Assigning Values to properties in Structures - vb.net

Question

In the below code "f" is an instance of the Class FORM which has a property "s" of type SIZE, a structure which has been defined in the code. My question is: When I try to assign values to the attributes of property "s" of the instance "t" directly it does not work: That is the statement f.s.height = 15 does not work. My confusion is arising from the fact that when I print the values of the property "s" of the instance "f", I am able to print the individual attributes of the structure SIZE but the same cannot be done while assignment of value. Assignment of values require me to call the constructor. Why is it so? What is preventing the assignment of the value to the attributes of "s": i.e. f.s.height & f.s.width?
Module Module1
Sub Main()
Dim f As New MyForm()
f.s = New Size(2, 5) 'Works Fine
f.colour = "Red" 'Assignment works just fine
'Below: Individual elements cannot be acceessed for assignment. WHY?
f.s.height = 15 'Doesn't Work
f.s.height = +2 'Doesn't work
'Individual elements can be accessed while printing
Console.WriteLine("Widht = {0}", f.s.width)
Console.WriteLine("Height = {0}", f.s.height)
Console.ReadLine()
End Sub
End Module
Class MyForm
Public Property s As Size
Public Property colour As String
End Class
Public Structure Size
Dim height As Integer
Dim width As Integer
Public Sub New(ByVal w As Integer, ByVal h As Integer)
width = w
height = h
End Sub
End Structure
Pls help.

Answer 1

The compiler should be indicating "Expression is a value and therefore cannot be the target of an assignment".
Change Size from a Structure to a Class (and Dim to Property) to fix the problem:
Public Class Size
Property height As Integer
Property width As Integer
Public Sub New(w As Integer, h As Integer)
width = w
height = h
End Sub
End Class
By the way, you'll also see this behaviour with the standard System.Drawing.Size which is defined as a Structure rather than a Class. (So is Point and probably others.)

Answer 2

This behavior is fundamental to value types (Structures). Conceptually, an instance of a value type is supposed to represent a single immutable value, and any instances with the same value are all supposed to be equivalent. As you have observed, you can get very surprising behavior if you try to change parts of an existing value type. It's really not intended for you to be able to alter them piecewise.
For this reason, I will always recommend that the members of a value type should be marked as ReadOnly so that you get blocked from trying to change them after construction.
If you want to be able to treat something like a mutable object instance instead of an immutable value, it needs to be a reference type (a Class). That's what they're designed to do.

Answer 3

After a lot of searching I came across the following article which probably explains the reason why we are not able to directly access the height/width attribute of the SIZE structure through an instance of the FORM class. Requesting people to go through this as the author has given a lot of details:
http://www.albahari.com/valuevsreftypes.aspx
PLs feel free to share any difference in opinion.