Refactor Invoice Payment Processing and Tests

RefactorThis.Application.UnitTest/FakeInvoiceRepository.cs

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+using RefactorThis.Application.Interfaces;
+using RefactorThis.Domain.Entities;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace RefactorThis.UnitTests
+{
+    public class FakeInvoiceRepository : IInvoiceRepository
+    {
+        private readonly List<Invoice> _invoices = new List<Invoice>();
+
+        public void Add(Invoice invoice)
+        {
+            _invoices.Add(invoice);
+        }
+
+        public Invoice GetInvoiceByPayment(Payment payment)
+        {
+            // For simplicity, return first invoice (or null if none)
+            return _invoices.FirstOrDefault();
+        }
+
+        public IEnumerable<Invoice> GetAllInvoices()
+        {
+            return _invoices;
+        }
+
+        public Invoice GetInvoice(string reference)
+        {
+            // Return invoice by reference if needed
+            return _invoices.FirstOrDefault();
+        }
+
+        public void SaveInvoice(Invoice invoice)
+        {
+            // In-memory, nothing needed. Could replace the invoice if needed.
+        }
+    }
+}

RefactorThis.Application.UnitTest/InvoicePaymentProcessorTests.cs

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+using NUnit.Framework;
+using RefactorThis.Application.Common;
+using RefactorThis.Application.Interfaces;
+using RefactorThis.Domain;
+using RefactorThis.Domain.Entities;
+using RefactorThis.Domain.Enums;
+using RefactorThis.Domain.PaymentStrategies;
+using RefactorThis.UnitTests;
+using System;
+using System.Collections.Generic;
+
+namespace RefactorThis.Application.UnitTests
+{
+    [TestFixture]
+    public class InvoicePaymentProcessorTests
+    {
+        private IInvoiceRepository _repo;
+        private IInvoicePaymentStrategyFactory _strategyFactory;
+        private InvoiceService _paymentProcessor;
+
+
+        [SetUp]
+        public void Setup()
+        {
+            _repo = new FakeInvoiceRepository();
+            _strategyFactory = new InvoicePaymentStrategyFactory();
+            _paymentProcessor = new InvoiceService(_repo, _strategyFactory);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ThrowException_When_NoInvoiceFoundForPaymentReference()
+        {
+            var payment = new Payment();
+            var ex = Assert.Throws<InvalidOperationException>(() => _paymentProcessor.ProcessPayment(payment));
+            Assert.AreEqual("There is no invoice matching this payment", ex.Message);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnFailureMessage_When_NoPaymentNeeded()
+        {
+            var invoice = CreateInvoice(0, 0, 0);
+            var payment = new Payment();
+
+            var result = ProcessInvoicePayment(invoice, payment);
+
+            Assert.AreEqual(PaymentResultMessages.ToMessage(PaymentResultCode.NoPaymentNeeded, invoice), result);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnFailureMessage_When_InvoiceAlreadyFullyPaid()
+        {
+
+            var invoice = CreateInvoice(10, 10, 10);
+            var payment = new Payment();
+
+            var result = ProcessInvoicePayment(invoice, payment);
+            Assert.AreEqual(PaymentResultMessages.ToMessage(PaymentResultCode.InvoiceAlreadyPaid, invoice), result);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnFailureMessage_When_PartialPaymentExistsAndAmountPaidExceedsAmountDue()
+        {
+            var invoice = CreateInvoice(10, 5, 5);
+            var payment = new Payment() { Amount = 6 };
+
+            var result = ProcessInvoicePayment(invoice, payment);
+            Assert.AreEqual(PaymentResultMessages.ToMessage(PaymentResultCode.PaymentGreaterThanRemaining, invoice), result);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnFailureMessage_When_NoPartialPaymentExistsAndAmountPaidExceedsInvoiceAmount()
+        {
+            var invoice = CreateInvoice(5, 0, 0);
+            var payment = new Payment() { Amount = 6 };
+
+            var result = ProcessInvoicePayment(invoice, payment);
+
+            Assert.AreEqual(PaymentResultMessages.ToMessage(PaymentResultCode.PaymentGreaterThanInvoiceAmount, invoice), result);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnFullyPaidMessage_When_PartialPaymentExistsAndAmountPaidEqualsAmountDue()
+        {
+            var invoice = CreateInvoice(10, 5, 5);
+            var payment = new Payment() { Amount = 5 };
+
+            var result = ProcessInvoicePayment(invoice, payment);
+
+            Assert.AreEqual(PaymentResultMessages.ToMessage(PaymentResultCode.FullyPaid, invoice), result);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnFullyPaidMessage_When_NoPartialPaymentExistsAndAmountPaidEqualsInvoiceAmount()
+        { 
+            var invoice = CreateInvoice(10, 0, 10);
+            var payment = new Payment() { Amount = 10 };
+
+            var result = ProcessInvoicePayment(invoice, payment);
+
+            Assert.AreEqual(PaymentResultMessages.ToMessage(PaymentResultCode.InvoiceAlreadyPaid, invoice), result);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnPartiallyPaidMessage_When_PartialPaymentExistsAndAmountPaidIsLessThanAmountDue()
+        {
+            var invoice = CreateInvoice(10, 5, 5);
+            var payment = new Payment() { Amount = 1 };
+
+            var result = ProcessInvoicePayment(invoice, payment);
+            Assert.AreEqual(PaymentResultMessages.ToMessage(PaymentResultCode.PartiallyPaid, invoice), result);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnPartiallyPaidMessage_WhenFirstPartialPayment()
+        {
+            var invoice = CreateInvoice(10, 0, 0);
+            var payment = new Payment() { Amount = 1 };
+
+            var result = ProcessInvoicePayment(invoice, payment);
+            Assert.AreEqual(PaymentResultMessages.ToMessage(PaymentResultCode.PartiallyPaid, invoice), result);
+        }
+
+        private Invoice CreateInvoice(decimal amount, decimal amountPaid = 0, params decimal[] payments)
+        {
+            var invoice = new Invoice
+            {
+                Amount = amount,
+                AmountPaid = amountPaid,
+                Payments = new List<Payment>()
+            };
+
+            foreach (var p in payments)
+            {
+                invoice.Payments.Add(new Payment { Amount = p });
+            }
+
+            return invoice;
+        }
+
+        private string ProcessInvoicePayment(Invoice invoice, Payment payment)
+        {
+            _repo.Add(invoice);
+            return _paymentProcessor.ProcessPayment(payment);
+        }
+    }
+}

RefactorThis.Application.UnitTest/RefactorThis.Application.UnitTests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net472</TargetFramework>
+
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="NUnit" Version="3.12.0" />
+    <PackageReference Include="NUnit3TestAdapter" Version="3.16.1" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.5.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\RefactorThis.Application\RefactorThis.Application.csproj" />
+    <ProjectReference Include="..\RefactorThis.Domain\RefactorThis.Domain.csproj" />
+    <ProjectReference Include="..\RefactorThis.Infrastructure\RefactorThis.Infrastructure.csproj" />
+  </ItemGroup>
+
+</Project>

RefactorThis.Application/Common/PaymentResultMessages.cs

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+using RefactorThis.Domain.Entities;
+using RefactorThis.Domain.Enums;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace RefactorThis.Application.Common
+{
+    public static class PaymentResultMessages
+    {
+        public static string ToMessage(PaymentResultCode code, Invoice invoice)
+        {
+            switch (code)
+            {
+                case PaymentResultCode.NoPaymentNeeded:
+                    return "No payment needed";
+
+                case PaymentResultCode.InvoiceAlreadyPaid:
+                    return "Invoice was already fully paid";
+
+                case PaymentResultCode.PaymentGreaterThanRemaining:
+                    return "The payment is greater than the partial amount remaining";
+
+                case PaymentResultCode.PaymentGreaterThanInvoiceAmount:
+                    return "The payment is greater than the invoice amount";
+
+                case PaymentResultCode.FullyPaid:
+                    return invoice.Payments != null && invoice.Payments.Count > 1
+                        ? "Final partial payment received, invoice is now fully paid"
+                        : "Invoice is now fully paid";
+
+                case PaymentResultCode.PartiallyPaid:
+                    return invoice.Payments != null && invoice.Payments.Count > 1
+                        ? "Another partial payment received, still not fully paid"
+                        : "Invoice is now partially paid";
+
+                default:
+                    return "Unknown payment status";
+            }
+        }
+    }
+}

RefactorThis.Application/Interfaces/IInvoiceRepository.cs

@@ -0,0 +1,16 @@
+using RefactorThis.Domain.Entities;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace RefactorThis.Application.Interfaces
+{
+    public interface IInvoiceRepository
+    {
+        Invoice GetInvoice(string reference);
+        void Add(Invoice invoice);
+        void SaveInvoice(Invoice invoice);
+    }
+}

RefactorThis.Application/Properties/AssemblyInfo.cs

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+using System.Reflection;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+// General Information about an assembly is controlled through the following
+// set of attributes. Change these attribute values to modify the information
+// associated with an assembly.
+[assembly: AssemblyTitle("RefactorThis.Application")]
+[assembly: AssemblyDescription("")]
+[assembly: AssemblyConfiguration("")]
+[assembly: AssemblyCompany("")]
+[assembly: AssemblyProduct("RefactorThis.Application")]
+[assembly: AssemblyCopyright("Copyright ©  2025")]
+[assembly: AssemblyTrademark("")]
+[assembly: AssemblyCulture("")]
+
+// Setting ComVisible to false makes the types in this assembly not visible
+// to COM components.  If you need to access a type in this assembly from
+// COM, set the ComVisible attribute to true on that type.
+[assembly: ComVisible(false)]
+
+// The following GUID is for the ID of the typelib if this project is exposed to COM
+[assembly: Guid("666b1ab5-1d20-40f8-a5f5-59bbcd655a58")]
+
+// Version information for an assembly consists of the following four values:
+//
+//      Major Version
+//      Minor Version
+//      Build Number
+//      Revision
+//
+[assembly: AssemblyVersion("1.0.0.0")]
+[assembly: AssemblyFileVersion("1.0.0.0")]

RefactorThis.Application/RefactorThis.Application.csproj

@@ -0,0 +1,62 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <ProjectGuid>{666B1AB5-1D20-40F8-A5F5-59BBCD655A58}</ProjectGuid>
+    <OutputType>Library</OutputType>
+    <RootNamespace>RefactorThis.Application</RootNamespace>
+    <AssemblyName>RefactorThis.Application</AssemblyName>
+    <TargetFrameworkVersion>v4.7.2</TargetFrameworkVersion>
+    <FileAlignment>512</FileAlignment>
+    <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
+    <Deterministic>true</Deterministic>
+    <TargetFrameworkProfile />
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
+    <PlatformTarget>AnyCPU</PlatformTarget>
+    <DebugSymbols>true</DebugSymbols>
+    <DebugType>full</DebugType>
+    <Optimize>false</Optimize>
+    <OutputPath>bin\Debug\</OutputPath>
+    <DefineConstants>DEBUG;TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
+    <PlatformTarget>AnyCPU</PlatformTarget>
+    <DebugType>pdbonly</DebugType>
+    <Optimize>true</Optimize>
+    <OutputPath>bin\Release\</OutputPath>
+    <DefineConstants>TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <PropertyGroup>
+    <StartupObject />
+  </PropertyGroup>
+  <ItemGroup>
+    <Reference Include="System" />
+    <Reference Include="System.Core" />
+    <Reference Include="System.Xml.Linq" />
+    <Reference Include="System.Data.DataSetExtensions" />
+    <Reference Include="Microsoft.CSharp" />
+    <Reference Include="System.Data" />
+    <Reference Include="System.Net.Http" />
+    <Reference Include="System.Xml" />
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Include="Common\PaymentResultMessages.cs" />
+    <Compile Include="Interfaces\IInvoiceRepository.cs" />
+    <Compile Include="Properties\AssemblyInfo.cs" />
+    <Compile Include="Services\InvoiceService.cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\RefactorThis.Domain\RefactorThis.Domain.csproj">
+      <Project>{5310b2fe-e26d-414e-b656-1f74c5a70368}</Project>
+      <Name>RefactorThis.Domain</Name>
+    </ProjectReference>
+  </ItemGroup>
+  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
+</Project>