Event Sourcing in .NET: Concepts and Implementation

namespace Bank.Domain;

public sealed record AccountOpened(
    Guid AccountId, string Owner, decimal OverdraftLimit, DateTimeOffset OpenedAt);
public sealed record MoneyDeposited(
    Guid AccountId, decimal Amount, string Reference, DateTimeOffset At);
public sealed record MoneyWithdrawn(
    Guid AccountId, decimal Amount, string Reference, DateTimeOffset At);
public sealed record AccountClosed(Guid AccountId, string Reason, DateTimeOffset ClosedAt);

public sealed record BankAccount(
    Guid Id, string Owner, decimal Balance, decimal OverdraftLimit, bool IsClosed)
{
    // Evolve: pure functions from (state, event) to new state.
    public static BankAccount Create(AccountOpened e) =>
        new(e.AccountId, e.Owner, 0m, e.OverdraftLimit, IsClosed: false);

    public BankAccount Apply(MoneyDeposited e) => this with { Balance = Balance + e.Amount };
    public BankAccount Apply(MoneyWithdrawn e) => this with { Balance = Balance - e.Amount };
    public BankAccount Apply(AccountClosed e) => this with { IsClosed = true };

    // Decide: validate a command against current state and return new events.
    public MoneyWithdrawn Withdraw(decimal amount, string reference, DateTimeOffset now)
    {
        if (IsClosed)
            throw new InvalidOperationException("The account is closed.");
        ArgumentOutOfRangeException.ThrowIfNegativeOrZero(amount);
        if (Balance - amount < -OverdraftLimit)
            throw new InvalidOperationException("The overdraft limit would be exceeded.");

        return new MoneyWithdrawn(Id, amount, reference, now);
    }

    // Rehydrate: a left fold over the stream.
    public static BankAccount Rehydrate(IReadOnlyList<object> history) =>
        history.Skip(1).Aggregate(
            Create((AccountOpened)history[0]),
            (state, e) => e switch
            {
                MoneyDeposited d => state.Apply(d),
                MoneyWithdrawn w => state.Apply(w),
                AccountClosed c => state.Apply(c),
                _ => state // Tolerate event types this version does not know.
            });
}

public class BankAccountTests
{
    private static readonly Guid Id = Guid.NewGuid();
    private static readonly DateTimeOffset Now = DateTimeOffset.UtcNow;

    [Fact]
    public void Withdrawal_beyond_the_overdraft_limit_is_rejected()
    {
        // Given
        var account = BankAccount.Rehydrate([
            new AccountOpened(Id, "Ada", OverdraftLimit: 100m, Now),
            new MoneyDeposited(Id, 50m, "salary", Now)]);

        // When / Then
        Assert.Throws<InvalidOperationException>(() => account.Withdraw(200m, "rent", Now));
    }
}