add simplification steps

csharp-fp1/Application/AccountApplicationService.cs

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+using CsharpFp1.Domain;
+
+namespace CsharpFp1.Application;
+
+public static class AccountApplicationService
+{
+    public static void WithdrawMoney(IAccountRepository repository, Guid accountId, decimal amount)
+    {
+        var account =
+            repository.GetById(new AccountId(accountId))
+            ?? throw new InvalidOperationException("Account not found.");
+
+        account.Withdraw(new Money(amount));
+
+        repository.Save(account);
+    }
+}

csharp-fp1/Domain/Account.cs

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+namespace CsharpFp1.Domain;
+
+/// Domain entity / aggregate root representing a bank account
+public sealed class Account : AggregateRoot<AccountId>
+{
+    public Money Balance { get; private set; }
+
+    // Often present to satisfy serializers / ORMs.
+    private Account()
+    {
+        Balance = new Money(0);
+    }
+
+    public Account(AccountId id, Money openingBalance)
+    {
+        if (openingBalance.Amount < 0)
+            throw new ArgumentOutOfRangeException(nameof(openingBalance));
+
+        Id = id;
+
+        Balance = openingBalance;
+    }
+
+    // Domain behaviour attached to the entity.
+    public void Withdraw(Money amount)
+    {
+        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 InsufficientBalanceException(Balance, amount);
+
+        Balance = Balance.Subtract(amount);
+    }
+}

csharp-fp1/Domain/AccountId.cs

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+namespace CsharpFp1.Domain;
+
+/// Value object used to wrap the aggregate identity
+public sealed record AccountId(Guid Value);

csharp-fp1/Domain/AggregateRoot.cs

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+namespace CsharpFp1.Domain;
+
+/// Conventional DDD base type for aggregates
+public abstract class AggregateRoot<TId>
+{
+    public TId Id { get; protected set; } = default!;
+}

csharp-fp1/Domain/IAccountRepository.cs

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+namespace CsharpFp1.Domain;
+
+/// Repository abstraction used to load and save accounts
+public interface IAccountRepository
+{
+    Account? GetById(AccountId id);
+    void Save(Account account);
+}

csharp-fp1/Domain/InsufficientBalanceException.cs

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+namespace CsharpFp1.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-fp1/Domain/Money.cs

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+namespace CsharpFp1.Domain;
+
+/// Value object used to represent money and enforce simple invariants.
+/// Note that this implementation uses immutable patterns for the data
+/// by returning a new instance for each modification. This is an early
+/// recommendation for DDD with OO, but not necessarily the common practice
+/// in many real-world implementations.
+public sealed class Money
+{
+    // Potentially with a setter - see note above
+    public decimal Amount { get; }
+
+    public Money(decimal amount)
+    {
+        Amount = amount;
+    }
+
+    // In many existing DDD/OO codebases you may actually see the use
+    // of mutable value types.
+    //
+    // public void Add(Money other)
+    // {
+    //     this.Amount += other.Amount;
+    // }
+
+    // On the other hand, sometimes these helpers may be left out
+    // and operations encoded directly "from the outside":
+    // newBalance = new Money(oldBalance.Amount - charge.Amount)
+    //
+    public Money Add(Money other) => new(Amount + other.Amount);
+
+    public Money Subtract(Money other) => new(Amount - other.Amount);
+}

csharp-fp1/Infrastructure/InMemoryAccountRepository.cs

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+using CsharpFp1.Domain;
+
+namespace CsharpFp1.Infrastructure;
+
+/// Simple in-memory repository for demonstration
+public class InMemoryAccountRepository : IAccountRepository
+{
+    private readonly Dictionary<AccountId, Account> _accounts = new();
+
+    public Account? GetById(AccountId id)
+    {
+        var found = _accounts.TryGetValue(id, out var account);
+        Console.WriteLine(
+            found ? $"[Repo] Loaded account {id.Value}" : $"[Repo] Account {id.Value} not found"
+        );
+
+        return found ? account : null;
+    }
+
+    public void Save(Account account)
+    {
+        _accounts[account.Id] = account;
+        Console.WriteLine(
+            $"[Repo] Saved account {account.Id.Value} with balance {account.Balance.Amount:0.00}"
+        );
+    }
+}

csharp-fp1/Program.cs

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+using CsharpFp1.Application;
+using CsharpFp1.Domain;
+using CsharpFp1.Infrastructure;
+
+namespace CsharpFp1;
+
+public class Program
+{
+    public static void Main()
+    {
+        Console.WriteLine("[csharp-fp1] Starting withdraw money demo...");
+
+        var repository = new InMemoryAccountRepository();
+
+        var accountId = Guid.NewGuid();
+        Console.WriteLine($"[csharp-fp1] Seeding account {accountId} with opening balance 200.00");
+        repository.Save(new Account(new AccountId(accountId), new Money(200m)));
+
+        decimal amount = 100m;
+        Console.WriteLine($"[csharp-fp1] Executing withdrawal {amount:0.00} from account {accountId}");
+        AccountApplicationService.WithdrawMoney(repository, accountId, amount);
+
+        Console.WriteLine("[csharp-fp1] Demo completed.");
+    }
+}

csharp-fp1/csharp-fp1.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net10.0</TargetFramework>
+    <RootNamespace>csharp_fp1</RootNamespace>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+</Project>