", "{E00F4E4F-8D52-49CE-8754-ED8378A5278F}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "csharp-oop.Tests", "csharp-oop.Tests\csharp-oop.Tests.csproj", "{9FD0C8F4-A23B-4C68-A365-86E7EF5623D8}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Debug|x64 = Debug|x64
+		Debug|x86 = Debug|x86
+		Release|Any CPU = Release|Any CPU
+		Release|x64 = Release|x64
+		Release|x86 = Release|x86
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
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+		{E00F4E4F-8D52-49CE-8754-ED8378A5278F}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{E00F4E4F-8D52-49CE-8754-ED8378A5278F}.Release|Any CPU.Build.0 = Release|Any CPU
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+		{E00F4E4F-8D52-49CE-8754-ED8378A5278F}.Release|x64.Build.0 = Release|Any CPU
+		{E00F4E4F-8D52-49CE-8754-ED8378A5278F}.Release|x86.ActiveCfg = Release|Any CPU
+		{E00F4E4F-8D52-49CE-8754-ED8378A5278F}.Release|x86.Build.0 = Release|Any CPU
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+		{9FD0C8F4-A23B-4C68-A365-86E7EF5623D8}.Debug|x64.Build.0 = Debug|Any CPU
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+		{9FD0C8F4-A23B-4C68-A365-86E7EF5623D8}.Debug|x86.Build.0 = Debug|Any CPU
+		{9FD0C8F4-A23B-4C68-A365-86E7EF5623D8}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{9FD0C8F4-A23B-4C68-A365-86E7EF5623D8}.Release|Any CPU.Build.0 = Release|Any CPU
+		{9FD0C8F4-A23B-4C68-A365-86E7EF5623D8}.Release|x64.ActiveCfg = Release|Any CPU
+		{9FD0C8F4-A23B-4C68-A365-86E7EF5623D8}.Release|x64.Build.0 = Release|Any CPU
+		{9FD0C8F4-A23B-4C68-A365-86E7EF5623D8}.Release|x86.ActiveCfg = Release|Any CPU
+		{9FD0C8F4-A23B-4C68-A365-86E7EF5623D8}.Release|x86.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(NestedProjects) = preSolution
+		{E00F4E4F-8D52-49CE-8754-ED8378A5278F} = {CEC0EC27-0CEC-90C6-CABA-E58AB278E4DA}
+		{9FD0C8F4-A23B-4C68-A365-86E7EF5623D8} = {CEC0EC27-0CEC-90C6-CABA-E58AB278E4DA}
+	EndGlobalSection
+EndGlobal