C# 8 Features in Visual Studio 2019

What Are New Features in C# 8.0? | C# 8 Features in VS 2019

What Are New Features in c# 8.0?

C# 8.0 adds the following features and enhancements to the C# language:
=>Read-only members
=>Default interface methods
=>Pattern matching enhancements:
          i) Witch expressions
          j) Property patterns
         k) Tuple patterns
         l) Positional patterns
=>Using declarations
=>Static local functions
=>Disposable ref structs
=>Nullable reference types
=>Asynchronous streams
=>Indices and ranges
=>Null-coalescing assignment
=>Unmanaged constructed types
=>Stackalloc in nested expressions
=>Enhancement of interpolated verbatim strings

You can see n-depth articles with examples –

Readonly Member  -
Provide a way to specify individual instance members on a struct do not modify state, in the same way that read-only struct specifies no instance members modify state. 

Allows you to apply the read-only modifier to any member of a struct.

public struct YValue
    private int Y { get; set; }
    public readonly int IncreaseY()
        // This will not compile: C# 8
        Y = Y + 1;
        var newY = Y + 1; // OK
        return newY;

Default Interface Methods  -

Allows you to add new functionality to your interfaces of your libraries and ensure the backward compatibility with code written for older versions of those interfaces.

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

Pattern Matching Enhancements  -

Provides the ability to deconstruct matched objects, and giving you access to parts of their data structures.

 C# offers a rich set of patterns that can be used for matching:  
1.      Switch expressions
2.      Property patterns
3.      Tuple patterns
4.      Positional patterns

There are several syntax improvements here:
1.      The variable comes before the switch keyword. The different order makes it visually easy to distinguish the switch expression from the switch statement.
2.      The case and : elements are replaced with =>. It's more concise and intuitive.
3.      The default case is replaced with a _ discard.
4.      The bodies are expressions, not statements.

static bool Positive(Point p) => p switch
            (0, 0) => true,
            (var x, var y) when x > 0 && y > 0 => true,
            _ => false

Nullable Reference Types -

Emits a compiler warning or error if a variable that must not be null is assigned to null.

string? nullableString = null;
// WARNING: may be null! Take care!

The goal of this feature is to:

Allow developers to express whether a variable, parameter or result of a reference type is intended to be null or not.

Provide warnings when such variables, parameters and results are not used according to that intent.

Using Declarations  -

The language will add two new capabilities around the using statement in order to make resource management simpler: using should recognize a disposable pattern in addition to IDisposable and add a using declaration to the language.

Enhances the ‘using’ operator to use with Patterns and make it more natural.

using var repository = new Repository();
// repository is disposed here!

Enhancement of interpolated verbatim strings

Order of the $ and @ tokens in interpolated verbatim strings can be any: both $@"..." and @$"..." are valid interpolated verbatim strings. In earlier C# versions, the $ token must appear before the @ token.

Allows @$““as a verbatim interpolated string,
var file = @$"c:\temp\{filename}";

Asynchronous Streams  -

Starting with C# 8.0, you can create and consume streams asynchronously. A method that returns an asynchronous stream has three properties:

1) It's declared with the async modifier.
2) It returns an IAsyncEnumerable<T>.
3) The method contains yield return statements to return successive elements in the asynchronous stream.

Allows having enumerators that support async operations.

await foreach (var x in enumerable)

Null-coalescing Assignment  -

C# 8.0 introduces the null-coalescing assignment operator ??=. You can use the ??= operator to assign the value of its right-hand operand to its left-hand operand only if the left-hand operand evaluates to null.

Simplifies a common coding pattern where a variable is assigned a value if it is null. It is common to see the code of the form:

//In C# 1 to 7
 if (variable == null)
   variable = expression; 

// In C# 8
variable ??= expression;

Static local functions

You can now add the static modifier to local functions to ensure that local function doesn't capture (reference) any variables from the enclosing scope. Allows you to add the 'static' modifier to the local functions.

int AddFiveAndSeven()
    int y = 1; int x = 2;
    return Add(x, y);

    static int Add(int o, int t) => o + t;

Unmanaged Constructed Types  -

Allows you to take a pointer to unmanaged constructed types, such as ValueTuple<int, int>, as long as all the elements of the generic type are unmanaged.

struct Foo<T> where T : unmanaged

public unsafe void Test()
    var foo = new Foo<int>();
    var bar = &foo;   // C# 8

Stackalloc in Nested Expressions

The result of a stackalloc expression is of the System.Span<T> or System.ReadOnlySpan<T> type.

Span<int> set = stackalloc[] { 1, 2, 3, 4, 5, 6};
var subSet = set.Slice(3, 2);

foreach (var n in subSet){
  Console.WriteLine(n);  // Output: 4  5

Disposable ref structs  -

A struct declared with the ref modifier may not implement any interfaces and so can't implement IDisposable. Therefore, to enable a ref struct to be disposed, it must have an accessible void Dispose() method. This feature also applies to read-only ref struct declarations.

Allows you to use the ‘using’ pattern with ref struct or readonly ref struct‘.

// Pattern-based using for ref struct
ref struct Test
    public void Dispose() { }
using var test = new Test();
// test is disposed here!
Indices and ranges  -
Indices and ranges provide a succinct syntax for accessing single elements or ranges in a sequence.

1) This language support relies on two new types, and two new operators:
2) System.Index represents an index into a sequence.
3) The index from end operator ^, which specifies that an index is relative to the end of the sequence.
4) System.Range represents a sub range of a sequence.
5) The range operator .., which specifies the start and end of a range as its operands.

Allows you to use more natural syntax for specifying subranges in an array or a collection.

int[] a = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

Index:  Used to obtain the collection from the beginning or from the end.

 // Number 4 from end of the collection
Index i2 = ^4;
Console.WriteLine($"{a[i2]}"); // "6"

//Range: Access a sub-collection(slice) from a collection.
var slice = a[i1..i2]; // { 3, 4, 5 }

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