C# Language

C# 7.0 Features

Introduction#

C# 7.0 is the seventh version of C#. This version contains some new features: language support for Tuples, local functions, out var declarations, digit separators, binary literals, pattern matching, throw expressions, ref return and ref local and extended expression bodied members list.

Official reference: What’s new in C# 7

out var declaration

A common pattern in C# is using bool TryParse(object input, out object value) to safely parse objects.

The out var declaration is a simple feature to improve readability. It allows a variable to be declared at the same time that is it passed as an out parameter.

A variable declared this way is scoped to the remainder of the body at the point in which it is declared.

Example

Using TryParse prior to C# 7.0, you must declare a variable to receive the value before calling the function:

int value;
if (int.TryParse(input, out value)) 
{
    Foo(value); // ok
}
else
{
    Foo(value); // value is zero
}

Foo(value); // ok

In C# 7.0, you can inline the declaration of the variable passed to the out parameter, eliminating the need for a separate variable declaration:

if (int.TryParse(input, out var value)) 
{
    Foo(value); // ok
}
else
{
    Foo(value); // value is zero
}

Foo(value); // still ok, the value in scope within the remainder of the body

If some of the parameters that a function returns in out is not needed you can use the discard operator _.

p.GetCoordinates(out var x, out _); // I only care about x

An out var declaration can be used with any existing function which already has out parameters. The function declaration syntax remains the same, and no additional requirements are needed to make the function compatible with an out var declaration. This feature is simply syntactic sugar.

Another feature of out var declaration is that it can be used with anonymous types.

var a = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
var groupedByMod2 = a.Select(x => new
                                  {
                                      Source = x,
                                      Mod2 = x % 2
                                  })
                     .GroupBy(x => x.Mod2)
                     .ToDictionary(g => g.Key, g => g.ToArray());
if (groupedByMod2.TryGetValue(1, out var oddElements))
{
    Console.WriteLine(oddElements.Length);
}

In this code we create a Dictionary with int key and array of anonymous type value. In the previous version of C# it was impossible to use TryGetValue method here since it required you to declare the out variable (which is of anonymous type!). However, with out var we do not need to explicitly specify the type of the out variable.

Limitations

Note that out var declarations are of limited use in LINQ queries as expressions are interpreted as expression lambda bodies, so the scope of the introduced variables is limited to these lambdas. For example, the following code will not work:

var nums = 
    from item in seq
    let success = int.TryParse(item, out var tmp)
    select success ? tmp : 0; // Error: The name 'tmp' does not exist in the current context

References

Binary literals

The 0b prefix can be used to represent Binary literals.

Binary literals allow constructing numbers from zeroes and ones, which makes seeing which bits are set in the binary representation of a number much easier. This can be useful for working with binary flags.

The following are equivalent ways of specifying an int with value 34 (=25 + 21):

// Using a binary literal:
//   bits: 76543210
int a1 = 0b00100010;          // binary: explicitly specify bits

// Existing methods:
int a2 = 0x22;                // hexadecimal: every digit corresponds to 4 bits
int a3 = 34;                  // decimal: hard to visualise which bits are set
int a4 = (1 << 5) | (1 << 1); // bitwise arithmetic: combining non-zero bits

Flags enumerations

Before, specifying flag values for an enum could only be done using one of the three methods in this example:

[Flags]
public enum DaysOfWeek
{
    // Previously available methods:
    //          decimal        hex       bit shifting
    Monday    =  1,    //    = 0x01    = 1 << 0
    Tuesday   =  2,    //    = 0x02    = 1 << 1
    Wednesday =  4,    //    = 0x04    = 1 << 2
    Thursday  =  8,    //    = 0x08    = 1 << 3
    Friday    = 16,    //    = 0x10    = 1 << 4
    Saturday  = 32,    //    = 0x20    = 1 << 5
    Sunday    = 64,    //    = 0x40    = 1 << 6

    Weekdays = Monday | Tuesday | Wednesday | Thursday | Friday,
    Weekends = Saturday | Sunday
}

With binary literals it is more obvious which bits are set, and using them does not require understanding hexadecimal numbers and bitwise arithmetic:

[Flags]
public enum DaysOfWeek
{
    Monday    = 0b00000001,
    Tuesday   = 0b00000010,
    Wednesday = 0b00000100,
    Thursday  = 0b00001000,
    Friday    = 0b00010000,
    Saturday  = 0b00100000,
    Sunday    = 0b01000000,

    Weekdays = Monday | Tuesday | Wednesday | Thursday | Friday,
    Weekends = Saturday | Sunday
}

Digit separators

The underscore _ may be used as a digit separator. Being able to group digits in large numeric literals has a significant impact on readability.

The underscore may occur anywhere in a numeric literal except as noted below. Different groupings may make sense in different scenarios or with different numeric bases.

Any sequence of digits may be separated by one or more underscores. The _ is allowed in decimals as well as exponents. The separators have no semantic impact - they are simply ignored.

int bin = 0b1001_1010_0001_0100;
int hex = 0x1b_a0_44_fe;
int dec = 33_554_432;
int weird = 1_2__3___4____5_____6______7_______8________9;
double real = 1_000.111_1e-1_000;

Where the _ digit separator may not be used:

  • at the beginning of the value (_121)
  • at the end of the value (121_ or 121.05_)
  • next to the decimal (10_.0)
  • next to the exponent character (1.1e_1)
  • next to the type specifier (10_f)
  • immediately following the 0x or 0b in binary and hexadecimal literals (might be changed to allow e.g. 0b_1001_1000)

Language support for Tuples

Basics

A tuple is an ordered, finite list of elements. Tuples are commonly used in programming as a means to work with one single entity collectively instead of individually working with each of the tuple’s elements, and to represent individual rows (ie. “records”) in a relational database.

In C# 7.0, methods can have multiple return values. Behind the scenes, the compiler will use the new ValueTuple struct.

public (int sum, int count) GetTallies() 
{
    return (1, 2);
}

Side note: for this to work in Visual Studio 2017, you need to get the System.ValueTuple package.

If a tuple-returning method result is assigned to a single variable you can access the members by their defined names on the method signature:

var result = GetTallies();
// > result.sum
// 1
// > result.count
// 2

Tuple Deconstruction

Tuple deconstruction separates a tuple into its parts.

For example, invoking GetTallies and assigning the return value to two separate variables deconstructs the tuple into those two variables:

(int tallyOne, int tallyTwo) = GetTallies();

var also works:

(var s, var c) = GetTallies();

You can also use shorter syntax, with var outside of ():

var (s, c) = GetTallies();

You can also deconstruct into existing variables:

int s, c;
(s, c) = GetTallies();

Swapping is now much simpler (no temp variable needed):

(b, a) = (a, b);

Interestingly, any object can be deconstructed by defining a Deconstruct method in the class:

class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public void Deconstruct(out string firstName, out string lastName)
    {
        firstName = FirstName;
        lastName = LastName;
    }
}

var person = new Person { FirstName = "John", LastName = "Smith" };
var (localFirstName, localLastName) = person;

In this case, the (localFirstName, localLastName) = person syntax is invoking Deconstruct on the person.

Deconstruction can even be defined in an extension method. This is equivalent to the above:

public static class PersonExtensions
{
    public static void Deconstruct(this Person person, out string firstName, out string lastName)
    {
        firstName = person.FirstName;
        lastName = person.LastName;
    }
}

var (localFirstName, localLastName) = person;

An alternative approach for the Person class is to define the Name itself as a Tuple. Consider the following:

class Person
{
    public (string First, string Last) Name { get; }

    public Person((string FirstName, string LastName) name)
    {
        Name = name;
    }
}

Then you can instantiate a person like so (where we can take a tuple as an argument):

var person = new Person(("Jane", "Smith"));

var firstName = person.Name.First; // "Jane"
var lastName = person.Name.Last;   // "Smith"

Tuple Initialization

You can also arbitrarily create tuples in code:

var name = ("John", "Smith");
Console.WriteLine(name.Item1);
// Outputs John

Console.WriteLine(name.Item2);
// Outputs Smith

When creating a tuple, you can assign ad-hoc item names to the members of the tuple:

var name = (first: "John", middle: "Q", last: "Smith");
Console.WriteLine(name.first);
// Outputs John

Type inference

Multiple tuples defined with the same signature (matching types and count) will be inferred as matching types. For example:

public (int sum, double average) Measure(List<int> items)
{
    var stats = (sum: 0, average: 0d);
    stats.sum = items.Sum();
    stats.average = items.Average();
    return stats;
}

stats can be returned since the declaration of the stats variable and the method’s return signature are a match.

Reflection and Tuple Field Names

Member names do not exist at runtime. Reflection will consider tuples with the same number and types of members the same even if member names do not match. Converting a tuple to an object and then to a tuple with the same member types, but different names, will not cause an exception either.

While the ValueTuple class itself does not preserve information for member names the information is available through reflection in a TupleElementNamesAttribute. This attribute is not applied to the tuple itself but to method parameters, return values, properties and fields. This allows tuple item names to be preserved across assemblies i.e. if a method returns (string name, int count) the names name and count will be available to callers of the method in another assembly because the return value will be marked with TupleElementNameAttribute containing the values “name” and “count”.

Use with generics and async

The new tuple features (using the underlying ValueTuple type) fully support generics and can be used as generic type parameter. That makes it possible to use them with the async/await pattern:

public async Task<(string value, int count)> GetValueAsync()
{
    string fooBar = await _stackoverflow.GetStringAsync();
    int num = await _stackoverflow.GetIntAsync();

    return (fooBar, num);
}

Use with collections

It may become beneficial to have a collection of tuples in (as an example) a scenario where you’re attempting to find a matching tuple based on conditions to avoid code branching.

Example:

private readonly List<Tuple<string, string, string>> labels = new List<Tuple<string, string, string>>()
{
    new Tuple<string, string, string>("test1", "test2", "Value"),
    new Tuple<string, string, string>("test1", "test1", "Value2"),
    new Tuple<string, string, string>("test2", "test2", "Value3"),
};

public string FindMatchingValue(string firstElement, string secondElement)
{
    var result = labels
        .Where(w => w.Item1 == firstElement && w.Item2 == secondElement)
        .FirstOrDefault();

    if (result == null)
        throw new ArgumentException("combo not found");

    return result.Item3;
}

With the new tuples can become:

private readonly List<(string firstThingy, string secondThingyLabel, string foundValue)> labels = new List<(string firstThingy, string secondThingyLabel, string foundValue)>()
{
    ("test1", "test2", "Value"),
    ("test1", "test1", "Value2"),
    ("test2", "test2", "Value3"),
}

public string FindMatchingValue(string firstElement, string secondElement)
{
    var result = labels
        .Where(w => w.firstThingy == firstElement && w.secondThingyLabel == secondElement)
        .FirstOrDefault();

    if (result == null)
        throw new ArgumentException("combo not found");

    return result.foundValue;
}

Though the naming on the example tuple above is pretty generic, the idea of relevant labels allows for a deeper understanding of what is being attempted in the code over referencing “item1”, “item2”, and “item3”.

Differences between ValueTuple and Tuple

The primary reason for introduction of ValueTuple is performance.

Type name ValueTuple Tuple
Class or structure struct class
Mutability (changing values after creation) mutable immutable
Naming members and other language support yes no (TBD)

References

Local functions

Local functions are defined within a method and aren’t available outside of it. They have access to all local variables and support iterators, async/await and lambda syntax. This way, repetitions specific to a function can be functionalized without crowding the class. As a side effect, this improves intellisense suggestion performance.

Example

double GetCylinderVolume(double radius, double height)
{
    return getVolume();

    double getVolume()
    {
        // You can declare inner-local functions in a local function 
        double GetCircleArea(double r) => Math.PI * r * r;

        // ALL parents' variables are accessible even though parent doesn't have any input. 
        return GetCircleArea(radius) * height;
    }
}

Local functions considerably simplify code for LINQ operators, where you usually have to separate argument checks from actual logic to make argument checks instant, not delayed until after iteration started.

Example

public static IEnumerable<TSource> Where<TSource>(
    this IEnumerable<TSource> source, 
    Func<TSource, bool> predicate)
{
    if (source == null) throw new ArgumentNullException(nameof(source));
    if (predicate == null) throw new ArgumentNullException(nameof(predicate));

    return iterator();

    IEnumerable<TSource> iterator()
    {
        foreach (TSource element in source)
            if (predicate(element))
                yield return element;
    }
}

Local functions also support the async and await keywords.

Example

async Task WriteEmailsAsync()
{
    var emailRegex = new Regex(@"(?i)[a-z0-9_.+-]+@[a-z0-9-]+\.[a-z0-9-.]+");
    IEnumerable<string> emails1 = await getEmailsFromFileAsync("input1.txt");
    IEnumerable<string> emails2 = await getEmailsFromFileAsync("input2.txt");
    await writeLinesToFileAsync(emails1.Concat(emails2), "output.txt");

    async Task<IEnumerable<string>> getEmailsFromFileAsync(string fileName)
    {
        string text;

        using (StreamReader reader = File.OpenText(fileName))
        {
            text = await reader.ReadToEndAsync();
        }

        return from Match emailMatch in emailRegex.Matches(text) select emailMatch.Value;
    }

    async Task writeLinesToFileAsync(IEnumerable<string> lines, string fileName)
    {
        using (StreamWriter writer = File.CreateText(fileName))
        {
            foreach (string line in lines)
            {
                await writer.WriteLineAsync(line);
            }
        }
    }
}

One important thing that you may have noticed is that local functions can be defined under the return statement, they do not need to be defined above it. Additionally, local functions typically follow the “lowerCamelCase” naming convention as to more easily differentiate themselves from class scope functions.

Pattern Matching

Pattern matching extensions for C# enable many of the benefits of pattern matching from functional languages, but in a way that smoothly integrates with the feel of the underlying language

switch expression

Pattern matching extends the switch statement to switch on types:

class Geometry {} 

class Triangle : Geometry
{
    public int Width { get; set; }
    public int Height { get; set; }
    public int Base { get; set; }
}

class Rectangle : Geometry
{
    public int Width { get; set; }
    public int Height { get; set; }
}

class Square : Geometry
{
    public int Width { get; set; }
}

public static void PatternMatching()
{
    Geometry g = new Square { Width = 5 }; 
    
    switch (g)
    {
        case Triangle t:
            Console.WriteLine($"{t.Width} {t.Height} {t.Base}");
            break;
        case Rectangle sq when sq.Width == sq.Height:
            Console.WriteLine($"Square rectangle: {sq.Width} {sq.Height}");
            break;
        case Rectangle r:
            Console.WriteLine($"{r.Width} {r.Height}");
            break;
        case Square s:
            Console.WriteLine($"{s.Width}");
            break;
        default:
            Console.WriteLine("<other>");
            break;
    }
}

is expression

Pattern matching extends the is operator to check for a type and declare a new variable at the same time.

Example

string s = o as string;
if(s != null)
{
    // do something with s
}

can be rewritten as:

if(o is string s)
{
    //Do something with s
};

Also note that the scope of the pattern variable s is extended to outside the if block reaching the end of the enclosing scope, example:

if(someCondition)
{
   if(o is string s)
   {
      //Do something with s
   }
   else
   {
     // s is unassigned here, but accessible 
   }

   // s is unassigned here, but accessible 
}
// s is not accessible here

ref return and ref local

Ref returns and ref locals are useful for manipulating and returning references to blocks of memory instead of copying memory without resorting to unsafe pointers.

Ref Return

public static ref TValue Choose<TValue>(
    Func<bool> condition, ref TValue left, ref TValue right)
{
    return condition() ? ref left : ref right;
}

With this you can pass two values by reference with one of them being returned based on some condition:

Matrix3D left = …, right = …;
Choose(chooser, ref left, ref right).M20 = 1.0;

Ref Local

public static ref int Max(ref int first, ref int second, ref int third)
{
    ref int max = first > second ? ref first : ref second;
    return max > third ? ref max : ref third;
}
…
int a = 1, b = 2, c = 3;
Max(ref a, ref b, ref c) = 4;
Debug.Assert(a == 1); // true
Debug.Assert(b == 2); // true
Debug.Assert(c == 4); // true

Unsafe Ref Operations

In System.Runtime.CompilerServices.Unsafe a set of unsafe operations have been defined that allow you to manipulate ref values as if they were pointers, basically.

For example, reinterpreting a memory address (ref) as a different type:

byte[] b = new byte[4] { 0x42, 0x42, 0x42, 0x42 };

ref int r = ref Unsafe.As<byte, int>(ref b[0]);
Assert.Equal(0x42424242, r);

0x0EF00EF0;
Assert.Equal(0xFE, b[0] | b[1] | b[2] | b[3]);

Beware of endianness when doing this, though, e.g. check BitConverter.IsLittleEndian if needed and handle accordingly.

Or iterate over an array in an unsafe manner:

int[] a = new int[] { 0x123, 0x234, 0x345, 0x456 };

ref int r1 = ref Unsafe.Add(ref a[0], 1);
Assert.Equal(0x234, r1);

ref int r2 = ref Unsafe.Add(ref r1, 2);
Assert.Equal(0x456, r2);

ref int r3 = ref Unsafe.Add(ref r2, -3);
Assert.Equal(0x123, r3);

Or the similar Subtract:

string[] a = new string[] { "abc", "def", "ghi", "jkl" };

ref string r1 = ref Unsafe.Subtract(ref a[0], -2);
Assert.Equal("ghi", r1);

ref string r2 = ref Unsafe.Subtract(ref r1, -1);
Assert.Equal("jkl", r2);

ref string r3 = ref Unsafe.Subtract(ref r2, 3);
Assert.Equal("abc", r3);

Additionally, one can check if two ref values are the same i.e. same address:

long[] a = new long[2];

Assert.True(Unsafe.AreSame(ref a[0], ref a[0]));
Assert.False(Unsafe.AreSame(ref a[0], ref a[1]));

Links

Roslyn Github Issue

System.Runtime.CompilerServices.Unsafe on github

throw expressions

C# 7.0 allows throwing as an expression in certain places:

class Person
{
    public string Name { get; }

    public Person(string name) => Name = name ?? throw new ArgumentNullException(nameof(name));

    public string GetFirstName()
    {
        var parts = Name.Split(' ');
        return (parts.Length > 0) ? parts[0] : throw new InvalidOperationException("No name!");
    }

    public string GetLastName() => throw new NotImplementedException();
}

Prior to C# 7.0, if you wanted to throw an exception from an expression body you would have to:

var spoons = "dinner,desert,soup".Split(',');

var spoonsArray = spoons.Length > 0 ? spoons : null;

if (spoonsArray == null) 
{
    throw new Exception("There are no spoons");
}

Or

var spoonsArray = spoons.Length > 0 
    ? spoons 
    : new Func<string[]>(() => 
      {
          throw new Exception("There are no spoons");
      })();

In C# 7.0 the above is now simplified to:

var spoonsArray = spoons.Length > 0 ? spoons : throw new Exception("There are no spoons");

Extended expression bodied members list

C# 7.0 adds accessors, constructors and finalizers to the list of things that can have expression bodies:

class Person
{
    private static ConcurrentDictionary<int, string> names = new ConcurrentDictionary<int, string>();

    private int id = GetId();

    public Person(string name) => names.TryAdd(id, name); // constructors

    ~Person() => names.TryRemove(id, out _);              // finalizers

    public string Name
    {
        get => names[id];                                 // getters
        set => names[id] = value;                         // setters
    }
}

Also see the out var declaration section for the discard operator.

ValueTask

Task<T> is a class and causes the unnecessary overhead of its allocation when the result is immediately available.

ValueTask<T> is a structure and has been introduced to prevent the allocation of a Task object in case the result of the async operation is already available at the time of awaiting.

So ValueTask<T> provides two benefits:

1. Performance increase

Here’s a Task<T> example:

  • Requires heap allocation

  • Takes 120ns with JIT

    async Task TestTask(int d) { await Task.Delay(d); return 10; }

Here’s the analog ValueTask<T> example:

  • No heap allocation if the result is known synchronously (which it is not in this case because of the Task.Delay, but often is in many real-world async/await scenarios)

  • Takes 65ns with JIT

    async ValueTask TestValueTask(int d) { await Task.Delay(d); return 10; }

2. Increased implementation flexibility

Implementations of an async interface wishing to be synchronous would otherwise be forced to use either Task.Run or Task.FromResult (resulting in the performance penalty discussed above). Thus there’s some pressure against synchronous implementations.

But with ValueTask<T>, implementations are more free to choose between being synchronous or asynchronous without impacting callers.

For example, here’s an interface with an asynchronous method:

interface IFoo<T>
{
    ValueTask<T> BarAsync();
}

…and here’s how that method might be called:

IFoo<T> thing = getThing();
var x = await thing.BarAsync();

With ValueTask, the above code will work with either synchronous or asynchronous implementations:

Synchronous implementation:

class SynchronousFoo<T> : IFoo<T>
{
    public ValueTask<T> BarAsync()
    {
        var value = default(T);
        return new ValueTask<T>(value);
    }
}

Asynchronous implementation

class AsynchronousFoo<T> : IFoo<T>
{
    public async ValueTask<T> BarAsync()
    {
        var value = default(T);
        await Task.Delay(1);
        return value;
    }
}

Notes

Although ValueTask struct was being planned to be added to C# 7.0, it has been kept as another library for the time being. https://stackoverflow.com/documentation/c%23/1936/c-sharp-7-0-features/28612/valuetaskt# System.Threading.Tasks.Extensions package can be downloaded from Nuget Gallery

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