Modest Proposal for C#

This is a trivial change to implement, and would turn what today is an error into useful behavior.

Consider the following C# program:

struct Rect {
	public int X, Y, Width, Height;
}

class Window {
	Rect bounds;

	public Rect Bounds {
		get { return bounds; }
		set {
			// Some code that needs to run when the	property is set
			WindowManager.Invalidate (bounds);
			WindowManager.Invalidate (value);
			bounds = value;
		}
	}
}

Currently, code like this:

Window w = new Window ();
w.Bounds.X = 10;

Produces the error:

Cannot modify the return value of "Window.Bounds.X" because it is not a variable

The reason is that the compiler returns a copy of the "bounds" structure and making changes to the returned value has no effect on the original property.

If we had used a public field for Bounds, instead of a property, the above code would compile, as the compiler knows how to get to the "Bounds.X" field and set its value.

My suggestion is to alter the C# compiler to turn what today is considered an error when accessing properties and doing what the developer expects.

The compiler would rewrite the above code into:

Window w = new Window ();
var tmp = w.Bounds;
tmp.X = 10;
w.Bounds = tmp;

Additionally, it should cluster all of the changes done in a single call, so:

Window w = new Window ();
w.Bounds.X = 10;
w.Bounds.Y = 20;

Will be compiled as:

Window w = new Window ();
var tmp = w.Bounds;
tmp.X = 10;
tmp.Y = 20;
w.Bounds = tmp;

To avoid calling the setter for each property set in the underlying structure.

The change is trivial and wont break any existing code.