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Added Multi stream projection exercise to introduction to Event Sourcing workshop
Renamed also the further exercises
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‎.claude/settings.local.json‎

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{
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"permissions": {
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"allow": [
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"Bash(ls:*)"
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]
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}
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}

‎EventSourcing.NetCore.slnx‎

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<Project Path="Workshops/IntroductionToEventSourcing/12-Projections.SingleStream/12-Projections.SingleStream.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/13-Projections.SingleStream.Idempotency/13-Projections.SingleStream.Idempotency.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/14-Projections.SingleStream.EventualConsistency/14-Projections.SingleStream.EventualConsistency.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/15-EventsDefinition/15-EventsDefinition.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/16-EntitiesDefinition/16-EntitiesDefinition.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/17-BusinessProcesses/17-BusinessProcesses.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/15-Projections.MultiStream/15-Projections.MultiStream.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/19-EventsDefinition/19-EventsDefinition.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/20-EntitiesDefinition/20-EntitiesDefinition.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/21-BusinessProcesses/21-BusinessProcesses.csproj" />
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</Folder>
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<Folder Name="/Workshops/IntroductionToEventSourcing/docker/">
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<File Path="Workshops/IntroductionToEventSourcing/docker-compose.yml" />
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<Project Path="Workshops/IntroductionToEventSourcing/Solved/12-Projections.SingleStream/12-Projections.SingleStream.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/Solved/13-Projections.SingleStream.Idempotency/13-Projections.SingleStream.Idempotency.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/Solved/14-Projections.SingleStream.EventualConsistency/14-Projections.SingleStream.EventualConsistency.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/Solved/15-EventsDefinition/15-EventsDefinition.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/Solved/16-EntitiesDefinition/16-EntitiesDefinition.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/Solved/17-BusinessProcesses/17-BusinessProcesses.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/Solved/15-Projections.MultiStream/15-Projections.MultiStream.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/Solved/19-EventsDefinition/19-EventsDefinition.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/Solved/20-EntitiesDefinition/20-EntitiesDefinition.csproj" />
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<Project Path="Workshops/IntroductionToEventSourcing/Solved/21-BusinessProcesses/21-BusinessProcesses.csproj" />
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</Folder>
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<Properties Name="RiderSharedRunConfigurations" Scope="PostLoad" />
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</Solution>

‎Workshops/IntroductionToEventSourcing/15-Projections.MultiStream/15-Projections.MultiStream.csproj‎

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<Project Sdk="Microsoft.NET.Sdk">
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<PropertyGroup>
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<TargetFramework>net10.0</TargetFramework>
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<RootNamespace>IntroductionToEventSourcing.Projections.MultiStream</RootNamespace>
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</PropertyGroup>
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<ItemGroup>
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<PackageReference Include="FluentAssertions" />
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<PackageReference Include="Microsoft.NET.Test.Sdk" />
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<PackageReference Include="GitHubActionsTestLogger">
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<PrivateAssets>all</PrivateAssets>
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<IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
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</PackageReference>
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<PackageReference Include="xunit" />
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<PackageReference Include="xunit.runner.visualstudio">
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<PrivateAssets>all</PrivateAssets>
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<IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
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</PackageReference>
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<PackageReference Include="coverlet.collector">
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<IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
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<PrivateAssets>all</PrivateAssets>
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</PackageReference>
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</ItemGroup>
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</Project>

‎Workshops/IntroductionToEventSourcing/15-Projections.MultiStream/ProjectionsTests.cs‎

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using FluentAssertions;
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using IntroductionToEventSourcing.Projections.MultiStream.Tools;
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using Xunit;
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namespace IntroductionToEventSourcing.Projections.MultiStream;
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// EVENTS
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public record PaymentRecorded(
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Guid PaymentId,
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Guid OrderId,
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decimal Amount
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);
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public record MerchantLimitsChecked(
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Guid PaymentId,
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Guid MerchantId,
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bool IsWithinLimits
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);
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public record FraudScoreCalculated(
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Guid PaymentId,
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decimal Score,
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bool IsAcceptable
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);
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public record PaymentVerificationCompleted(
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Guid PaymentId,
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bool IsApproved
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);
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// ENUMS
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public enum VerificationStatus
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{
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Pending,
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Passed,
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Failed
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}
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public enum PaymentStatus
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{
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Pending,
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Approved,
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Rejected
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}
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// READ MODEL
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public class PaymentVerification
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{
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public Guid Id { get; set; }
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public PaymentStatus Status { get; set; }
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}
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public class ProjectionsTests
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{
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[Fact]
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[Trait("Category", "SkipCI")]
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public void MultiStreamProjection_ForPaymentVerification_ShouldSucceed()
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{
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var payment1Id = Guid.CreateVersion7();
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var payment2Id = Guid.CreateVersion7();
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var payment3Id = Guid.CreateVersion7();
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var payment4Id = Guid.CreateVersion7();
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var order1Id = Guid.CreateVersion7();
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var order2Id = Guid.CreateVersion7();
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var order3Id = Guid.CreateVersion7();
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var order4Id = Guid.CreateVersion7();
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var merchant1Id = Guid.CreateVersion7();
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var merchant2Id = Guid.CreateVersion7();
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var fraudCheck1Id = Guid.CreateVersion7();
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var fraudCheck2Id = Guid.CreateVersion7();
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var fraudCheck3Id = Guid.CreateVersion7();
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var eventStore = new EventStore();
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var database = new Database();
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// TODO:
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// 1. Create a PaymentVerificationProjection class that handles each event type.
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// Events arrive on different streams (payment, merchant, fraud check),
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// but they share PaymentId — use PaymentId as the read model key.
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// 2. Register your event handlers using `eventStore.Register`.
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// Payment 1: Approved — all checks pass
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eventStore.Append(payment1Id, new PaymentRecorded(payment1Id, order1Id, 100m));
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eventStore.Append(merchant1Id, new MerchantLimitsChecked(payment1Id, merchant1Id, true));
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eventStore.Append(fraudCheck1Id, new FraudScoreCalculated(payment1Id, 0.1m, true));
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eventStore.Append(payment1Id, new PaymentVerificationCompleted(payment1Id, true));
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// Payment 2: Merchant rejected — exceeds merchant limits
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eventStore.Append(payment2Id, new PaymentRecorded(payment2Id, order2Id, 5000m));
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eventStore.Append(merchant2Id, new MerchantLimitsChecked(payment2Id, merchant2Id, false));
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eventStore.Append(fraudCheck2Id, new FraudScoreCalculated(payment2Id, 0.2m, true));
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eventStore.Append(payment2Id, new PaymentVerificationCompleted(payment2Id, false));
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// Payment 3: Fraud rejected — high fraud score
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eventStore.Append(payment3Id, new PaymentRecorded(payment3Id, order3Id, 200m));
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eventStore.Append(merchant1Id, new MerchantLimitsChecked(payment3Id, merchant1Id, true));
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eventStore.Append(fraudCheck3Id, new FraudScoreCalculated(payment3Id, 0.95m, false));
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eventStore.Append(payment3Id, new PaymentVerificationCompleted(payment3Id, false));
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// Payment 4: Pending — still awaiting fraud check and final decision
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eventStore.Append(payment4Id, new PaymentRecorded(payment4Id, order4Id, 50m));
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eventStore.Append(merchant1Id, new MerchantLimitsChecked(payment4Id, merchant1Id, true));
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// Assert Payment 1: Approved
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var payment1 = database.Get<PaymentVerification>(payment1Id)!;
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payment1.Should().NotBeNull();
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payment1.Id.Should().Be(payment1Id);
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payment1.Status.Should().Be(PaymentStatus.Approved);
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// Assert Payment 2: Merchant rejected
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var payment2 = database.Get<PaymentVerification>(payment2Id)!;
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payment2.Should().NotBeNull();
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payment2.Id.Should().Be(payment2Id);
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payment2.Status.Should().Be(PaymentStatus.Rejected);
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// Assert Payment 3: Fraud rejected
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var payment3 = database.Get<PaymentVerification>(payment3Id)!;
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payment3.Should().NotBeNull();
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payment3.Id.Should().Be(payment3Id);
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payment3.Status.Should().Be(PaymentStatus.Rejected);
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// Assert Payment 4: Pending
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var payment4 = database.Get<PaymentVerification>(payment4Id)!;
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payment4.Should().NotBeNull();
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payment4.Id.Should().Be(payment4Id);
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payment4.Status.Should().Be(PaymentStatus.Pending);
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}
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}

‎Workshops/IntroductionToEventSourcing/15-Projections.MultiStream/README.md‎

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‎Workshops/IntroductionToEventSourcing/15-Projections.MultiStream/Tools/Database.cs‎

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using System.Text.Json;
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namespace IntroductionToEventSourcing.Projections.MultiStream.Tools;
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public class Database
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{
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private readonly Dictionary<string, object> storage = new();
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public void Store<T>(Guid id, T obj) where T: class =>
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storage[GetId<T>(id)] = obj;
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public void Delete<T>(Guid id) =>
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storage.Remove(GetId<T>(id));
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public T? Get<T>(Guid id) where T: class =>
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storage.TryGetValue(GetId<T>(id), out var result) ?
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// Clone to simulate getting new instance on loading
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JsonSerializer.Deserialize<T>(JsonSerializer.Serialize((T)result))
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: null;
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private static string GetId<T>(Guid id) => $"{typeof(T).Name}-{id}";
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}

‎Workshops/IntroductionToEventSourcing/15-Projections.MultiStream/Tools/EventEnvelope.cs‎

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namespace IntroductionToEventSourcing.Projections.MultiStream.Tools;
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public record EventMetadata(
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string EventId,
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ulong StreamPosition,
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ulong LogPosition
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)
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{
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public static EventMetadata For(ulong streamPosition, ulong logPosition) =>
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new(Guid.CreateVersion7().ToString(), streamPosition, logPosition);
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}
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public record EventEnvelope(
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object Data,
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EventMetadata Metadata
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);
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public record EventEnvelope<T>(
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T Data,
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EventMetadata Metadata
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): EventEnvelope(Data, Metadata) where T : notnull
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{
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public new T Data => (T)base.Data;
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}

‎Workshops/IntroductionToEventSourcing/15-Projections.MultiStream/Tools/EventStore.cs‎

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namespace IntroductionToEventSourcing.Projections.MultiStream.Tools;
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public class EventStore
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{
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private readonly Dictionary<Type, List<Action<EventEnvelope>>> handlers = new();
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private readonly Dictionary<Guid, List<EventEnvelope>> events = new();
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public void Register<TEvent>(Action<EventEnvelope<TEvent>> handler) where TEvent : notnull
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{
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var eventType = typeof(TEvent);
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void WrappedHandler(object @event) => handler((EventEnvelope<TEvent>)@event);
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if (handlers.TryGetValue(eventType, out var handler1))
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handler1.Add(WrappedHandler);
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else
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handlers.Add(eventType, [WrappedHandler]);
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}
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public void Append<TEvent>(Guid streamId, TEvent @event) where TEvent : notnull
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{
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if (!events.ContainsKey(streamId))
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events[streamId] = [];
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var eventEnvelope = new EventEnvelope<TEvent>(@event,
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EventMetadata.For(
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(ulong)events[streamId].Count + 1,
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(ulong)events.Values.Sum(s => s.Count)
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)
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);
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events[streamId].Add(eventEnvelope);
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if (!handlers.TryGetValue(eventEnvelope.Data.GetType(), out var eventHandlers)) return;
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foreach (var handle in eventHandlers)
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{
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handle(eventEnvelope);
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}
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}
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}

Workshops/IntroductionToEventSourcing/15-EventsDefinition/15-EventsDefinition.csproj renamed to Workshops/IntroductionToEventSourcing/19-EventsDefinition/19-EventsDefinition.csproj

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Workshops/IntroductionToEventSourcing/15-EventsDefinition/EventsDefinitionTests.cs renamed to Workshops/IntroductionToEventSourcing/19-EventsDefinition/EventsDefinitionTests.cs

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