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Technically DomainDrivenDesign #TechoramaNLNL @pelshoff

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Questions?

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Context Technical building blocks Quiz Case study I Case study II

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Context Technical building blocks Quiz Case study I Case study II

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Value objects

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public class Name { public String LastName { get; } public String FirstName { get; } public Name (String lastName, String firstName) { LastName = lastName; FirstName = firstName; NamePartsMayNotBeNull(); LastNameIsRequired(); } private void NamePartsMayNotBeNull() { if (null == LastName || null == FirstName) { throw NameIsInvalid.BecausePartsAreMissing(); } } private void LastNameIsRequired() { if (LastName.Length < 1) { throw NameIsInvalid.BecauseLastNameIsTooShort(); } ...

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public class Email { public String EmailAsString { get; } public Email(String email) { EmailAsString = email; EmailMustBeAnActualEmailAddress(); } private void EmailMustBeAnActualEmailAddress() { if (String.IsNullOrWhiteSpace(EmailAsString)) { throw EmailIsInvalid.BecauseEmailIsMissing(); } if (!EmailAsString.Contains('@')) { throw EmailIsInvalid.BecauseAtIsMissing(); } } }

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Value objects ● Represent a value ● Define allowed values ● Are immutable ● Are easy to test

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Entities

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public class Attendee { public Guid Id { get; } private Name _Name; public Name Name { get { return _Name; } set { _Name = value; ValuesMayNotBeNull(); } } private Email _Email; public Email Email { get { return _Email; } set { _Email = value; ValuesMayNotBeNull(); } } public Attendee(Guid id, Name name, Email email) { Id = id; _Name = name; _Email = email; ValuesMayNotBeNull(); } private void ValuesMayNotBeNull() { if (null == _Name || null == _Email) { throw AttendeeIsInvalid.BecauseValuesAreNull(); } } }

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Entities ● Have identity ● Are more than the sum of their attributes ● Evolve over time ● Are slightly harder to test

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Services

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public class Registration { private RegistrationRepository Repository; public Registration(RegistrationRepository repository) { Repository = repository; } public void RegisterNew(Attendee Attendee) { EmailAddressMustBeUnique(Attendee.Email); Repository.Add(Attendee); } private void EmailAddressMustBeUnique(Email Email) { try { Repository.FindByEmail(Email); } catch (AttendeeNotFound ex) { return; } throw RegistrationFailed.BecauseEmailAddressIsNotUnique(); } }

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Services ● Don’t have attributes or identity (Are not a “thing”) ● Are suitable for cross-concern cutting operations (involving multiple entities) ● Are harder to test

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Context Technical building blocks Quiz Case study I Case study II

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class Address

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class BicycleService

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class StreetAddress

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class Investment

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Context Technical building blocks Quiz Case study I Case study II

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public class Meeting { public Meeting( Guid id, string title, string description, string code, string startDate, string startTime, string endDate, string endTime, bool isPublished, string subTitle, List program ) { // ... } }

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new Meeting( Guid.NewGuid(), "Techorama 2019", "...", "#TechoramaNL", "2019-10-01", "07:30", "2019-10-02", "17:30", true, "Deep Knowledge IT Conference", new List(){ new ProgramSlot("2019-10-01", "07:30", "08:45", "Registration and Breakfast", "All of Pathé"), new ProgramSlot("2019-10-01", "17:45", "18:45", "Technically DDD", "Room 5"), } );

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Meetings cannot end before they start

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The Approach™ 1. Implement in Entity 2. Extract Value Object 3. Refactor Value Object

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public class Meeting { public Meeting(/* */) { // ... MeetingsCannotEndBeforeStart(); } private void MeetingsCannotEndBeforeStart() { if (StartDate.CompareTo(EndDate) == -1) { return; } if (StartDate.CompareTo(EndDate) == 1) { throw MeetingIsInvalid.BecauseMeetingEndsBeforeStarting(); } if (StartTime.CompareTo(EndTime) == 1) { throw MeetingIsInvalid.BecauseMeetingEndsBeforeStarting(); } } }

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public class Meeting { // ... private MeetingDuration Duration; // ... public Meeting( Guid id, string title, string description, string code, MeetingDuration duration, bool isPublished, string subTitle, List program ) { // ... Duration = duration; // ... } }

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public class MeetingDuration { // ... public MeetingDuration(string startDate, string startTime, string endDate, string endTime) { // ... MeetingsCannotEndBeforeStart(); } private void MeetingsCannotEndBeforeStart() { if (StartDate.CompareTo(EndDate) == -1) { return; } if (StartDate.CompareTo(EndDate) == 1) { throw MeetingIsInvalid.BecauseMeetingEndsBeforeStarting(); } if (StartTime.CompareTo(EndTime) == 1) { throw MeetingIsInvalid.BecauseMeetingEndsBeforeStarting(); } } }

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public class MeetingDuration { private DateTime Start; private DateTime End; public MeetingDuration(DateTime start, DateTime end) { Start = start; End = end; MeetingsCannotEndBeforeStart(); } private void MeetingsCannotEndBeforeStart() { if (Start.CompareTo(End) == 1) { throw MeetingIsInvalid.BecauseMeetingEndsBeforeStarting(); } } }

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new Meeting( Guid.NewGuid(), "Techorama 2019", "...", "#TechoramaNL", new MeetingDuration( new DateTime(2019, 10, 1, 7, 30, 0), new DateTime(2019, 10, 2, 17, 30, 0) ), true, "Deep Knowledge IT Conference", new List(){ new ProgramSlot("2019-10-01", "07:30", "08:45", "Registration and Breakfast", "All of Pathé"), new ProgramSlot("2019-10-01", "17:45", "18:45", "Technically DDD", "Room 5"), } );

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Context Technische bouwblokken Quiz Case study I Case study II

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Program slots cannot occur in the same room at the same time

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new Meeting( Guid.NewGuid(), "Techorama 2019", "...", "#TechoramaNL", new MeetingDuration( new DateTime(2019, 10, 1, 7, 30, 0), new DateTime(2019, 10, 2, 17, 30, 0) ), true, "Deep Knowledge IT Conference", new List(){ new ProgramSlot("2019-10-01", "07:30", "08:45", "Registration and Breakfast", "All of Pathé"), new ProgramSlot("2019-10-01", "17:45", "18:45", "Technically DDD", "Room 5"), } );

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private void ProgramSlotsCannotOccurInTheSameRoomAtTheSameTime() { foreach (var slotA in Program) { foreach (var slotB in Program) { if (slotA == slotB) { continue; } if (!slotA.Room.Equals(slotB.Room)) { continue; } if (!slotA.Date.Equals(slotB.Date)) { continue; } if (slotA.StartTime.CompareTo(slotB.EndTime) == 1) { continue; } if (slotA.EndTime.CompareTo(slotB.StartTime) == -1) { continue; } throw MeetingIsInvalid.BecauseProgramSlotsOverlap(); } } // ...

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public class Meeting { // ... private Program Program; public Meeting( Guid id, string title, string description, string code, MeetingDuration duration, bool isPublished, string subTitle, Program program ) { // ... Program = program; } }

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public class Program { private List Slots; public Program(List program) { Slots = program; ProgramSlotsCannotOccurInTheSameRoomAtTheSameTime(); } private void ProgramSlotsCannotOccurInTheSameRoomAtTheSameTime() { foreach (var slotA in Slots) { foreach (var slotB in Slots) { if (slotA == slotB) { continue; } // ... } } }

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public class Program { private List Slots; public Program(List program) { Slots = program; ProgramSlotsCannotOccurInTheSameRoomAtTheSameTime(); } private void ProgramSlotsCannotOccurInTheSameRoomAtTheSameTime() { foreach (var slotA in Slots) { foreach (var slotB in Slots) { if (slotA == slotB) { continue; } if (slotA.OverlapsWith(slotB)) { throw MeetingIsInvalid.BecauseProgramSlotsOverlap(); } } } } }

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public class ProgramSlot { public MeetingDuration Duration; public string Title; public string Room; public ProgramSlot(MeetingDuration duration, string title, string room) { Duration = duration; Title = title; Room = room; } public bool OverlapsWith(ProgramSlot that) { return this.Room == that.Room && this.Duration.OverlapsWith(that.Duration); } }

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public class MeetingDuration { private DateTime Start; private DateTime End; public MeetingDuration(DateTime start, DateTime end) { Start = start; End = end; MeetingsCannotEndBeforeStart(); } private void MeetingsCannotEndBeforeStart() { /*...*/ } public bool OverlapsWith(MeetingDuration that) { return this.Start.CompareTo(that.Start) >= 0 && this.Start.CompareTo(that.End) < 1 || this.End.CompareTo(that.Start) >= 0 && this.End.CompareTo(that.End) < 1; } }

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public class MeetingDuration { private DateTime Start; private DateTime End; public MeetingDuration(DateTime start, DateTime end) { Start = start; End = end; MeetingsCannotEndBeforeStart(); } private void MeetingsCannotEndBeforeStart() { /*...*/ } public bool OverlapsWith(MeetingDuration that) { return this.Contains(that.Start) || this.Contains(that.End); } private bool Contains(DateTime Date) { return this.Start.CompareTo(Date) < 1 && this.End.CompareTo(Date) >= 0; } }

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1. 3. 2. 4. This slot That slot This slot That slot This slot That slot This slot That slot Time

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This slot That slot This slot That slot Time 1. 2.

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public class MeetingDuration { private DateTime Start; private DateTime End; public MeetingDuration(DateTime start, DateTime end) { Start = start; End = end; MeetingsCannotEndBeforeStart(); } private void MeetingsCannotEndBeforeStart() { /*...*/ } public bool OverlapsWith(MeetingDuration that) { return !(this.Start.CompareTo(that.End) == 1 || this.End.CompareTo(that.Start) == -1); } }

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public class MeetingDuration { private DateTime Start; private DateTime End; public MeetingDuration(DateTime start, DateTime end) { Start = start; End = end; MeetingsCannotEndBeforeStart(); } private void MeetingsCannotEndBeforeStart() { /*...*/ } public bool OverlapsWith(MeetingDuration that) { return !this.IsBefore(that) && !that.IsBefore(this); } private bool IsBefore(MeetingDuration that) { return this.Start.CompareTo(that.End) == 1; } }

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new ProgramSlot("2019-10-01", "17:45", "18:45", "Technically DDD", "Room 5") private bool IsBefore(MeetingDuration that) { return this.Start.CompareTo(that.End) == 1; }

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public class SlotDuration { private DateTime Start; private DateTime End; public SlotDuration(DateTime start, DateTime end) { Start = start; End = end; MeetingsCannotEndBeforeStart(); SlotMustStartAndEndOnSameDay(); } /*...*/ public bool OverlapsWith(SlotDuration that) { return !this.IsBefore(that) && !that.IsBefore(this); } private bool IsBefore(SlotDuration that) { return this.Start.CompareTo(that.End) >= 0; } }

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new Meeting( Guid.NewGuid(), "Techorama 2019", "...", "#TechoramaNL", "2019-10-01", "07:30", "2019-10-02", "17:30", true, "Deep Knowledge IT Conference", new List(){ new ProgramSlot("2019-10-01", "07:30", "08:45", "Registration and Breakfast", "All of Pathé"), new ProgramSlot("2019-10-01", "17:45", "18:45", "Technically DDD", "Room 5"), } );

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new Meeting( Guid.NewGuid(), "Techorama 2019", "...", "#TechoramaNL", new MeetingDuration(new DateTime(2019, 10, 1, 7, 30, 0), new DateTime(2019, 10, 2, 17, 30, 0)), True, "Deep Knowledge IT Conference", new Program( new List(){ new ProgramSlot( new SlotDuration(new DateTime(2019, 10, 1, 7, 30, 0), new DateTime(2019, 10, 1, 8, 45, 0)), "Registration and Breakfast", "All of Pathé" ), new ProgramSlot( new SlotDuration(new DateTime(2019, 10, 1, 17, 45, 0), new DateTime(2019, 10, 1, 18, 45, 0)), "Technically DDD", "Room 5" ), } ) );

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A meeting must be able to be rescheduled

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Would you like to know more? “Domain-Driven Design: Tackling Complexity in the Heart of Software”, Eric Evans “Domain-Driven Design Distilled”, Vaughn Vernon

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Pim Elshoff developer.procurios.com @pelshoff https://speakerdeck.com/pelshoff/technically-ddd-c-v10