assign to variable of struct through property

Question

I have the following

Public Structure Foo
   dim i as integer
End Structure

Public Class Bar

Public Property MyFoo as Foo
Get
   return Foo
End Get
Set(ByVal value as Foo)
   foo  = value
End Set

dim foo as Foo    
End Class

Public Class Other

   Public Sub SomeFunc()    
     dim B as New Bar()    
     B.MyFoo = new Foo()    
     B.MyFoo.i = 14 'Expression is a value and therefore cannot be the target of an assignment ???    
   End Sub
End Class

My question is, why can I not assign to i through my property in the Bar class? What have I done wrong?

Answer 1

The answer is found here, it says the following:

' Assume this code runs inside Form1.
Dim exitButton As New System.Windows.Forms.Button()
exitButton.Text = "Exit this form"
exitButton.Location.X = 140
' The preceding line is an ERROR because of no storage for Location.

The last statement of the preceding example fails because it creates only a temporary allocation for the Point structure returned by the Location property. A structure is a value type, and the temporary structure is not retained after the statement runs. The problem is resolved by declaring and using a variable for Location, which creates a more permanent allocation for the Point structure. The following example shows code that can replace the last statement of the preceding example.

This is because the struct is only a temporary variable. So the solution is to create a new struct of the type you need, assign it all it's internal variables and then assign that struct to the struct property of the class.

Answer 2

You could do

Dim b as New Bar()
Dim newFoo As New Foo()
newFoo.i = 14
b.MyFoo = newFoo

To work around the problem.

Trying the same in C# with the code

class Program
{
    public void Main()
    {
        Bar bar = new Bar();
        bar.foo = new Foo();
        bar.foo.i = 14;
        //You get, Cannot modify the return value of ...bar.foo
        //    because it is not a variable
    }
}
struct Foo
{
    public int i { get; set; }
}

class Bar
{
    public Foo foo { get; set; }
}

Which I think, is a more direct way of saying the same thing as

Expression is a value and therefore cannot be the target of an assignment