[Proposal] Object initializers for factory methods

Problem

This is another one of the issues when a change to logic significantly changes code structure. Let’s say I had the following:

var x = new X {
    A = a,
    B = b,
    C = { D = { d1, d2 } }
};

Now I decided all X should be created through an XFactory instead. So I get:

var x = factory.CreateX();
x.A = a;
x.B = b;
x.C.D.Add(d1);
x.C.D.Add(d2);

One small logical change causes a total restructure of not-really-related code.

Potential solution

Provide a way to use object initializer with any existing object. Example:

var x = factory.CreateX() with {
    A = a,
    B = b,
    C = { D = { d1, d2 } }
};

Keyword with here is provided to remove any syntax ambiguities.

Pros

1. More consistent syntax everywhere. Shorter syntax for setting a few properties when you need that.

Cons

1. New kind of expression-with-side-effects when used on an existing object.

[MadsTorgersen]

It’s desirable to allow the same initializers on factories as constructor calls. We need to think this through in conjunction with with expressions, and with object initializers for immutable objects.

[gafter]

Issue moved to dotnet/csharplang #803 via ZenHub