Accessing value of a Variable in parent from custom control

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Guest

Is it possible to access value of a variable in Parent Form within control at
runtime?
Please provide an example (possibly in VB).
 
It isnt very object oriented as the control isnt part of the form and could
be used on a form without that variable. Unless the controls code definition
was inside the forms, in which case it would be a able to access privates
after casting the Parent property to the type of the parent form.

Ciaran O'Donnell
 
You can create a property in the control and then, when the parent form
initializes the control or changes the value of the variable, set the value
of the property in the control.

Though, like Ciaran says, it is not very object oriented, I have had cases
where I had to pass the parent form as a property to a control. To get
around the issue he identifies of the control being used in a different form,
then you create an interface for the data that the control requires from its
parent and implement that interface in all parents that use that control. In
that way, by casting to the interface type, you can make any form that uses
your control into something you can use inside your control.

HTH

Dale
 
First I would like to thank both of you for replying to my question. As I do
not have any experience with interfaces, therefore if you can provide an
example I will be greatfull.

Zahid.
 
Unfortunately, I am not a VB.Net developer but you should hopefully be able
to figure this out from the C#.

Let's say that your control is MyControl and your form is MyForm. The
instance, in my example, of MyControl is named myControl. Assume your form
needs a string variable we'll call formString and an int variable we'll call
formInt that are defined in the parent form. Create an interface called, for
instance, iUsesMyControl.

In your form class declaration, change
public class MyForm : Form

to
public class MyForm : Form, iUsesMyControl

I think the VB would look like
Public Class MyForm : Extends iUsesMyControl

Your interface, iUsesMyControl, would look like:
internal interface iUsesMyControl
{
int FormInt { get; set; }
string FormString { get; set; }
}

Basically, we're defining two properties in the interface that all classes
implementing iUsesMyControl must implement. Check your VB documentation for
how to define properties in an interface in VB.

Your implementation of iUsesMyControl in your form would look like

private int formInt; // though this may have been previously defined
elsewhere.
internal int FormInt
{
get { return formInt; }
set { formInt = value; }
}

private string formString;
internal string FormString
{
get { return formString; }
st { formString = value; }
}

Check your VB documentation for how to define the variables and expose them
as properties in VB to accomplish the above C# code in VB.

In your control, add a property:

private iUsesMyControl parentForm;
internal iUsesMyControl ParentForm
{
set { parentForm = value; }
}

Somewhere early in your parent form, perhaps the load event, add
myControl.ParentForm = (iUsesMyControl)this;

This passes the MyForm to MyControl but by casting to iUsesMyControl it
tells MyControl only about the two variables defined in iUsesMyControl. I
think this, in VB would be:
myControl.ParentForm = CType(Me, iUsesMyControl)

Now, in your control, you have available:
parentForm.FormInt
and
parentForm.FormString
 
I think this would be the VB code for that:

In your form, add an Implements for the interface, and add the properties.
You will want to have private variables to keep the current value,
you are exposing them through a property.

public class MyFrom
Implements iUsesMyControl

Private _FormInt as Integer
Public Property FormInt() As Integer Implements
IUsesMyControl.FormInt
Get
return _FormInt
End Get
Set(ByVal value As String)
_FormInt = value
End Set
End Function
Private _FormString as String
Public Property FormString() As String Implements
IUsesMyControl.FormString
Get
return _FormString
End Get
Set(ByVal value As String)
_FormString = value
End Set
End Function
....(other form code)
End Class

Public Interface IUsesMyControl
Property FormInt as Integer
Property FormString as String
End Interface

In your control:
'**I'm not sure about this; Either I have it wrong,
'** or there's some way to define something as
'** an interface. If this doesn't work, try
'** "implements IUsesMyControl" instead of "as IUsesMyControl".
Private _parentForm as IUsesMyControl
Public Readonly Property ParentForm as IUsesMyControl
Set
_parentForm = value
End Set
End Property

Somewhere early in your parent form
myControl.ParentForm = DirectCast(me, IUsesMyControl)

I think that's right; feel free to correct me if I got any of it wrong.
Robin S.
-------------------------
 
My parent Form looks like this:
=======================
Public Class Form1
Implements iUsesMyControl
Private _formInt As Integer
Private _formString As String
Public var1 As Integer
Private mycontrol As ContainerControl

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles MyBase.Load

_formInt = 123
_formString = "Zahid"
mycontrol = DirectCast(Me, iUsesMyControl)

End Sub

Public Property FormInt() As Integer Implements iUsesMyControl.FormInt
Get
Return _formInt
End Get
Set(ByVal value As Integer)
_formInt = value
End Set
End Property

Public Property FormString() As String Implements
iUsesMyControl.FormString
Get
Return _formString
End Get
Set(ByVal value As String)
_formString = value
End Set
End Property
End Class
============

I do not understand the implements of Control itself. Do we need to
implement another interface 'iUsesMyControl' as we did in the parent from
(Form1)?
 
Well, I was just trying to translate from C# to VB for you, but I've gone
back and reread the whole thread. If all you want is to be able to
get some kind of information from the parent form in which the control
resides, i'd think you could just add a public property to the parent form.

If Form1 implements an interface, it means that you must provide the
code for the properties defined in the interface. I think the line
myControl = DirectCast(Me, iUsesMyControl)
Click to expand...
allows you to access the methods and properties in the interface
from the control. I think what this allows you to do is set the
properties in the form that implement the interface to private.
Then casting myControl to that interface allows it to see the
private properties. So only the control can see the properties,
not the whole project. So in the control, maybe you could access
myControl.FormInt and myControl.FormString and see the
properties from the form.

Can anybody verify that I'm understanding that right?

However, I don't really see the need for something this complicated
if all you are trying to do is provide the ability for the control
to see something in the form. I would think you could just add a
property to the form, and let the control access it. The code
you have for the properties (which are public) should work okay.
Then in your control, I would think you could access the
properties as Form1.FormInt and Form1.FormString.
I guess it depends on whether it's okay to make those
properties public to the rest of the project or not.

Hope that helps. If anybody can shed some wisdom here,
that would be great.

Thanks,
Robin S.
 
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