Cometary.Composition

Cometary.Composition

This library brings true composition to the C# world.

Installation

Install-Package Cometary.Composition

Overview

The Compose attribute is at the core of this library, and allows a class to implement multiple components.

Components are classes marked with the Component attribute.

Example

[Component]
public class Named
{
    public string Name { get; set; }
}

[Compose(typeof(Named))]
public class Person
{
    public int Age { get; set; }

    public override string ToString() => $"{this.Name}, aged {this.Age}";
}

Every single member of the Named component will be copied to the Person class.

Dynamic components

Another way to define a component is to create a class that inherits the Component class, and that defines an empty constructor. A component defined this way is known as a "dynamic" component.
Dynamic components must declare an Apply method that allows a dynamically add members to a class.

Example

public class Immutable : Component
{
    public override ClassDeclarationSyntax Apply(ClassDeclarationSyntax node, INamedTypeSymbol symbol, CancellationToken cancellationToken)
    {
        return node.AddMembers(...);
    }
}

[Compose(typeof(Immutable))]
public class ImmutablePerson
{
    public string Name { get; }
    public int Age { get; }

	public ImmutablePerson(string name)
	{
		Name = name;
	}
}

// The immutable person now declares WithName(string name) and WithAge(int age).
ImmutablePerson bob = new ImmutablePerson("Bob");
ImmutablePerson dan = bob.WithName("Dan");

ReferenceEquals(bob, dan); // => false
bob.Name; // => "Bob"
dan.Name; // => "Dan"

Note:
The full content of the Immutable class is available in the Immutable.cs file.