C# 9.0 on the record

Mads Torgersen

Mads

C# 9.0 on the record

It’s official: C# 9.0 is out! Back in May I blogged about the C# 9.0 plans, and the following is an updated version of that post to match what we actually ended up shipping.

With every new version of C# we strive for greater clarity and simplicity in common coding scenarios, and C# 9.0 is no exception. One particular focus this time is supporting terse and immutable representation of data shapes.

Init-only properties

Object initializers are pretty awesome. They give the client of a type a very flexible and readable format for creating an object, and they are especially great for nested object creation where a whole tree of objects is created in one go. Here’s a simple one:

var person = new Person { FirstName = "Mads", LastName = "Torgersen" };

Object initializers also free the type author from writing a lot of construction boilerplate – all they have to do is write some properties!

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

The one big limitation today is that the properties have to be mutable for object initializers to work: They function by first calling the object’s constructor (the default, parameterless one in this case) and then assigning to the property setters. Init-only properties fix that! They introduce an init accessor that is a variant of the set accessor which can only be called during object initialization:

public class Person
{
    public string? FirstName { get; init; }
    public string? LastName { get; init; }
}

With this declaration, the client code above is still legal, but any subsequent assignment to the FirstName and LastName properties is an error:

var person = new Person { FirstName = "Mads", LastName = "Nielsen" }; // OK
person.LastName = "Torgersen"; // ERROR!

Thus, init-only properties protect the state of the object from mutation once initialization is finished.

Init accessors and readonly fields

Because init accessors can only be called during initialization, they are allowed to mutate readonly fields of the enclosing class, just like you can in a constructor.

public class Person
{
    private readonly string firstName = "<unknown>";
    private readonly string lastName = "<unknown>";
    
    public string FirstName 
    { 
        get => firstName; 
        init => firstName = (value ?? throw new ArgumentNullException(nameof(FirstName)));
    }
    public string LastName 
    { 
        get => lastName; 
        init => lastName = (value ?? throw new ArgumentNullException(nameof(LastName)));
    }
}

Records

At the core of classic object-oriented programming is the idea that an object has strong identity and encapsulates mutable state that evolves over time. C# has always worked great for that, But sometimes you want pretty much the exact opposite, and here C#’s defaults have tended to get in the way, making things very laborious.

If you find yourself wanting the whole object to be immutable and behave like a value, then you should consider declaring it as a record:

public record Person
{
    public string? FirstName { get; init; }
    public string? LastName { get; init; }
}

A record is still a class, but the record keyword imbues it with several additional value-like behaviors. Generally speaking, records are defined by their contents, not their identity. In this regard, records are much closer to structs, but records are still reference types.

While records can be mutable, they are primarily built for better supporting immutable data models.

With-expressions

When working with immutable data, a common pattern is to create new values from existing ones to represent a new state. For instance, if our person were to change their last name we would represent it as a new object that’s a copy of the old one, except with a different last name. This technique is often referred to as non-destructive mutation. Instead of representing the person over time, the record represents the person’s state at a given time. To help with this style of programming, records allow for a new kind of expression; the with-expression:

var person = new Person { FirstName = "Mads", LastName = "Nielsen" };
var otherPerson = person with { LastName = "Torgersen" };

With-expressions use object initializer syntax to state what’s different in the new object from the old object. You can specify multiple properties.

The with-expression works by actually copying the full state of the old object into a new one, then mutating it according to the object initializer. This means that properties must have an init or set accessor to be changed in a with-expression.

Value-based equality

All objects inherit a virtual Equals(object) method from the object class. This is used as the basis for the Object.Equals(object, object) static method when both parameters are non-null. Structs override this to have "value-based equality", comparing each field of the struct by calling Equals on them recursively. Records do the same. This means that in accordance with their "value-ness" two record objects can be equal to one another without being the same object. For instance if we modify the last name of the modified person back again:

var originalPerson = otherPerson with { LastName = "Nielsen" };

We would now have ReferenceEquals(person, originalPerson) = false (they aren’t the same object) but Equals(person, originalPerson) = true (they have the same value). Along with the value-based Equals there’s also a value-based GetHashCode() override to go along with it. Additionally, records implement IEquatable<T> and overload the == and != operators, so that the value-based behavior shows up consistently across all those different equality mechanisms.

Value equality and mutability don’t always mesh well. One problem is that changing values could cause the result of GetHashCode to change over time, which is unfortunate if the object is stored in a hash table! We don’t disallow mutable records, but we discourage them unless you have thought through the consequences!

Inheritance

Records can inherit from other records:

public record Student : Person
{
    public int ID;
}

With-expressions and value equality work well with record inheritance, in that they take the whole runtime object into account, not just the type that it’s statically known by. Say that I create a Student but store it in a Person variable:

Person student = new Student { FirstName = "Mads", LastName = "Nielsen", ID = 129 };

A with-expression will still copy the whole object and keep the runtime type:

var otherStudent = student with { LastName = "Torgersen" };
WriteLine(otherStudent is Student); // true

In the same manner, value equality makes sure the two objects have the same runtime type, and then compares all their state:

Person similarStudent = new Student { FirstName = "Mads", LastName = "Nielsen", ID = 130 };
WriteLine(student != similarStudent); //true, since ID's are different

Positional records

Sometimes it’s useful to have a more positional approach to a record, where its contents are given via constructor arguments, and can be extracted with positional deconstruction. It’s perfectly possible to specify your own constructor and deconstructor in a record:

public record Person 
{ 
    public string FirstName { get; init; } 
    public string LastName { get; init; }
    public Person(string firstName, string lastName) 
      => (FirstName, LastName) = (firstName, lastName);
    public void Deconstruct(out string firstName, out string lastName) 
      => (firstName, lastName) = (FirstName, LastName);
}

But there’s a much shorter syntax for expressing exactly the same thing (modulo casing of parameter names):

public record Person(string FirstName, string LastName);

This declares the public init-only auto-properties and the constructor and the deconstructor, so that you can write:

var person = new Person("Mads", "Torgersen"); // positional construction
var (f, l) = person;                        // positional deconstruction

If you don’t like the generated auto-property you can define your own property of the same name instead, and the generated constructor and deconstructor will just use that one. In this case, the parameter is in scope for you to use for initialization. Say, for instance, that you’d rather have the FirstName be a protected property:

public record Person(string FirstName, string LastName)
{
    protected string FirstName { get; init; } = FirstName; 
}

A positional record can call a base constructor like this:

public record Student(string FirstName, string LastName, int ID) : Person(FirstName, LastName);

Top-level programs

Writing a simple program in C# requires a remarkable amount of boilerplate code:

using System;
class Program
{
    static void Main()
    {
        Console.WriteLine("Hello World!");
    }
}

This is not only overwhelming for language beginners, but clutters up the code and adds levels of indentation. In C# 9.0 you can just write your main program at the top level instead:

using System;

Console.WriteLine("Hello World!");

Any statement is allowed. The program has to occur after the usings and before any type or namespace declarations in the file, and you can only do this in one file, just as you can have only one Main method today. If you want to return a status code you can do that. If you want to await things you can do that. And if you want to access command line arguments, args is available as a "magic" parameter.

using static System.Console;
using System.Threading.Tasks;

WriteLine(args[0]);
await Task.Delay(1000);
return 0;

Local functions are a form of statement and are also allowed in the top level program. It is an error to call them from anywhere outside of the top level statement section.

Improved pattern matching

Several new kinds of patterns have been added in C# 9.0. Let’s look at them in the context of this code snippet from the pattern matching tutorial:

public static decimal CalculateToll(object vehicle) =>
    vehicle switch
    {
       ...
       
        DeliveryTruck t when t.GrossWeightClass > 5000 => 10.00m + 5.00m,
        DeliveryTruck t when t.GrossWeightClass < 3000 => 10.00m - 2.00m,
        DeliveryTruck _ => 10.00m,

        _ => throw new ArgumentException("Not a known vehicle type", nameof(vehicle))
    };

Simple type patterns

Previously, a type pattern needs to declare an identifier when the type matches – even if that identifier is a discard _, as in DeliveryTruck _ above. But now you can just write the type:

DeliveryTruck => 10.00m,

Relational patterns

C# 9.0 introduces patterns corresponding to the relational operators <, <= and so on. So you can now write the DeliveryTruck part of the above pattern as a nested switch expression:

DeliveryTruck t when t.GrossWeightClass switch
{
    > 5000 => 10.00m + 5.00m,
    < 3000 => 10.00m - 2.00m,
    _ => 10.00m,
},

Here > 5000 and < 3000 are relational patterns.

Logical patterns

Finally you can combine patterns with logical operators and, or and not, spelled out as words to avoid confusion with the operators used in expressions. For instance, the cases of the nested switch above could be put into ascending order like this:

DeliveryTruck t when t.GrossWeightClass switch
{
    < 3000 => 10.00m - 2.00m,
    >= 3000 and <= 5000 => 10.00m,
    > 5000 => 10.00m + 5.00m,
},

The middle case there uses and to combine two relational patterns and form a pattern representing an interval. A common use of the not pattern will be applying it to the null constant pattern, as in not null. For instance we can split the handling of unknown cases depending on whether they are null:

not null => throw new ArgumentException($"Not a known vehicle type: {vehicle}", nameof(vehicle)),
null => throw new ArgumentNullException(nameof(vehicle))

Also not is going to be convenient in if-conditions containing is-expressions where, instead of unwieldy double parentheses:

if (!(e is Customer)) { ... }

You can just say

if (e is not Customer) { ... }

And in fact, in an is not expression like that we allow you to name the Customer for subsequent use:

if (e is not Customer c) { throw ... } // if this branch throws or returns...
var n = c.FirstName; // ... c is definitely assigned here

Target-typed new expressions

"Target typing" is a term we use for when an expression gets its type from the context of where it’s being used. For instance null and lambda expressions are always target typed.

new expressions in C# have always required a type to be specified (except for implicitly typed array expressions). In C# 9.0 you can leave out the type if there’s a clear type that the expression is being assigned to.

Point p = new (3, 5);

This is particularly nice when you have a lot of repetition, such as in an array or object initializer:

Point[] ps = { new (1, 2), new (5, 2), new (5, -3), new (1, -3) }; 

Covariant returns

It’s sometimes useful to express that a method override in a derived class has a more specific return type than the declaration in the base type. C# 9.0 allows that:

abstract class Animal
{
    public abstract Food GetFood();
    ...
}
class Tiger : Animal
{
    public override Meat GetFood() => ...;
}

And much more…

The best place to check out the full set of C# 9.0 features is the "What’s new in C# 9.0" docs page.

72 comments

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  • Avatar
    Ian Marteens

    I only have one thing to ask: please, keep GvR as far as possible from C#. I don’t want to deal with duck typing in my so far fav language. Btw, adding duck type is an excelent example that shows how silly is the “you don’t have to use it” argument.

  • Avatar
    Steve John

    I have Visual Studio 16.8.1 and the dotnet 5.0.100 SDK installed. The IDE highlights C# 9 features with red squiggles, but the project still builds.

  • Avatar
    Ben Jones

    This is a particularly useful construct for those of use that use a lot of immutable DTO classes cutting out a lot of boiler plate code. However, how can you declare XML comments for a public record to ensure compliance with CS1591?! I’ve tried setting them up in a similar style to constructor commenting but it still warns me I am missing XML comment for publicly visible type or member. Any advice?

    • Avatar
      Ben Jones

      For clarity, I’m specifically talking about positional record declarations here, i.e.

      public record SomeImmutableClass(int Id, bool IsDefault, string Title);
      
  • Avatar
    Conrad Akunga

    Why does the record construct require properties to have an init setter at all? Is this not implied?

    public record Person
    {
        public string FirstName { get; }
        public string Surname { get; }
        public DateTime DateOfBirth { get; }
        public int YearOfBirth => DateOfBirth.Year;
    
    }

    Would this not be enough to signify intent that this class is purely a record, and get the benefits of init without having to explicitly indicate it?

  • Avatar
    christof jans

    I think there is a problem with this sample code that Mads wrote:

    DeliveryTruck t when t.GrossWeightClass switch
    {
        > 5000 => 10.00m + 5.00m,
         10.00m - 2.00m,
        _ => 10.00m,
    },

    Instead, this should be :

    DeliveryTruck t => t.GrossWeightClass switch
    {
        > 5000 => 10.00m + 5.00m,
         10.00m - 2.00m,
        _ => 10.00m,
    },

    (I replaced the “when” with an arrow “=>” )

  • Avatar
    Terry Foster

    Just chiming in on these new features. I’ve been using C# since pretty much the beginning (almost 20 years now) and generally love the language/platform. I’m very excited about many of these features – actually more of the little ones than the big ones, such as being able to write if (e is not Customer) { ... } (I actually avoid these kinds of tests because I hate dealing with those extra parentheses), target-type new expressions (finally!), and co-variant returns (I’ve hit this limitation so many times, and I oh how I hate it). I’ve also been waiting for some form of records for a very long time. It drives me crazy how much (redundant) code is required to define a simple class that can be instantiated in a concise way. I’m not sure how I feel yet about how this actually got implemented – some aspects are kind of strange to me, and time will tell – but I’m excited to start trying them out. The ‘top-level program’ thing seems rather unnecessary and too ‘magicky’ (with too many special rules) to me, but I probably won’t have to deal with it much. All the pattern matching stuff, though, is really starting to lose me. It might just be me, but I’m really struggling to follow some of the examples. Readability doesn’t seem great, though I can generally decipher what’s happening, but I feel like I’d have quite a difficult time actually constructing these kinds of statements. Again, I guess time will tell on how accessible and useful the syntax will actually be. Thanks for all the hard work on an overall excellent language!

  • Avatar
    Craig Barrett

    What is required to be able to use these features in Visual Studio 2019 without it reporting them all as errors? I found (I think on StackOverflow) that to get init working without targeting .Net 5 required an static class System.Runtime.CompilerServices.IsExternalInit. But VS 2019 (16.8.2) is unable to recognise records correctly. It shows the record type name as an error and says it can’t find the type. If I declare a record inside a class then the class becomes riddled with errors – gets and sets, member variable names, uses of properties and method arguments and other things all get marked as errors. The code compiles, but it’s impossible in the editor to tell what’s really an error and what’s not. If I move that record declaration outside the class then all those errors (except the ones saying the record type isn’t found when I create instances) disappear. The code formatting also doesn’t work for records – indentation is removed.

    Target-typed instantiation doesn’t work at all. The code just doesn’t compile when I use that.

    Which of these features are actually only properly enabled when targeting .Net 5 (or declaring things like the IsExternalInit static class)?

    • Avatar
      Morten Maxild

      Add

      namespace System.Runtime.CompilerServices {
          public class IsExternalInit {}
      }
      

      to your code. Then init-only props and records can compile.

      Only needed if multitargeting (i.e. for TFM other than net5.0)

  • Avatar
    Morten Maxild

    Do positional records support adding xml-doc on the (required, positional, tuple-like) parameters?

    I am converting

    /// ….
    public string SomeProp { get; }

    to

    public record SomeRecord(string SomeProp);