[2005] Really Basic Question [Location Property]

[Fromethius]

I feel like such a noob asking this

Suppose I am in an event handler of a form and type this:

Me.Location.X = 5;

I get the error:

Cannot modify the return value of 'System.Windows.Forms.Form.Location' because it is not a variable

For a while now, whenever I wanted to change the Location property of an object, I would just instantiate a new Point. However, if I am changing the location 1,000 times a second, such as in a game, wouldn't that be considered bad practice?

Surely there is a way to just change the X value to something else without instantiating a new Point o-O

[Atheist]

You could use the Top/Left properties

EDIT: Or keep a point object declared at class level. Modify its value and assign it to the Location property every time you need to move the form.

Public Class Form1
    Private formLocation As Point
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        formLocation = Me.Location
    End Sub
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        formLocation.X += 1
        formLocation.Y += 1
        Me.Location = formLocation
    End Sub
End Class

[Fromethius]

Yes, exactly! I help out the VB students in during my lunch period at school, and I tell them to use the method with the class level point and assigning the point to the location. They basically always ask, "why can't we just assign to the point value directly?"

That's kind of weird of VB to do that..

[jmcilhinney]

No it's not weird at all. Here's why. The Location property is type Point. Point is a structure, so it's a value type. When you get the Location property you are getting a copy of the Point value stored in the control. If you set the X property of that copy that will have no effect on the original Point value and therefore no effect on the control's location. That's why the Top and Left properties exist: so you can set them and then internally they will create a new value and assign it to the relevant variable internally.

Now, if Point was a class this would not be the case. The Location property would return a reference to the same Point object so setting its X property directly would be fine. The moral of the story is this: if a property returns a value, rather than a reference, then setting a property of that value will not affect the original. The copy is immediately discarded so any change you make to it is useless so the compiler doesn't even let you try.

If you want to set one component of the Location only you should be using the Top and Left properties. If you want to set both you should be creating a Point and setting the Location. If you want to set the Location and Size you should using the Bounds property or the SetBounds method.

[Fromethius]

Thanks

What of the Vector3 struct then? It doesn't have those Bounds methods and properties. Should I just make a new point each time? I suppose.

Yet again, thanks. I'd rep you but I need to rep someone else first.. as usual..

[jmcilhinney]

It's just a general rule that you follow in all cases. If you have objectA and you want to set objectA.PropertyA.PropertyB then the type of objectA and PropertyB are irrelevant. The only thing that matters is whether PropertyA is a value type or a reference type. If it's a value type then setting PropertyB directly is not allowed. You'd have to get PropertyA's value into a local variable, set PropertyB on that variable, then assign the variable back to PropertyA.

[jmcilhinney]

To illustrate that last post with code, Let's say that SomeType.SomeProperty is a reference type. In that case this is legal:

Dom obj As New SomeType
 
obj.SomeProperty.SomeOtherProperty = someValue

If SomeProperty is value type though, that is not allowed and the compiler will tell you so. In that case you'd need to do this:

Dom obj As New SomeType
 
Dim propVal As SomeValueType = obj.SomeProperty
 
propVal.SomeOtherProperty = someValue
obj.SomeProperty = propVal

[bigMeUp]

(In a complete aside) where are you boys getting that VB.Net code tag? I can only see this tag

Code

.

Go on, give it up!

[jmcilhinney]

The VBCode button adds a [HIGHLIGHT=vb] tag. If you change that to [HIGHLIGHT=vb.net] you'll get that effect. You can also use =vb, =C#, =CSharp, =php and various other languages. I'm not sure of the full list.

[bigMeUp]

Dim s As String = rtbLetter.Rtf
Dim i As Int32 = s.IndexOf("\", 3)
s = s.Substring(i)

Humph! It worked a treat. Thanks!