book

Head First C#, 5th Edition

by Andrew Stellman, Jennifer Greene

July 2024

Beginner

838 pages

27h 13m

English

O'Reilly Media, Inc.

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Table of Contents (the real thing)
How to use this Book Intro
Who is this book for?Who should probably back away from this book?We know what you’re thinkingWe know what your brain is thinkingMetacognition: thinking about thinkingHere’s what WE did:Here’s what YOU can do to bend your brain into submissionREADME.mdThe technical review team“Shoulders of giants”Acknowledgments...and a few more folks who have been so incredibly supportive over the years
1. Start Building Apps with C# Build something great...fast!
Learn C#...and learn to become a great developer...with a learning system that’s effective and funWrite code and explore C# with Visual StudioVisual Studio is your free gateway to C#Install Visual Studio Community EditionRun Visual StudioCreate and run your first C# project in Visual StudioYou can use Visual Studio Code with Head First C#Install the C# extensionsChange the C# debug console settingCreate and run your first project in Visual Studio CodeSet up Visual Studio Code for the next projectFirst open a folder, then add a projectThe Command PaletteInstall .NET MAUI before reading the rest of this chapterIf you’re using Linux, you’ll need an Android device for the .NET MAUI projectsLet’s build a game!Break up large projects into smaller partsHere’s how you’ll build your gameCreate a .NET MAUI project in Visual StudioCreate a .NET MAUI project in Visual Studio CodeRun your new .NET MAUI appStop your MAUI appMAUI apps work on all of your devicesHere’s the page that you’ll buildStart editing your XAML codeAdd the XAML for a Button and a LabelUse a FlexLayout to make a grid of animal buttonsWrite C# code to add the animals to the buttonsStart editing the PlayAgainButton event handler methodAdd a C# statement to the event handler methodAdd more statements to your event handlerAdd animals to your buttonsRun your app!Here are just a few things Git can do for youVisual Studio makes it easy to use GitAdd C# code to handle mouse clicksEnter the code for the event handlerRun your app and find all the pairsUh-oh—there’s a bug in your codeWhen you find a bug, you need to sleuth it outFinish the game by adding a timerAdd a timer to your game’s codeFinish the code for your gameAdd a field to hold the time elapsedFinish your TimerTick methodEven better ifs...
2. Variables, Statements, and Methods Dive into C# code
Take a closer look at the files in your console appA statement performs one single actionStatements are the building blocks for your appsStatements live inside methodsMethods do somethingMethods help you organize your codeYour methods use variables to work with dataDeclare your variablesVariables varyYou need to assign values to variables before you use themA few useful typesGenerate a new method to work with variablesAdd code that uses operators to your methodUse the debugger to watch your variables changeUse code snippets to help write loopsUse operators to work with variablesif statements make decisionsif/else statements also do something if a condition isn’t trueLoops perform an action over and overwhile loops keep looping statements while a condition is truedo/while loops run the statements then check the conditionfor loops run a statement after each loopSome useful things to keep in mind about C# codeControls drive the mechanics of your user interfacesMeet some of the controls you’ll use in this bookOther controls you’ll use in this bookBuild a .NET MAUI app to experiment with controlsCreate a new app to experiment with controlsExplore your new MAUI app and figure out how it worksAdd an Entry control to your appAdd properties to your Entry controlMake your Entry control update a Label controlCombine horizontal and vertical stack layoutsYou’ll nest one Layout inside anotherAdd a Picker control to display a list of choices
Unity Lab #1 Explore C# with Unity
Unity is a powerful tool for game designDownload Unity HubUse Unity Hub to create a new projectWork with your project in the Unity editorTake control of the Unity layoutChoose the Wide layout to match our screenshotsSet up Unity to work with Visual StudioYour scene is a 3D environmentUnity games are made with GameObjectsUse the Move Gizmo to move your GameObjectsThe Inspector shows your GameObject’s componentsAdd a material to your Sphere GameObjectRotate your sphereMove the Scene view camera with the View Tool and Scene GizmoGet creative!
3. Namespaces and Classes Organizing your code
Classes help you organize your codeIf code is useful, classes can help you reuse itSome methods take parameters and return a valueVisual Studio helps you explore parameters and return valuesLet’s build an app that picks random cardsYou’ll use an array to store the cardsCreate an app with a Main methodCreate your PickRandomCards app without top-level statementsAdd a class called CardPicker to your appUse Quick Actions to remove unnecessary using linesConvert between namespace stylesUse the new keyword to create an array of stringsHere’s the code for your finished CardPicker classAna’s working on her next gameAna’s game is evolving......so how can Ana make things easier for herself?Build a paper prototype for a classic gameBuild a MAUI version of your random card appMake your app accessible!Make your MAUI app pick random cardsReuse your CardPicker classAdd a using directive to use code in another namespaceAna’s prototypes look great......but what if she wants more than one enemy?Ana can use objects to solve her problemYou use a class to build an objectAn object gets its methods from its classWhen you create a new object from a class, it’s called an instance of that classA better solution for Ana...brought to you by objectsTheory and practiceAn instance uses fields to keep track of thingsMethods are what an object does. Fields are what an object knows.Thanks for the memoryWhat’s on your app’s mindSometimes code can be difficult to readExtremely compact code can be especially problematicMost code doesn’t come with a manualUse intuitive class and method namesBuild a class to work with some guysThere’s an easier way to initialize objects with C#Use the C# Interactive window or csi to run C# code
4. Data, Types, Objects, and References Managing your app’s data
Owen could use our help!Storytelling, fantasy, and mechanicsCharacter sheets store different types of data on paperA variable’s type determines what kind of data it can storeC# has several types for storing integersTypes for storing really HUGE and really tiny numbersLet’s talk about stringsA literal is a value written directly into your codeUse suffixes to give your literals typesA variable is like a data to-go cupUse the Convert class to explore bits and bytesOther types come in different sizes too10 pounds of data in a 5-pound bagCasting lets you copy values that C# can’t automatically convert to another typeSo what happened?When you cast a value that’s too big, C# adjusts it to fit its new containerC# does some conversions automaticallyWhen you call a method, the arguments need to be compatible with the types of the parametersOwen is constantly improving his game......but the trial and error can be time-consumingLet’s help Owen experiment with ability scoresFix the compiler error by adding a castAdd a cast to get the AbilityScoreCalculator class to compile......but there’s still a bug!Now we can finally fix Owen’s bug—and get the REAL Sharpen answerUse reference variables to access your objectsReferences are like sticky notes for your objectsIf there aren’t any more references, your object gets garbage-collectedMultiple references and their side effectsTwo references mean TWO variables that can change the same object’s dataObjects use references to talk to each otherArrays hold multiple valuesUse each element in an array like it’s a normal variableArrays can contain reference variablesnull means a reference points to nothingConsole.ReadLine returns a null when there are no lines availableUse the string? type when a string might be nullint.TryParse takes a string? parameterWelcome to Sloppy Joe’s Budget House o’ Discount Sandwiches!Sloppy Joe’s menu app uses a Grid layoutGrid controlsUse Grid properties to put a control in a cellDefine the rows and columns for a GridCreate the Sloppy Joe’s menu app and set up the gridHere’s the XAML for the appThe C# code for the main pageCan we make the app more accessible?Set the main header so the screen reader narrates itTry setting the item1 label’s SemanticProperties.Description insteadUse the SetValue method to change a control’s semantic properties
Unity Lab #2 Write C# Code for Unity
C# scripts add behavior to your GameObjectsAdd a C# script to your GameObjectWrite C# code to rotate your sphereAdd a breakpoint and debug your gameUse a hit count to skip framesUse the debugger to understand Time.deltaTimeAdd a cylinder to show where the Y axis isAdd fields to your class for the rotation angle and speedUse Debug.DrawRay to explore how 3D vectors workUse Unity to visualize vectors in 3DRun the game to see the ray in the Scene viewAdd a duration to the ray so it leaves a trailRotate your ball around a point in the sceneUse Unity to take a closer look at rotation and vectorsGet creative!

5. Encapsulation How objects keep their Secrets
Let’s help Owen roll for damageCreate a console app to calculate damageDesign a MAUI version of the damage calculator appHow your damage calculator app will workTabletop talk (or maybe...dice discussion?)Let’s try to fix that bugOops! It’s still not workingUse Debug.WriteLine to print diagnostic informationIt’s easy to accidentally misuse your objectsEncapsulation means keeping some data in a class privateWhen in doubt, make it privateUse encapsulation to control access to your class’s methods and fieldsBut is the RealName field REALLY protected?Private fields and methods can only be accessed from instances of the same classWhy encapsulation? Think of an object as an opaque box...Encapsulation makes your classes...Let’s use encapsulation to improve the SwordDamage classIs every member of the SwordDamage class public?Are fields or methods being misused?Is there calculation required after setting a field?So what fields and methods really need to be public?Encapsulation keeps your data safeLet’s use encapsulation in a classWrite a console app to test the PaintballGun classOur class is well-encapsulated, but...Properties make encapsulation easierReplace the GetBalls and SetBalls methods with a propertyModify your top-level statements to use the Balls propertyDebug your PaintballGun class to understand how the property worksAuto-implemented properties simplify your codeUse the prop snippet to create an auto-implemented propertyUse a private setter to create a read-only propertyMake the BallsLoaded setter privateWhat if we want to change the magazine size?But there’s a problem...how do we initialize MagazineSize?Use a constructor with parameters to initialize propertiesSpecify arguments when you use the new keywordInitialize fields and properties inline or in the constructorMake the screen reader announce each rollA few useful facts about methods and properties
6. Inheritance Your Object’s Family Tree
Calculate damage for MORE weaponsUse a switch statement to match several candidatesOne more thing...can we calculate damage for a dagger? And a mace? And a staff? and...When your classes use inheritance, you only need to write your code onceBuild up your class model by starting general and getting more specificHow would you design a zoo simulator?Different animals have different behaviorsEvery subclass extends its base classC# always calls the most specific methodAny place where you can use a base class, you can use one of its subclasses insteadUse a colon to extend a base classWe know that inheritance adds the base class fields, properties, and methods to the subclass......but some birds don’t fly!A subclass can override methods to change or replace members it inheritedSome members are only implemented in a subclassUse the debugger to understand how overriding worksBuild an app to explore virtual and overrideA subclass can hide methods in the base classHiding methods vs. overriding methodsUse the new keyword when you’re hiding methodsUse different references to call hidden methodsUse the override and virtual keywords to inherit behaviorA subclass can access its base class using the base keywordWhen a base class has a constructor, your subclass needs to call itA subclass and base class can have different constructorsIt’s time to finish the job for OwenUse the debugger to really understand how these classes workA class should do one thingBuild a beehive management systemHow the Beehive Management System app worksThe page uses a grid to lay out the controls for the UIThe Bee Jobs box uses a Border with a VerticalStackLayoutThe Beehive Management System class modelAll bees in the system extend the Bee classAll the constants are in their own static classThe worker bees extend the Bee classThe Queen class: how she manages the worker beesHere’s the code-behind for MainPage.xaml.csFeedback drives your beehive management gameWorkers and honey are in a feedback loopThe Beehive Management System is turn-based... now let’s convert it to real-timeSome classes should never be instantiatedAn abstract class is an intentionally incomplete classLet’s plan a trip to another planetLike we said, some classes should never be instantiatedSolution: use an abstract classAn abstract method doesn’t have a bodyAbstract properties work just like abstract methodsThe Deadly Diamond of Death
Unity Lab #3 GameObject Instances
Let’s build a game in Unity!Create a new material inside the Materials folderSpawn a billiard ball at a random point in the sceneUse the debugger to understand Random.valueTurn your GameObject into a prefabCreate a script to control the gameAttach the GameController script to the Main CameraPress Play to run your codeWatch the live instances in the Hierarchy windowUse the Inspector to work with GameObject instancesUse physics to keep balls from overlappingGet creative!
7. Interfaces, Casting, and “is” Making classes keep their Promises
The beehive is under attack!HiveDefender needs a DefendHive method because enemies can attack at any timeWe could use casting to call the DefendHive method……but what if we add more Bee subclasses that can defend?An interface defines methods and properties that a class must implement……but there’s no limit to the number of interfaces a class can implementInterfaces let unrelated classes do the same jobGet a little practice using interfacesFireside ChatsYou can’t instantiate an interface, but you can reference an interfaceIf you try to instantiate an interface, your code won’t buildUse the interface to reference an object you already havePool PuzzlePool Puzzle SolutionInterface references are ordinary object referencesThe RoboBee 4000 can do a worker bee’s job without using valuable honeyThe IWorker’s Job property is a hackUse is to check the type of an objectUse is to access methods in a subclassWhat if we want different animals to swim or hunt in packs?Use interfaces to work with classes that do the same jobUse the “is” keyword to check if the Animal is a swimmer or pack hunterSafely navigate your class hierarchy with isC# has another tool for safe type conversion: the as keywordUse upcasting and downcasting to move up and down a class hierarchyA quick example of upcastingUpcasting turns your CoffeeMaker into an ApplianceDowncasting turns your Appliance back into a CoffeeMakerUpcasting and downcasting work with interfaces tooInterfaces can inherit from other interfacesInterfaces can have static membersDefault implementations give bodies to interface methodsAdd a ScareAdults method with a default implementationData binding updates MAUI controls automaticallyAdd data binding to the default MAUI appMake Moods implement the INotifyPropertyChanged interfaceUse the PropertyChanged event to make data binding workPolymorphism means that one object can take many different formsKeep your eyes open for polymorphism!The four core principles of object-oriented programming
8. Enums and Collections Organizing your data
If a constructor just sets fields, use a primary constructor insteadA primary constructor can extend a base constructorOverloaded constructors use this to call the primary constructorStrings don’t always work for storing categories of dataEnums let you work with a set of valid valuesAn enum defines a new typeEnums let you represent numbers with namesWe could use an array to create a deck of cards……but what if you wanted to do more?Arrays can be annoying to work withLists make it easy to store collections of…anythingLists are more flexible than arraysLet’s build an app to store shoesGeneric collections can store any typeGeneric lists are declared using <angle brackets>You can use collection expressions to create ListsLet’s create a List of DucksUse a collection expression to create a List of DucksSorting lists can be trickyLists know how to sort themselvesIComparable<Duck> helps your List sort its DucksAn object’s CompareTo method compares it to another objectUse IComparer to tell your List how to sortAdd an IComparer to your projectCreate an instance of your comparer objectMultiple IComparer implementations, multiple ways to sort your objectsComparers can do complex comparisonsOverriding a ToString method lets an object describe itselfOverride the ToString method to see your Ducks in the IDEUpdate your foreach loops to let your Ducks and Cards write themselves to the consoleAdd a ToString method to your Card object tooYou can upcast an entire list using IEnumerable<T>Use a Dictionary to store keys and valuesHow to use a DictionaryThe Dictionary functionality rundownYour key and value can be different typesWrite an app that uses a DictionaryUse a collection initializer to create the DictionaryAnd yet MORE collection types…Generic .NET collections implement IEnumerableA queue is FIFO—first in, first outA stack is LIFO—last in, first outCollectionView is a MAUI control built for displaying collectionsObservableCollection is a collection made for data bindingAdd your Card class to the projectUse XAML to instantiate your objects for data bindingModify your app to use a resource DictionaryModify the event handlers to use the resource DictionaryUse what you’ve learned to build an app with two decks
Unity Lab #4 User Interfaces
Add a score that goes up when the player clicks a ballAdd two different modes to your gameAdd game mode to your gameAdd a UI to your gameUse the 2D view to work with the CanvasSet up the Text that will display the score in the UIAdd a Button that calls a method to start the gameMake the Play Again button and Score Text workFinish the code for the gameGet creative!
9. LINQ and Lambdas Get Control of your data
Jimmy’s a Captain Amazing superfan......but his collection’s all over the placeUse LINQ to query your collectionsLINQ works with any sequenceLINQ methods enumerate your sequences—just like foreach loopsLINQ’s query syntaxLINQ queries are built from clausesLINQ works with objectsUse a LINQ query to finish the app for JimmyThe var keyword lets C# figure out variable types for youWhen you use var, C# figures out the variable’s type automaticallyLINQ is versatileLINQ queries aren’t run until you access their resultsUse a group query to separate your sequence into groupsUse join queries to merge data from two sequencesUse the new keyword to create anonymous typesUnit tests help you make sure your code worksAdd a second project to your solution for the unit testsStart writing your first test methodRun the unit testGive your unit tests access to the classes they’re testingAdd a project reference so the unit tests can access the classes they need to testOne project can only access public classes in another projectMake your JimmyLinq classes and enums publicUse the Arrange-Act-Assert pattern to write an effective testUse Assert.AreEqual to check that a calculated value matches an expected valueFinish your first unit testWrite a unit test for the GetReviews methodWrite unit tests to handle edge cases and weird dataUse the => operator to create lambda expressionsRefactor a clown with lambdasUse the ?: operator to make your lambdas make choicesLINQ queries are made up of methodsYou can use lambda expressions with methods that take a Func parameterLINQ declarative syntax can be refactored into chained methodsUse the => operator to create switch expressionsExplore the Enumerable classEnumerable.Empty creates an empty sequence of any typeEnumerable.Repeat repeats a value a specific number of timesSo what exactly is an IEnumerable<T>?Create an enumerable sequence by handUse yield return to create your own sequencesUse the debugger to explore yield returnUse yield return to refactor ManualSportSequenceAdd an indexer to BetterSportSequenceCollectioncrossCollectioncross solution
10. Reading and Writing Files Save the last byte for me
.NET uses streams to read and write dataDifferent streams read and write different thingsThings you can do with a stream:A FileStream reads and writes bytes in a fileWrite text to a file in three simple stepsThe Swindler launches another diabolical planStreamWriter.WriteLine works just like Console.WriteLineStreamWriter MagnetsStreamWriter Magnets SolutionUse a StreamReader to read a fileData can go through more than one streamUse the static File and Directory classes to work with files and directoriesThings you can do with the static File class:Things you can do with the static Directory class:IDisposable makes sure objects are closed properlyUse the IDE to explore IDisposableAvoid filesystem errors with using statementsUse multiple using statements for multiple objectsUse a MemoryStream to stream data to memoryUse Encoding.UTF8.GetString to convert byte arrays to stringsWhat happens to an object when it’s serialized?But what exactly IS an object’s state? What needs to be saved?When an object is serialized, all of the objects it refers to get serialized too…Use JsonSerializer to serialize your objectsJSON only includes data, not specific C# typesNext up: we’ll take a deep dive into our dataC# strings are encoded with UnicodeVisual Studio works really well with Unicode.NET uses Unicode to store characters and textC# can use byte arrays to move data aroundUse a BinaryWriter to write binary dataUse BinaryReader to read the data back inA hex dump lets you see the bytes in your filesHow to make a hex dump of some plain textUse StreamReader to build a hex dumperUse Stream.Read to read bytes from a streamModify your hex dumper to read directly from the streamRun your app from the command lineUse the args variable to access command-line argumentsPass command-line arguments to an app run in the IDE
Unity Lab #5 Raycasting
Create a new Unity project and start to set up the sceneSet up the cameraCreate a GameObject for the playerIntroducing Unity’s navigation systemInstall the AI Navigation packageThings you’ll do with navigationSet up the NavMeshFix the carved-out hole in your NavMeshMake your player automatically navigate the play area
11. Captain Amazing the Death of the Object
The life and death of an objectUse the GC class (with caution) to force garbage collectionYour last chance to DO something... your object’s finalizerWhen EXACTLY does a finalizer run?You can SUGGEST to .NET that it’s time to collect the garbageFinalizers can’t depend on other objectsDon’t use finalizers for serializationA struct looks like an object......but isn’t an object Values get copied; references get assignedStructs are value types; objects are reference typesHere’s what happened...The stack vs. the heap: more on memoryUse out parameters to make a method return more than one valuePass by reference using the ref modifier Use optional parameters to set default valuesA null reference doesn’t refer to any objectNon-nullable reference types help you avoid NREsUse encapsulation to prevent your property from ever being nullNullable value types can be null...and handled safelyThe null-coalescing operator ?? checks for nulls automatically?? checks for null and returns an alternative ??= assigns a value to a variable only if it’s null“Captain” Amazing...not so muchRecords give your objects value equality automaticallyDon’t modify records—copy themUse the with keyword to create a modified copy of your DuckExtension methods add new behavior to EXISTING classesExtending a fundamental type: string
12. Exception Handling Putting out fires gets old
Your hex dumper reads a filename from the command lineBut what happens if you give HexDump an invalid filename?When your program throws an exception, the CLR generates an Exception objectAll Exception objects inherit from System.ExceptionThere are some files you just can’t dumpWhat happens when a method you want to call is risky?Handle exceptions with try and catchUse the debugger to follow the try/catch flowIf you have code that ALWAYS needs to run, use a finally blockCatch-all exceptions handle System.ExceptionAvoid catch-all exception with multiple catch blocksPool PuzzlePool Puzzle SolutionUse the right exception for the situationCatch custom exceptions that extend System.ExceptionException MagnetsException Magnets SolutionException filters help you create precise handlersThe worst catch block EVER: catch-all plus commentsYou should handle your exceptions, not bury themTemporary solutions are OK (temporarily)Use NuGet to add a logging library to your appPackages add DLLs with classesAdd logging to your ExceptionExperiment app
Unity Lab #6 Scene Navigation
Let’s pick up where the last Unity Lab left offAdd a platform to your sceneUse bake options to make the platform walkableInclude the stairs and ramp in the NavMeshMake the player navigate around the obstaclesFix the moving obstacleGet creative!Thank you for reading our book!But wait, there’s more! Your journey’s just begun...Check out these essential (and amazing!) books written by our friends and colleagues, also published by
Index
The Authors

Content preview from Head First C#, 5th Edition

Chapter 12. Exception Handling Putting out fires gets old

When you have to deal with error after error, it’s called “putting out fires.”

Imagine: it’s a few years from now. You spent all that time working on your C# skills and continuing to learn and improve, and now you’re one of the most senior developers at a big tech company. Now you’re getting panicked phone calls in the middle of the night from your support team because your app crashes, or doesn’t behave like it’s supposed to. You want to spend your time writing code, not putting out fires! Because nothing pulls you out of the programming groove like having to fix a strange bug you don’t recognize. Luckily, C# has exception handling, which lets you write code to deal with problems that come up. Better yet, you can even plan for those problems, and keep things running when they happen.

Your hex dumper reads a filename from the command line

At the end of Chapter 10, you built a hex dumper that uses command-line arguments to dump any file. You used the project properties in the IDE to set the arguments for the debugger, and you saw how to call it from a Windows command prompt or macOS Terminal window.

But what happens if you give HexDump an invalid filename?

When you modified your HexDump app to use command-line arguments, ...

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