[Proposal]: Factory methods

Factory methods

Specification: proposals/factory-methods.md Discussion: https://github.com/dotnet/csharplang/discussions/6602

Summary

This proposal will extend the scenarios in which object initializers can be used to include type factory methods in addition to constructors.

Motivation

Records need to support initializer style expressions in scenarios other than standard object construction to support with expressions. This though is simply adding a special property to a specific named method in a type. Records though are not alone in wanting to denote methods as being used exclusively for construction. Factory methods serve the same purpose in .NET and are common throughout our framework.

The ability to mark methods as being used exclusively for creation should become a standalone feature. Developers will be able to mark members which exist soley to create objects with [Factory] and the language will give such methods the same flexibility as object constructors. That means those methods can now be used in combination with object and collection initializers.

public class Widget
{
    public string Name { get; set; }
    public int Id { get; init; }

    internal Widget() { }
}

public static class WidgetFactory
{
    [Factory]
    public static Widget Create() => new Widget();
}

var w = WidgetFactory.Create()
{
    Name = "JaredPar",
    Id = 42;
}

Design meetings

• https://github.com/dotnet/csharplang/blob/main/meetings/2020/LDM-2020-04-20.md