Guided by practical examples this talk conveys the value of how code can become clearer when focusing on behavior, language, responsibility and the chosen model to implement. The clarity that ensues can lead to a more supple design allowing to implement - or assess the effort required to implement - future functionality faster.

4. public class Group {
public Guid Id { get; set; }
public string Name { get; set; }
public string Objective { get; set; }
public GroupType Type { get; set; }
//...
}
//Usage
var @group = new Group {
Id = message.GroupId,
Name = message.Name,
Objective = message.Objective,
Type = GroupType.Private
};

5. public class Group {
public Group(
Guid id,
string name,
string objective,
GroupType type)
{
//...
}
}
//Usage
var @group = new Group(
message.GroupId,
message.Name,
message.Objective,
GroupType.Private
);

6. public class PrivateGroup {
private PrivateGroup(/* ... */)
{
/* ... */
}
public static PrivateGroup Start(
GroupId id,
GroupName name,
GroupObjective objective)
{
//...
}
}
//Usage
var @group = PrivateGroup.Start(
new GroupId(message.GroupId),
new GroupName(message.Name),
GroupObjective.InferFrom(message.Objective)
);

7. public class Group {
public Group(
Guid id,
string name,
string objective,
GroupType type)
{
if(id == Guid.Empty)
throw new ArgumentException("...", "id");
if(string.IsNullOrEmpty(name))
throw new ArgumentException("...", "name");
if(name.Length > 255)
throw new ArgumentException("...", "name");
if(objective != null && objective.Length == 0)
throw new ArgumentException("...", "objective");
if(objective != null && objective.Length > 500)
throw new ArgumentException("...", "objective");
if(type != GroupType.Private && type != GroupType.Public)
throw new ArgumentException("...", "type");
//...
}
}

8. public class PrivateGroup {
public static PrivateGroup Start(
GroupId id,
GroupName name,
GroupObjective objective)
{
//...
}
}

9. public class GroupId {
private readonly Guid value;
public GroupId(Guid value)
{
if(value == Guid.Empty)
throw new ArgumentException("...", "value");
this.value = value;
}
public override bool Equals(object other)
{
if(other == null || other.GetType() != this.GetType()) return false;
return ((GroupId)other).value.Equals(this.value);
}
public override int GetHashCode()
{
return this.value.GetHashCode();
}
//...
}

10. public class GroupName {
private readonly string value;
public GroupName(string value)
{
if(string.IsNullOrEmpty(value))
throw new ArgumentException("...", "value");
if(value.Length > Metadata.MaximumGroupNameLength)
throw new ArgumentException("...", "value");
this.value = value;
}
//...
}
//Usage
@group.Rename(new GroupName(message.Name));

11. public class GroupObjective {
private static readonly GroupObjective NotSpecified = new GroupObjective(null);
private readonly string value;
private GroupObjective(string value)
{
this.value = value;
}
public static GroupObjective InferFrom(string value)
{
if(value == null) return NotSpecified;
if(value.Length == 0)
throw new ArgumentException("...", "value");
if(value.Length > Metadata.MaximumGroupObjectiveLength)
throw new ArgumentException("...", "value");
return new GroupObjective(value);
}
}

12. public class GroupObjective {
//...
public static GroupObjective InferFromMarkdown(string value)
{
if(value == null) return NotSpecified;
MarkDownDocument parsedDocument;
if(!MarkDownParser.TryParse(value, out parsedDocument))
throw new ArgumentException("...", "value");
var document = parsedDocument.Sanitize();
if(document.Text.Length == 0)
throw new ArgumentException("...", "value");
if(document.Text.Length > Metadata.MaximumGroupObjectiveLength)
throw new ArgumentException("...", "value");
return new GroupObjective(document.ToString());
}
}

13. public class GroupObjective {
private static readonly GroupObjective NotSpecified =
new GroupObjective(MimeType.PlainText, null);
private readonly MimeType mimeType;
private readonly string value;
private GroupObjective(MimeType mimeType, string value)
{
this.mimeType = mimeType;
this.value = value;
}
public static GroupObjective InferFromMarkdown(MarkDownDocument value)
{
if(value == null) return NotSpecified;
var document = value.Sanitize();
if(document.Text.Length == 0)
throw new ArgumentException("...", "value");
if(document.Text.Length > Metadata.MaximumGroupObjectiveLength)
throw new ArgumentException("...", "value");
return new GroupObjective(MimeType.Markdown, document.ToString());
}
}

14. public class GroupObjective {
public static GroupObjective InferFromMarkdown(SanitizedMarkDownDocument document)
{
if(value == null) return NotSpecified;
if(document.Text.Length == 0)
throw new ArgumentException("...", "value");
if(document.Text.Length > Metadata.MaximumGroupObjectiveLength)
throw new ArgumentException("...", "value");
return new GroupObjective(MimeType.Markdown, document.ToString());
}
}

17. public class FillPercentage {
private readonly double value;
public FillPercentage(double value)
{
if(value < 0)
throw new ArgumentOutOfRangeException("value", value,
"The fill percentage must be greater than or equal to 0.");
this.value = value;
}
public static FillPercentage FromPercentNotation(double value)
{
return new FillPercentage(value / 100d);
}
public static implicit operator double(FillPercentage instance)
{
return instance.value;
}
}

18. public class MaximumFillPercentage {
private readonly double value;
public MaximumFillPercentage(double value)
{
if(value < 1)
throw new ArgumentOutOfRangeException("value", value,
"The maximum fill percentage must be greater than or equal to 1.");
this.value = value;
}
public static MaximumFillPercentage FromPercentNotation(double value)
{
return new MaximumFillPercentage(value / 100d);
}
public static implicit operator double(MaximumFillPercentage instance)
{
return instance.value;
}
}

19. public class MaximumFillPercentage : IComparable<FillPercentage> {
//...
public int CompareTo(FillPercentage other)
{
return this.value.Compare(other);
}
}

20. public class MaximumFillPercentage : IComparable<FillPercentage> {
//...
public static bool operator <
(MaximumFillPercentage left, FillPercentage right) {
return left.CompareTo(right) == -1;
}
public static bool operator <=
(MaximumFillPercentage left, FillPercentage right) {
return left.CompareTo(right) <= 0;
}
public static bool operator >
(MaximumFillPercentage left, FillPercentage right) {
return left.CompareTo(right) == 1;
}
public static bool operator >=
(MaximumFillPercentage left, FillPercentage right) {
return left.CompareTo(right) >= 0;
}
}

21. public class FillPercentage {
//...
public bool Exceeds(MaximumFillPercentage maximum) {
return maximum < this;
}
}

22. public class Session
{
// ...
public Session Add(Timeslot timeslot)
{
return new Session(this.StartTime, this.EndTime,
this.MaximumFillPercentage, this.Timeslots.Add(timeslot));
}
public MaximumFillPercentage MaximumFillPercentage { get; }
public FillPercentage CurrentFillPercentage
{
get
{
var sessionDuration = this.EndTime.Subtract(this.StartTime);
var totalTimeslotDuration = this.Timeslots.Aggregate(Duration.Zero,
(current, timeslot) =>
current.Add(timeslot.DurationBetween(this.StartTime, this.EndTime)))
return new FillPercentage(
totalTimeslotDuration / sessionDuration);
}
}
}

23. public class ScheduleDay
{
public void Book(AppointmentTimeslot timeslot)
{
var beforeBooking = FindSessionToBookOn(timeslot);
var afterBooking = beforeBooking.Add(timeslot);
if (afterBooking
.CurrentFillPercentage
.Exceeds(beforeBooking.MaximumFillPercentage))
{
throw new MaximumFillPercentageExceededException(/* ... */);
}
// ...
}
}

24. •
•
•
•
•
•

25. •
•
•
•
•

28. public class PrivateGroupBehavior : BehaviorModule {
public PrivateGroupBehavior(
ITenantRepository tenantRepository,
IPrivateGroupRepository groupRepository)
{
Receive<StartPrivateGroup>(async message =>
{
var tenant = await tenantRepository.Get(new TenantId(message.TenantId));
var @group = tenant.StartPrivateGroup(
new GroupId(message.GroupId),
new GroupName(message.GroupName);
GroupObjective.InferFromMarkdown(message.GroupObjective));
groupRepository.Add(@group);
});
}
}

29. public class PrivateGroupScenarios {
[Fact] public async Task when_starting_a_private_group() {
using(var context = new GroupContext()) {
//Arrange
context.Tenants.Add(new TenantData { TenantId = 123, Subdomain = "nespresso" });
var module = new PrivateGroupBehavior(
new TenantRepository(context),
new PrivateGroupRepository(context)
);
//Act
await module.Send(new StartPrivateGroup {
TenantId = 123,
GroupId = new Guid("a9a620ca-c8c2-41b7-b7c0-fc03881ed00d"),
Name = "Prototype Retail Machines"
});
//Assert
Assert.Equal(1, context.PrivateGroups.Count());
GroupData actual = context.PrivateGroups.Single();
Assert.Equal(123, actual.TenantId);
Assert.Equal(new Guid("a9a620ca-c8c2-41b7-b7c0-fc03881ed00d"), actual.GroupId);
Assert.Equal("Prototype Retail Machines", actual.Name);
}
}
}

30. public class PrivateGroupScenarios {
[Fact] public Task when_starting_a_private_group() {
var tenantId = new Random().Next(1, 100);
var groupId = Guid.NewGuid();
return new Scenario()
.Given(tenantId, new TenantSnapshot {
TenantId = tenantId,
Subdomain = "nespresso"
})
.When(new StartPrivateGroup {
TenantId = tenantId,
GroupId = groupId,
Name = "Prototype Retail Machines"
})
.Then(groupId, new PrivateGroupSnapshot {
TenantId = tenantId,
GroupId = groupId,
Name = "Prototype Retail Machines"
})
.Assert(); // <- the boring stuff happens here
} //example of testing a state-based model
}

31. public class PrivateGroupScenarios {
[Fact] public Task when_starting_a_private_group() {
var tenantId = new Random().Next(1, 100);
var groupId = Guid.NewGuid();
return new Scenario()
.Given(tenantId, new RegisteredAsTenant {
TenantId = tenantId,
Subdomain = "nespresso"
})
.When(new StartPrivateGroup {
TenantId = tenantId,
GroupId = groupId,
Name = "Prototype Retail Machines"
})
.Then(groupId, new PrivateGroupStarted {
TenantId = tenantId,
GroupId = groupId,
Name = "Prototype Retail Machines"
})
.Assert(); // <- the boring stuff happens here
} //example of testing a event-driven model
}

32. public class PrivateGroupScenarios {
[Fact] public Task when_starting_a_private_group() {
var tenantId = new TenantId(new Random().Next(1, 100));
var subdomain = new Subdomain("nespresso");
var groupId = new GroupId(Guid.NewGuid());
var groupName = new GroupName("Prototype Retail Machines");
return new Scenario()
.Given(tenantId, new RegisteredAsTenant {
TenantId = tenantId,
Subdomain = subdomain
})
.When(new StartPrivateGroup {
TenantId = tenantId,
GroupId = groupId,
Name = groupName
})
.Then(groupId, new PrivateGroupStarted {
TenantId = tenantId,
GroupId = groupId,
Name = groupName
})
.Assert();
} //introducing value objects in your tests
}

33. public class PrivateGroupScenarios {
[Fact] public Task when_starting_a_private_group() {
var tenantId = new TenantId(new Random().Next(1, 100));
var subdomain = new Subdomain("nespresso");
var groupId = new GroupId(Guid.NewGuid());
var groupName = new GroupName("Prototype Retail Machines");
return new Scenario()
.Given(tenantId, new RegisteredAsTenant {
TenantId = tenantId,
Subdomain = subdomain
})
.When(new StartPrivateGroup {
TenantId = tenantId,
GroupId = groupId,
Name = groupName
})
.Then(groupId, new PrivateGroupStarted {
TenantId = tenantId,
GroupId = groupId,
Name = groupName
})
.Assert();
} //introducing value objects in your tests
}

34. public class PrivateGroupScenarios {
[Fact] public Task when_starting_a_private_group() {
var fixture = new ScenarioFixture();
var tenantId = fixture.Create<TenantId>();
var groupId = fixture.Create<GroupId>();
var groupName = fixture.Create<GroupName>();
return new Scenario()
.Given(tenantId, new RegisteredAsTenant {
TenantId = tenantId, Subdomain = fixture.Create<Subdomain>()
})
.When(new StartPrivateGroup {
TenantId = tenantId,
GroupId = groupId,
Name = groupName
})
.Then(groupId, new PrivateGroupStarted {
TenantId = tenantId,
GroupId = groupId,
Name = groupName
})
.Assert();
} //generate anonymous, random test values
}

35. •
•
•
•
•

51. public class MembershipRequest {
public void Approve(PersonId approver) {
if (this.Archived)
throw new MembershipRequestWasArchived(this.GroupId, this.RequestId);
if (this.Declined)
throw new MembershipRequestWasDeclined(this.GroupId, this.RequestId);
if (this.Cancelled)
throw new MembershipRequestWasCancelled(this.GroupId, this.RequestId);
if (this.Approved) return;
// ... happy path ...
}
}

52. public class MembershipRequest {
private enum State {
Requested, Approved, Declined, Cancelled, Archived
}
public void Approve(PersonId approver) {
if (this.CurrentState == State.Archived)
throw new MembershipRequestWasArchived(this.GroupId, this.RequestId);
if (this.CurrentState == State.Declined)
throw new MembershipRequestWasDeclined(this.GroupId, this.RequestId);
if (this.CurrentState == State.Cancelled)
throw new MembershipRequestWasCancelled(this.GroupId, this.RequestId);
if (this.CurrentState == State.Approved) return;
// ... happy path ...
this.CurrentState = State.Approved;
}
}

53. public class MembershipRequest {
private enum State {
Initial, Requested, Approved, Declined, Cancelled, Archived
}
private enum Trigger {
Request, Approve, Decline, Cancel, Archive
}
public void Approve(PersonId approver) {
this.Machine.Fire(Trigger.Approve); // <- Magic
// ... happy path ...
}
}

54. public class MembershipRequest {
public MembershipRequest() { this.Machine = ConfigureStatemachine(); }
private StateMachine<State, Trigger> ConfigureStatemachine() {
return new StateMachine<State, Trigger>(State.Initial)
.Configure(State.Initial).Permit(Trigger.Request, State.Requested)
.Configure(State.Requested)
.Permit(Trigger.Approve, State.Approved)
.Permit(Trigger.Decline, State.Declined)
.Permit(Trigger.Cancel, State.Cancelled)
.Permit(Trigger.Archive, State.Archived)
.Configure(State.Approved).Ignore(Trigger.Approve)
.GuardAnyOf(Trigger.Cancel, Trigger.Decline, Trigger.Archive,
new MembershipRequestWasApproved(this.GroupId, this.RequestId))
.Configure(State.Cancelled).Ignore(Trigger.Cancel)
.GuardAnyOf(Trigger.Approve, Trigger.Decline, Trigger.Archive,
new MembershipRequestWasCancelled(this.GroupId, this.RequestId))
.Configure(State.Declined).Ignore(Trigger.Decline)
.GuardAnyOf(Trigger.Approve, Trigger.Cancel, Trigger.Archive,
new MembershipRequestWasDeclined(this.GroupId, this.RequestId))
.Configure(State.Archived).Ignore(Trigger.Archive)
.GuardAnyOf(Trigger.Approve, Trigger.Cancel, Trigger.Decline,
new MembershipRequestWasArchived(this.GroupId, this.RequestId));
}
}

55. •
•
•
•