General form of C# Object Initializers

C# has a language feature that allows several properties of an object to be assigned to as a suffix to a new expression, namely the object initializer:

var myObj = new MyClass
            {
                SomeProperty = 5,
                Another = true,
                Complain = str => MessageBox.Show(str),
            };

As properties can have hand-coded setters, this is an opportunity to call several methods on the newly constructed object, without having to make each method return the same object.

The limitations on property setters are:

• They can only accept one argument
• They cannot be generic

I would like it if we could call methods and enlist in events, as well as assign to properties, inside an object initializer block.

var myObj = new MyClass
            {
                SomeProperty = 5,
                Another = true,
                Complain = str => MessageBox.Show(str),
                DoSomething(),
                Click += (se, ev) => MessageBox.Show("Clicked!"),
            };

And why should such a block of modifications only be applicable immediately after construction? We could have:

myObj with
{
    SomeProperty = 5,
    Another = true,
    Complain = str => MessageBox.Show(str),
    DoSomething(),
    Click += (se, ev) => MessageBox.Show("Clicked!"),
}

The with would be a new keyword that operates on an object of some type and produces the same object and type - note that this would be an expression, not a statement.

So you could use initializer-style syntax regardless of whether you'd got the object from a new expression or from an IOC or factory method, etc.

In fact you could use with after a complete new and it would be equivalent to the current style of object initializer:

var myObj = new MyClass() with
            {
                SomeProperty = 5,
                Another = true,
                Complain = str => MessageBox.Show(str),
                DoSomething(),
                Click += (se, ev) => MessageBox.Show("Clicked!")
            };

I mused about this in a Stack Overflow answer, and Charlie Flowers pointed out something I should have realised immediately - we can implement with as an extension method.

public static T With(this T with, Action<T> action)
{
    if (with != null)
        action(with);
    return with;
}

Equivalent of normal object initializer, but with event enlisting:

var myObj = new MyClass().With(w =>
            {
                w.SomeProperty = 5;
                w.Another = true;
                w.Click += (se, ev) => MessageBox.Show("Clicked!");
            };

And on a factory method instead of a new:

var myObj = Factory.Alloc().With(w =>
            {
                w.SomeProperty = 5;
                w.Another = true;
                w.Click += (se, ev) => MessageBox.Show("Clicked!");
            };

I couldn't resist giving it the "maybe monad"-style check for null as well, so if you have something that might return null, you can still apply With to it and then check it for null-ness.

Readonly Fields and Thread Safety

In C# you can mark the fields of a type as readonly (indeed you generally should if it's a struct). By doing so, you make it illegal to change the fields except in a constructor of the type.

The advantage of this is that readonly data can be shared freely between threads without any thread causing updates that may be seen "in progress" by other threads, simply because there never will be any updates.

But is this strictly speaking true? Nope. It would be true if it were impossible for another thread to gain access to a partially constructed object. This seems true at first because the constructor of an object effectively "returns" a reference to itself via the new operator, which then becomes available to the caller of new, at which point the object is available to be passed around and is also fully constructed.

But a constructor can call methods, and it can pass this to methods. Those methods could pass that information to other threads. So you have to be careful what you do in a constructor. Maybe the compiler will help to check this somehow in a future version of the language.

This ties in nicely with some general advice about constructors that Brad Abrams gives in the Annotated C# Programming Language: do as little as possible in the constructor, and do everything else "lazily" - that is, the first time you need it. If you find yourself requiring non-readonly fields to achieve that, then you potentially have a problem with your design, because your class as a whole is not going to be readonly and so will not be trivially shareable in the way you originally assumed.

JavaScript Invaders!

Click in the small text field in the top left corner to give allow the game to receive keyboard events.

• Z = left
• X = right
• M = fire

Compatible with IE, Firefox and Safari, pure JavaScript and HTML

I wrote this during a train journey, just to see if it was practically possible. Only then did I google for other examples, and found quite a few going back years. This version of Pacman has excellent attention to detail.