Richiban

My proposed syntax for record types in C#

Posted on 2014-12-08 Updated on 2016-04-06

With the impending release of C# 6.0 and all its new features, I have been thinking about what might come in the next version of C# (v7.0, I’m guessing). There has been talk in the Roslyn forums about things such as pattern matching and record types (the latter being especially important since the primary constructors feature was dropped from C# 6.0).

Below is a quick sketch of how I think record classes should work in C#. You can probably tell that I spend a lot of time programming functionally, because I’ve included a number of syntax features here to make immutability a very easy option.

Note: I’m not sure that the “class” keyword is even needed any more. Contrast: “public record class Person” vs “public record Person”

public record class Person
{
	string Name { get; set; }
	DateTime DoB { get; }

	bool IsRobot { get; set; } = false;
}
//	Note the lack of visibility modifiers. Like an interface, everything is public in a record class

var john = new Person
{
	Name = "John"	//	Compiler error: A value for "DoB" has not been supplied
}

//	This is legal. IsRobot is "optional" because it has a default value in the class definition
var joe = new Person
{
	Name = "Joe",
	DoB = DateTime.Yesterday
}

joe.Name = "Suzy"	//	Legal: Name is "mutable" because it has a public setter in the class definition.
joe.DoB = DateTime.Now	//	Compiler error: "DoB" has no public setter. DoB is "immutable"


public record class Point
{
	double X { get; }
	double Y { get; }
}

var origin = new Point { X = 0.0, Y = 0.0 }
var location = new Point { X = 0.0, Y = 0.0 }

origin == location // => true. Records have auto-generated structural equality and GetHashcode methods

origin.ToString() // => "Point { X = 0.0, Y = 0.0 }". Records have an auto-generated ToString() method (can be overridden).


//	Custom members can be defined, both properties and methods. Note that only auto-implemented properties are required
//  when instantiating the record.
public record class Point3D
{
	double X { get; }
	double Y { get; }
	double Z { get; }

	override String ToString()
	{
		return $"({X}, {Y}, {Z})";
	}

	double Dist => Sqrt(X * X + Y * Y + Z * Z);
}