Difference between modifying a variable and a property

Question

There is probably a quite logical explanation to this, but I have a question.

Let's say I have a variable of type Rectangle called _rect. I can now say _rect.X = 50; without any problems.

Now I have a class with a property called Rect that exposes the internal variable _rect.

Then, if I try to write Rect.X = 50; I get the following compilation error:

Cannot modify the return value of 'TestClass.Rect' because it is not a variable.

I can write Rect = new Rectangle( 50, Rect.Y, Rect.Width, Rect.Height) like for a immutable type, but for non-immutable types, are there any other way of doing this?

I want to use auto-properties for this rectangle field, but it's really annoying not being able to modify it inside the class itself.

Are there any way short of making a backing field and dropping the auto-property?

Answer 1

The reason for this error is because Rectangle is a value type (struct) in contrast to reference types (classes). You cannot modify the X property because when you use the Rect property getter a new value for the rectangle is returned (the getter is a function). If it was a reference type you are manipulating the pointer and this would be possible.

This is an important aspect of value vs reference types to be aware of.

Answer 2

Accessing the property is really a function call which returns a copy of the value type. Rect.X = 50; would only modify this temporary copy, not the backing field itself.

When the property is not auto-implemented, you could create an additional property RectX, which can be used to get and set the X property of the rectangle.