Jon Bock - Development Tech Blog

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Added new readonly modifiers to be used in structs, lets you define more granular properties that do not modify state.

eg consider the following mutable struct:-

public struct Point
{
    public double X { get; set; }
    public double Y { get; set; }
    public double Distance => Math.Sqrt(X * X + Y * Y);

    public override string ToString() =>
        $"({X}, {Y}) is {Distance} from the origin";
}

We can decorate the ToString() method as readonly as it doesnt modify state.

public readonly override string ToString() =>
    $"({X}, {Y}) is {Distance} from the origin";

when built this would produce a compile time error like:-

warning CS8656: Call to non-readonly member 'Point.Distance.get' from a 'readonly' member results in an implicit copy of 'this'

and would require the following change to the property so that it doesnt change the state, it doesnt assume that get accessors do not modify state.

public readonly double Distance => Math.Sqrt(X * X + Y * Y);

Default Interface Methods

You are now able to add functionality to interfaces without breaking functionality to older versions of implemented interfaces.

interface IWriteLine  
{  
 public void WriteLine()  
 {  
 Console.WriteLine("Wow C# 8!");  
 }  
}  

This has to be used carefully as it will easily violate the Single Responsibility principles

Nullable reference type

Allow you to induce compiler warnings then using reference types for more defensive coding.

string? nullableString = null;  
Console.WriteLine(nullableString.Length); // WARNING: may be null! Take care!  

Async streams

Allow the use of Async to be now used over collections and can be iterated over.

await foreach (var x in enumerable)  
{  
  Console.WriteLine(x);  
}

Now its possible to the client consumer to be able to consume async streams in chunks as they are returned and are available.

Ranges

You can now access data sequences in 2 new ways and get various slices of data that you require.

Indices

Access to collections by taking from the beginning or the end.

Index i1 = 3; // number 3 from beginning  
Index i2 = ^4; // number 4 from end  
int[] a = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };  
Console.WriteLine($"{a[i1]}, {a[i2]}"); // "3, 6" 

Sub collection

Obtain a sub collection from the data

var slice = a[i1..i2]; // { 3, 4, 5 } 

Null-coalescing assignment

New feature that allows you to null-coalesce assign with the ??= operator so if the value on the right is assigned to the left only if the left hand is null.

List<int> numbers = null;
int? i = null;

numbers ??= new List<int>();
numbers.Add(i ??= 17);
numbers.Add(i ??= 20);

Console.WriteLine(string.Join(" ", numbers));  // output: 17 17
Console.WriteLine(i);  // output: 17

Default in deconstruction

Allows the following syntax (int i, string s) = default; and (i, s) = default

(int x, string y) = (default, default); // C# 7  
(int x, string y) = default;               // C# 8  

Using declarations

Enhance the using operator to be more inline such as 

// C# Old Style  
using (var repository = new Repository())    
{    
} // repository is disposed here!    
     
// vs.C# 8    
     
using var repository = new Repository();    
Console.WriteLine(repository.First());    
// repository is disposed here! 

Here is a quick summary of design patterns that everyone should know:-

Factory Patterns

• Builder - Separate the creation of a complex object from its representation so that we can use the same construction process to create different representations.
• Abstract factory - Using a interface to create families of related or dependent objects without specifying their concrete classes.
• Factory method - Define a interface for creating an object but let their subclasses decide how they are implemented.
• Prototype - Design a prototype instance and use it to create new objects by copying this prototype.
•  Singleton - Create only one instance of a object and ensure that it is globally accessible.

Structural design patterns

• Adapter - Create a interface that lets you interact with another interface the client expects, it lets incompatible classes work together.
• Builder - Decouple the abstraction from its implementation so the two can vary independently.
• Composite - Compose objects into tree structures to enable recursive composition. 
• Decorator - Add additional responsibilities to an object dynamically.
• Facade - Provide a unified interface to a set of interfaces in a subsystem. Makes it easier to use subsystems by using this higher level interface.
• Flyweight - Support the sharing of large number of fine grained objects efficiently.
• Proxy - Provide a placeholder for another object to control access to it. 

Behavioral design patterns

• Chain of responsibility - Avoid coupling the sender from the receiver by allowing other objects a chance to handle it.
• Command - Encapsulate a request as a object and let you parametrize with different request, queues or logs. Supports undoable operations.
• Interpreter - Given a langauge, define a representation for its grammar along with interpreter that uses the representation to interpret sentences in the language.
• Iterator - Provide ways to access items in a object in a sequential manner without exposing its underlying representation.
• Mediator - Promotes loose coupling between objects and prevents them from referring to each other directly, encapsulates how a set of objects interact.
• Memento - Without violating encapsulation, capture and externalize and objects internal state so that it can later be returned to this state later.
• Observer - Define a one to many relationship between objects so then when one objects state changes all its dependencies are notified and updated.
• State - Allow an object to be able to change its behaviour when its internal state changes so that it appears that the objects class has changed.
• Strategy - Define a family of algorithms, encapsulate each one and make them interchangeable. Allows you to vary the algorithm independently from the clients that use it.
• Template method - Define a skeleton of algorithm in a operation whilst deferring some steps to client subclasses. Allows you to redefine steps in the algorithm without changing the algorithms structure.
• Visitor - Represent an operation to be performed on elements of a object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.

Lets look at some new features recently release with the C# 7.2 minor releases, I will explain what it was trying to solve and how.

A bit of background, with structs in C#, its possible to expose public properties as well as setting private properties, the example below shows its structure:-

public struct Document
{
public string Title { get; set; }
public string Author { get; set; }

public Document( string title, string author )
{
Title = title;
Author = author;
}

public void ChangeDocument(Document newDoc){
this = newDoc;
}
}

 So from this we can see that it causes the struct to be in a mutable state whereby we allow the public properties to be modified using the "this" key.

Moving for a more immutable solution we can leverage the new C# 7.2 feature and decorate the struct with readonly.

public readonly struct Document
{
public string Title { get; set; }
public string Author { get; set; }

public Document( string title, string author )
{
Title = title;
Author = author;
}

public void ChangeDocument(Document newDoc){
this = newDoc;
}
}

 This will cause a compile error with the this keyword as a result.

The code can then be shortened to refactor towards immutability as follows:-

public readonly struct Document
{
public string Title { get; set; }
public string Author { get; set; }

public Document (string title, string author)
{
Title = title;
Author = author;
}
}

So in the usage in the code, it can be updated from:-

var doc = new Document();
doc.Title = "SomeTitle";
doc.Author = "SomeAuthor";

to:-

var doc = new Document
{
Title = "SomeTitle",
Author = "SomeAuthor"
}

So with this new change, it encourages developers to program with immutability in mind and even though the default parameterless constructor is still available, readonly will cause a compile time error.