What’s New in C # 9

Friday, 14/08/2020

Tram Ho

(1) Visual Studio

Need to use the latest Visual Studio preview, fix the * .csproj file to

<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>Project</span> <span class="token attr-name">Sdk</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>Microsoft.NET.Sdk<span class="token punctuation">"</span></span><span class="token punctuation">&gt;</span></span>
  <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>PropertyGroup</span><span class="token punctuation">&gt;</span></span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>OutputType</span><span class="token punctuation">&gt;</span></span>Exe<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>OutputType</span><span class="token punctuation">&gt;</span></span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>TargetFramework</span><span class="token punctuation">&gt;</span></span>net5.0<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>TargetFramework</span><span class="token punctuation">&gt;</span></span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>LangVersion</span><span class="token punctuation">&gt;</span></span>9.0<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>LangVersion</span><span class="token punctuation">&gt;</span></span>
  <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>PropertyGroup</span><span class="token punctuation">&gt;</span></span>
<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>Project</span><span class="token punctuation">&gt;</span></span>

</PropertyGroup>

</Project>

(2) Feature Target Typing

File Program.cs

Old syntax

<span class="token keyword">namespace</span> <span class="token namespace">csharp9<span class="token punctuation">.</span>old1</span>
<span class="token punctuation">{</span>
    <span class="token keyword">class</span> <span class="token class-name">Program1Old</span>
    <span class="token punctuation">{</span>

        <span class="token return-type class-name"><span class="token keyword">void</span></span> <span class="token function">MyMethod</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
        <span class="token punctuation">{</span>
            <span class="token class-name">Person</span> person <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token constructor-invocation class-name">Person</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//&lt;-- Trong C# 8.</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">class</span> <span class="token class-name">Person</span>
    <span class="token punctuation">{</span>
        <span class="token keyword">public</span> <span class="token return-type class-name"><span class="token keyword">string</span></span> FirstName <span class="token punctuation">{</span> <span class="token keyword">get</span><span class="token punctuation">;</span> <span class="token keyword">set</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>

<span class="token punctuation">}</span>

namespace csharp9.old1

{

class Program1Old

{

void MyMethod()

{

Person person = new Person(); //<-- Trong C# 8.

}

class Person

{

public string FirstName { get; set; }

}

New syntax

<span class="token keyword">namespace</span> <span class="token namespace">csharp9<span class="token punctuation">.</span>new1</span>
<span class="token punctuation">{</span>

    <span class="token keyword">class</span> <span class="token class-name">ProgramNew</span>
    <span class="token punctuation">{</span>
        <span class="token return-type class-name"><span class="token keyword">void</span></span> <span class="token function">MyMethod</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
        <span class="token punctuation">{</span>
            <span class="token class-name">Person</span> person <span class="token operator">=</span> <span class="token keyword">new</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//&lt;-- Trong C# 9.</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">class</span> <span class="token class-name">Person</span>
    <span class="token punctuation">{</span>
        <span class="token keyword">public</span> <span class="token return-type class-name"><span class="token keyword">string</span></span> FirstName <span class="token punctuation">{</span> <span class="token keyword">get</span><span class="token punctuation">;</span> <span class="token keyword">set</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>

<span class="token punctuation">}</span>

namespace csharp9.new1

{

class ProgramNew

{

Person person = new(); //<-- Trong C# 9.

}

class Person

{

}

(3) Feature Init-only property

The old way

<span class="token keyword">using</span> <span class="token namespace">System</span><span class="token punctuation">;</span>

<span class="token keyword">namespace</span> <span class="token namespace">csharp9<span class="token punctuation">.</span>old2</span>
<span class="token punctuation">{</span>

    <span class="token keyword">class</span> <span class="token class-name">Program2Old</span>
    <span class="token punctuation">{</span>
        <span class="token keyword">static</span> <span class="token return-type class-name"><span class="token keyword">void</span></span> <span class="token function">Main</span><span class="token punctuation">(</span><span class="token class-name"><span class="token keyword">string</span><span class="token punctuation">[</span><span class="token punctuation">]</span></span> args<span class="token punctuation">)</span>
        <span class="token punctuation">{</span>
            <span class="token class-name"><span class="token keyword">var</span></span> person <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token constructor-invocation class-name">Person</span><span class="token punctuation">(</span><span class="token string">"Nguyễn Văn A"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            person<span class="token punctuation">.</span>Name <span class="token operator">=</span> <span class="token string">"xx"</span><span class="token punctuation">;</span>
            Console<span class="token punctuation">.</span><span class="token function">WriteLine</span><span class="token punctuation">(</span>person<span class="token punctuation">.</span>Name<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Person</span>
    <span class="token punctuation">{</span>
        <span class="token keyword">public</span> <span class="token function">Person</span><span class="token punctuation">(</span><span class="token class-name"><span class="token keyword">string</span></span> name<span class="token punctuation">)</span>
        <span class="token punctuation">{</span>
            <span class="token keyword">this</span><span class="token punctuation">.</span>Name <span class="token operator">=</span> name<span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        <span class="token keyword">public</span> <span class="token return-type class-name"><span class="token keyword">string</span></span> Name <span class="token punctuation">{</span> <span class="token keyword">get</span><span class="token punctuation">;</span> <span class="token keyword">set</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token comment">// Trong C# 8.</span>
    <span class="token punctuation">}</span>

<span class="token punctuation">}</span>

using System;

namespace csharp9.old2

{

class Program2Old

{

static void Main(string[] args)

{

var person = new Person("Nguyễn Văn A");

person.Name = "xx";

Console.WriteLine(person.Name);

}

public class Person

{

public Person(string name)

{

this.Name = name;

}

public string Name { get; set; } // Trong C# 8.

}

New way in C# 9

<span class="token keyword">using</span> <span class="token namespace">System</span><span class="token punctuation">;</span>

<span class="token keyword">namespace</span> <span class="token namespace">csharp9<span class="token punctuation">.</span>new2</span>
<span class="token punctuation">{</span>
    <span class="token keyword">class</span> <span class="token class-name">Program</span>
    <span class="token punctuation">{</span>
        <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token return-type class-name"><span class="token keyword">void</span></span> <span class="token function">Main</span><span class="token punctuation">(</span><span class="token class-name"><span class="token keyword">string</span><span class="token punctuation">[</span><span class="token punctuation">]</span></span> args<span class="token punctuation">)</span>
        <span class="token punctuation">{</span>
            <span class="token class-name"><span class="token keyword">var</span></span> person <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token constructor-invocation class-name">Person</span><span class="token punctuation">(</span><span class="token string">"Tran Van B"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            Console<span class="token punctuation">.</span><span class="token function">WriteLine</span><span class="token punctuation">(</span>person<span class="token punctuation">.</span>Name<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Person</span>
    <span class="token punctuation">{</span>
        <span class="token keyword">public</span> <span class="token function">Person</span><span class="token punctuation">(</span><span class="token class-name"><span class="token keyword">string</span></span> name<span class="token punctuation">)</span>
        <span class="token punctuation">{</span>
            <span class="token keyword">this</span><span class="token punctuation">.</span>Name <span class="token operator">=</span> name<span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        <span class="token keyword">public</span> <span class="token return-type class-name"><span class="token keyword">string</span></span> Name <span class="token punctuation">{</span> <span class="token keyword">get</span><span class="token punctuation">;</span> init<span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token comment">// Trong C# 9.</span>
    <span class="token punctuation">}</span>

<span class="token punctuation">}</span>

using System;

namespace csharp9.new2

{

class Program

{

public static void Main(string[] args)

{

var person = new Person("Tran Van B");

}

public class Person

{

this.Name = name;

}

public string Name { get; init; } // Trong C# 9.

}

(4) Key word “is” và “and”

<span class="token keyword">using</span> <span class="token namespace">System</span><span class="token punctuation">;</span>

<span class="token keyword">namespace</span> <span class="token namespace">csharp9<span class="token punctuation">.</span>new3</span>
<span class="token punctuation">{</span>
    <span class="token keyword">class</span> <span class="token class-name">Program</span>
    <span class="token punctuation">{</span>
        <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token return-type class-name"><span class="token keyword">void</span></span> <span class="token function">Main</span><span class="token punctuation">(</span><span class="token class-name"><span class="token keyword">string</span><span class="token punctuation">[</span><span class="token punctuation">]</span></span> args<span class="token punctuation">)</span>
        <span class="token punctuation">{</span>
            <span class="token class-name"><span class="token keyword">int</span></span> <span class="token keyword">value</span> <span class="token operator">=</span> <span class="token number">42</span><span class="token punctuation">;</span>

            <span class="token keyword">switch</span> <span class="token punctuation">(</span><span class="token keyword">value</span><span class="token punctuation">)</span>
            <span class="token punctuation">{</span>
                <span class="token keyword">case</span> <span class="token operator">&lt;=</span> <span class="token number">0</span><span class="token punctuation">:</span>
                    Console<span class="token punctuation">.</span><span class="token function">WriteLine</span><span class="token punctuation">(</span><span class="token string">"Less than or equal to 0"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                    <span class="token keyword">break</span><span class="token punctuation">;</span>
                <span class="token keyword">case</span> <span class="token operator">&gt;</span> <span class="token number">0</span> and <span class="token operator">&lt;=</span> <span class="token number">10</span><span class="token punctuation">:</span>
                    Console<span class="token punctuation">.</span><span class="token function">WriteLine</span><span class="token punctuation">(</span><span class="token string">"More than 0 but less than or equal to 10"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                    <span class="token keyword">break</span><span class="token punctuation">;</span>
                <span class="token keyword">default</span><span class="token punctuation">:</span>
                    Console<span class="token punctuation">.</span><span class="token function">WriteLine</span><span class="token punctuation">(</span><span class="token string">"More than 10"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                    <span class="token keyword">break</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>

            <span class="token class-name"><span class="token keyword">var</span></span> message <span class="token operator">=</span> <span class="token keyword">value</span> <span class="token keyword">switch</span>
            <span class="token punctuation">{</span>
                <span class="token operator">&lt;=</span> <span class="token number">0</span> <span class="token operator">=&gt;</span> <span class="token string">"Less than or equal to 0"</span><span class="token punctuation">,</span>
                <span class="token operator">&gt;</span> <span class="token number">0</span> and <span class="token operator">&lt;=</span> <span class="token number">10</span> <span class="token operator">=&gt;</span> <span class="token string">"More than 0 but less than or equal to 10"</span><span class="token punctuation">,</span>
                _ <span class="token operator">=&gt;</span> <span class="token string">"More than 10"</span>
            <span class="token punctuation">}</span><span class="token punctuation">;</span>
            Console<span class="token punctuation">.</span><span class="token function">WriteLine</span><span class="token punctuation">(</span>message<span class="token punctuation">)</span><span class="token punctuation">;</span>

            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">value</span> <span class="token operator">&gt;</span> <span class="token number">0</span> <span class="token operator">&amp;&amp;</span> <span class="token keyword">value</span> <span class="token operator">&lt;=</span> <span class="token number">10</span><span class="token punctuation">)</span> <span class="token comment">// Trong C# 8.</span>
            <span class="token punctuation">{</span>
                Console<span class="token punctuation">.</span><span class="token function">WriteLine</span><span class="token punctuation">(</span><span class="token string">"More than 0 but less than or equal to 10"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>

            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">value</span> <span class="token keyword">is</span> <span class="token operator">&gt;</span> <span class="token number">0</span> and <span class="token operator">&lt;=</span> <span class="token number">10</span><span class="token punctuation">)</span> <span class="token comment">// Trong C# 9.</span>
            <span class="token punctuation">{</span>
                Console<span class="token punctuation">.</span><span class="token function">WriteLine</span><span class="token punctuation">(</span><span class="token string">"More than 0 but less than or equal to 10"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>

<span class="token punctuation">}</span>

using System;

namespace csharp9.new3

{

class Program

{

int value = 42;

switch (value)

{

case <= 0:

Console.WriteLine("Less than or equal to 0");

break;

case > 0 and <= 10:

Console.WriteLine("More than 0 but less than or equal to 10");

break;

default:

Console.WriteLine("More than 10");

break;

}

var message = value switch

{

<= 0 => "Less than or equal to 0",

> 0 and <= 10 => "More than 0 but less than or equal to 10",

_ => "More than 10"

};

Console.WriteLine(message);

if (value > 0 && value <= 10) // Trong C# 8.

{

Console.WriteLine("More than 0 but less than or equal to 10");

}

if (value is > 0 and <= 10) // Trong C# 9.

{

Console.WriteLine("More than 0 but less than or equal to 10");

}

(5) Feature Record Object

<span class="token keyword">using</span> <span class="token namespace">System</span><span class="token punctuation">;</span>

<span class="token keyword">namespace</span> <span class="token namespace">csharp9</span>
<span class="token punctuation">{</span>
    <span class="token keyword">class</span> <span class="token class-name">RecordObject</span>
    <span class="token punctuation">{</span>
        <span class="token keyword">static</span> <span class="token return-type class-name"><span class="token keyword">void</span></span> <span class="token function">Main</span><span class="token punctuation">(</span><span class="token class-name"><span class="token keyword">string</span><span class="token punctuation">[</span><span class="token punctuation">]</span></span> args<span class="token punctuation">)</span>
        <span class="token punctuation">{</span>
            <span class="token class-name"><span class="token keyword">var</span></span> book <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token constructor-invocation class-name">Book</span> <span class="token punctuation">{</span> Title <span class="token operator">=</span> <span class="token string">"Chí Phèo"</span><span class="token punctuation">,</span> Author <span class="token operator">=</span> <span class="token string">"Nam Cao"</span><span class="token punctuation">,</span> ISBN <span class="token operator">=</span> <span class="token string">"8935092514741"</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
            <span class="token class-name"><span class="token keyword">var</span></span> newBook <span class="token operator">=</span> <span class="token return-type class-name">book</span> with <span class="token punctuation">{</span> Title <span class="token operator">=</span> <span class="token string">"Đôi lứa xứng đôi"</span> <span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// Trong C# 9. Record Object.</span>
            Console<span class="token punctuation">.</span>OutputEncoding <span class="token operator">=</span> System<span class="token punctuation">.</span>Text<span class="token punctuation">.</span>Encoding<span class="token punctuation">.</span>UTF8<span class="token punctuation">;</span>
            Console<span class="token punctuation">.</span><span class="token function">WriteLine</span><span class="token punctuation">(</span>newBook<span class="token punctuation">.</span>Title<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token return-type class-name">record</span> Book
    <span class="token punctuation">{</span>
        <span class="token keyword">public</span> <span class="token return-type class-name"><span class="token keyword">string</span></span> Title <span class="token punctuation">{</span> <span class="token keyword">get</span><span class="token punctuation">;</span> init<span class="token punctuation">;</span> <span class="token punctuation">}</span>
        <span class="token keyword">public</span> <span class="token return-type class-name"><span class="token keyword">string</span></span> Author <span class="token punctuation">{</span> <span class="token keyword">get</span><span class="token punctuation">;</span> init<span class="token punctuation">;</span> <span class="token punctuation">}</span>
        <span class="token keyword">public</span> <span class="token return-type class-name"><span class="token keyword">string</span></span> ISBN <span class="token punctuation">{</span> <span class="token keyword">get</span><span class="token punctuation">;</span> init<span class="token punctuation">;</span> <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>

<span class="token punctuation">}</span>

using System;

namespace csharp9

{

class RecordObject

{

var book = new Book { Title = "Chí Phèo", Author = "Nam Cao", ISBN = "8935092514741" };

var newBook = book with { Title = "Đôi lứa xứng đôi" }; // Trong C# 9. Record Object.

Console.OutputEncoding = System.Text.Encoding.UTF8;

Console.WriteLine(newBook.Title);

}

public record Book

{

public string Title { get; init; }

public string Author { get; init; }

public string ISBN { get; init; }

}

Source code: https://github.com/donhuvy/csharp9.git

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