Csharp 8

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Switch expression

Csharp.svg
string myVar;
switch (i)
{
    case 1:
        myVar = "One";
        break;
    default:
        throw new ArgumentOutOfRangeException(nameof(i));
}

var myVar = i switch
{
    1 => "One",
    _ => throw new ArgumentOutOfRangeException(nameof(i))
};

Using declaration

Csharp.svg
using (var file = new StreamWriter("MyFile.txt"))
{
    // some code
} // file is disposed here

void MyMethod()
{
    using var file = new StreamWriter("MyFile.txt");
    // some code

    // file is disposed here, at the end of the enclosing scope
}

Static local function

Csharp.svg
int x = 5;
int y = 7;
return Add(x, y);

// Can be static because it doesn't access any variables in the enclosing scope
static int Add(int left, int right) => left + right;

Asynchronous streams / async yield

Csharp.svg
public static async IAsyncEnumerable<int> GenerateSequence()
{
    for (int i = 0; i < 20; i++)
    {
        await Task.Delay(100);
        yield return i;
    }
}

await foreach (var number in GenerateSequence())
{
    Console.WriteLine(number);
}

Indices and ranges

Csharp.svg
var latest = myList[^1];
var secondToFour = myList[2..5];
var lastTwo = myList[^2..^0];
var firstThree = myList[..4];
var allExceptFirst = myList[2..];

Null-coalescing assignment / ?? =

Csharp.svg
int? i = null;

i = i == null ? 20 : i;
i = i ?? 20;

i ??= 20;  // assign 20 to i if i is null

Nullable reference type

MyProject.csproj
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <!-- add this line to enable it -->
    <Nullable>enable</Nullable>

Now by default everything is non nullable. If you want to declare a type as accepting null values, you need to add ? after the type.

Csharp.svg
string s1 = null;  // Warning: Converting null literal or possible null value to non-nullable type.
string? s2 = null; // ok

// null-forgiving operator
Console.WriteLine(s2!.Length);  // in this case we know that s2 is not null, so we can use "!" to instruct the compiler "s2" is not null here

// allow null even if the type is not nullable
public void MyMethod([AllowNull]ref string value)
{ }

// disallow null even if the type is nullable
public void MyMethod([DisallowNull]ref string? value)
{ }

private string s1 = "";
// allow null for setter even if the type is not nullable
[AllowNull]
public string S1
{
    get => s1;
    set => s1 = value ?? "";
}

.NET Framework

The C# 8 / .NET Framework combination is not officially supported by Microsoft.
It can be forced by editing the csproj file.

MyProject.csproj
<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <TargetFrameworkVersion>v4.7.2</TargetFrameworkVersion>
    <LangVersion>8.0</LangVersion>
  • default interface members won't work
  • asynchronous streams and indices and ranges will need polyfill nuget package