Inheritance/Casting question in vb.net

[Sam Kuehn]

What is the correct way to "widen" the interface of an
object? Let me explain (this is not a real example but
should illustrate my point). Say you have a person
object with some properties. And you also have a
customer object that inherits from the person object and
adds some properties. I know that you cannot directly
cast from a person to a customer. But, I am in a
situation where I need to do exactly that. I could go
and copy all of the properties of the person object into
the customer object but that doesn't seem like a very
good solution. Any ideas?

[Herfried K. Wagner [MVP]]

"Sam Kuehn" <sa******@hotma il.com> schrieb:

What is the correct way to "widen" the interface of an
object? Let me explain (this is not a real example but
should illustrate my point). Say you have a person
object with some properties. And you also have a
customer object that inherits from the person object and
adds some properties. I know that you cannot directly
cast from a person to a customer. But, I am in a
situation where I need to do exactly that.

But how would you fill the properties customers have but persons don't?

--
Herfried K. Wagner [MVP]
<URL:http://dotnet.mvps.org/>

[A Traveler]

One way you could do it would be to put a Shared method on the Customer
class which takes a Person object as its argument and returns back a
Customer object for you. You would still be copying over properties, but it
would be encapsulated at least, and you could treat it in a more OO way.
So something like:
Class Customer
Public Shared Function Widen(Who As Person) As Customer
Dim c As New Customer

... copy over props here ...
c.myprop = Who.myprop
... end copying props ...

Return c
End Function
End Class

Then you could call it easily enough,

Dim cust As Customer = Customer.FromPe rson(myPerson)

Hope this may help.
"Sam Kuehn" <sa******@hotma il.com> wrote in message
news:09******** *************** *****@phx.gbl.. .

What is the correct way to "widen" the interface of an
object? Let me explain (this is not a real example but
should illustrate my point). Say you have a person
object with some properties. And you also have a
customer object that inherits from the person object and
adds some properties. I know that you cannot directly
cast from a person to a customer. But, I am in a
situation where I need to do exactly that. I could go
and copy all of the properties of the person object into
the customer object but that doesn't seem like a very
good solution. Any ideas?

-----Original Message-----
"Sam Kuehn" <sa******@hotma il.com> schrieb:

What is the correct way to "widen" the interface of an
object? Let me explain (this is not a real example but
should illustrate my point). Say you have a person
object with some properties. And you also have a
customer object that inherits from the person object and adds some properties. I know that you cannot directly
cast from a person to a customer. But, I am in a
situation where I need to do exactly that.
But how would you fill the properties customers have but

persons don't?
--
Herfried K. Wagner [MVP]
<URL:http://dotnet.mvps.org/>
.

Obviously, you cannot fill in the properties of a
customer that a person doesn't have. Most of those
properties are going to be calculated properties. Let me
explain my exact situation and it may make more sense. I
have a stored procedure that returns some numeric
statistics on a book of business (insurance). I have a
class representing these values. Since I have a common
business object that hydrates the BookStats object I can
only have the same number of properties as I have columns
returned from the stored proc. So basically I have a
stored proc that fills in the properties of the
BookStats. Now what I need to do is extend this object.
I need to provide some other stats that should not be
returned by the stored proc they are calculated values
that I would like to keep in my BLL. So I was hoping
that I could have an ExtendedBookSta ts class that would
take a BookStats class and its current values and give me
the extra properties and presumably methods that I need.
Hope this makes sense.

[Sam Kuehn]

-----Original Message-----
One way you could do it would be to put a Shared method on the Customerclass which takes a Person object as its argument and returns back aCustomer object for you. You would still be copying over properties, but itwould be encapsulated at least, and you could treat it in a more OO way.So something like:
Class Customer
Public Shared Function Widen(Who As Person) As Customer Dim c As New Customer

... copy over props here ...
c.myprop = Who.myprop
... end copying props ...

Return c
End Function
End Class

Then you could call it easily enough,

Dim cust As Customer = Customer.FromPe rson(myPerson)

Hope this may help.
"Sam Kuehn" <sa******@hotma il.com> wrote in message
news:09******* *************** ******@phx.gbl. ..

What is the correct way to "widen" the interface of an
object? Let me explain (this is not a real example but
should illustrate my point). Say you have a person
object with some properties. And you also have a
customer object that inherits from the person object and adds some properties. I know that you cannot directly
cast from a person to a customer. But, I am in a
situation where I need to do exactly that. I could go
and copy all of the properties of the person object into the customer object but that doesn't seem like a very
good solution. Any ideas?

.

Looks like an acceptable solution. Thank you.

-----Original Message-----
"Sam Kuehn" <sa******@hotma il.com> schrieb:

What is the correct way to "widen" the interface of an
object? Let me explain (this is not a real example but
should illustrate my point). Say you have a person
object with some properties. And you also have a
customer object that inherits from the person object and adds some properties. I know that you cannot directly
cast from a person to a customer. But, I am in a
situation where I need to do exactly that.
But how would you fill the properties customers have but

persons don't?
--
Herfried K. Wagner [MVP]
<URL:http://dotnet.mvps.org/>
.

Obviously, you cannot fill in the properties of a
customer that a person doesn't have. Most of those
properties are going to be calculated properties. Let me
explain my exact situation and it may make more sense. I
have a stored procedure that returns some numeric
statistics on a book of business (insurance). I have a
class representing these values. Since I have a common
business object that hydrates the BookStats object I can
only have the same number of properties as I have columns
returned from the stored proc. So basically I have a
stored proc that fills in the properties of the
BookStats. Now what I need to do is extend this object.
I need to provide some other stats that should not be
returned by the stored proc they are calculated values
that I would like to keep in my BLL. So I was hoping
that I could have an ExtendedBookSta ts class that would
take a BookStats class and its current values and give me
the extra properties and presumably methods that I need.
Hope this makes sense.

[Sam Kuehn]

-----Original Message-----
One way you could do it would be to put a Shared method on the Customerclass which takes a Person object as its argument and returns back aCustomer object for you. You would still be copying over properties, but itwould be encapsulated at least, and you could treat it in a more OO way.So something like:
Class Customer
Public Shared Function Widen(Who As Person) As Customer Dim c As New Customer

... copy over props here ...
c.myprop = Who.myprop
... end copying props ...

Return c
End Function
End Class

Then you could call it easily enough,

Dim cust As Customer = Customer.FromPe rson(myPerson)

Hope this may help.
"Sam Kuehn" <sa******@hotma il.com> wrote in message
news:09******* *************** ******@phx.gbl. ..

What is the correct way to "widen" the interface of an
object? Let me explain (this is not a real example but
should illustrate my point). Say you have a person
object with some properties. And you also have a
customer object that inherits from the person object and adds some properties. I know that you cannot directly
cast from a person to a customer. But, I am in a
situation where I need to do exactly that. I could go
and copy all of the properties of the person object into the customer object but that doesn't seem like a very
good solution. Any ideas?

.

Looks like an acceptable solution. Thank you.

[Jay B. Harlow [MVP - Outlook]]

Sam,
In addition to the other comments.

I would consider making the Customer object contain a reference to the
Person object, then delegate all the "Person" properties to the underlying
Person object.

Alternatively rather then having Customer inherit from Person, I would
consider using the Role Pattern and make Customer a Role that Person can
have. This allows Person to change its Role (to Customer) without needed to
copy the Person attributes.

' quick sample of the Role Pattern:

Public MustInherit Class PersonRole

Private Class NullRole
Inherits PersonRole

' default behavior & attributes for a Person without a specific
role.

End Class

Public Shared ReadOnly Null As PersonRole = New NullRole

' behavior & attributes common to roles

End Class

Public Class Person

Private m_role As PersonRole

Public Sub New()
m_role = PersonRole.Null
End Sub

Public Property Role() As PersonRole
Get
Return m_role
End Get
Set(ByVal value As PersonRole)
If value Is Nothing Then Throw New
ArgumentNullExc eption("Role")
m_role = value
End Set
End Property

' behavior & attributes specific to a Person

End Class

Public Class CustomerRole
Inherits PersonRole

' behavior & attributes specific to a Customer

End Class

Person.Role could be a collection if a Person supported multiple
simultaneous roles.

Hope this helps
Jay

"Sam Kuehn" <sa******@hotma il.com> wrote in message
news:09******** *************** *****@phx.gbl.. .

What is the correct way to "widen" the interface of an
object? Let me explain (this is not a real example but
should illustrate my point). Say you have a person
object with some properties. And you also have a
customer object that inherits from the person object and
adds some properties. I know that you cannot directly
cast from a person to a customer. But, I am in a
situation where I need to do exactly that. I could go
and copy all of the properties of the person object into
the customer object but that doesn't seem like a very
good solution. Any ideas?

[Jay B. Harlow [MVP - Outlook]]

Sam,
In addition to the other comments.

I would consider making the Customer object contain a reference to the
Person object, then delegate all the "Person" properties to the underlying
Person object.

Alternatively rather then having Customer inherit from Person, I would
consider using the Role Pattern and make Customer a Role that Person can
have. This allows Person to change its Role (to Customer) without needed to
copy the Person attributes.

' quick sample of the Role Pattern:

Public MustInherit Class PersonRole

Private Class NullRole
Inherits PersonRole

' default behavior & attributes for a Person without a specific
role.

End Class

Public Shared ReadOnly Null As PersonRole = New NullRole

' behavior & attributes common to roles

End Class

Public Class Person

Private m_role As PersonRole

Public Sub New()
m_role = PersonRole.Null
End Sub

Public Property Role() As PersonRole
Get
Return m_role
End Get
Set(ByVal value As PersonRole)
If value Is Nothing Then Throw New
ArgumentNullExc eption("Role")
m_role = value
End Set
End Property

' behavior & attributes specific to a Person

End Class

Public Class CustomerRole
Inherits PersonRole

' behavior & attributes specific to a Customer

End Class

Person.Role could be a collection if a Person supported multiple
simultaneous roles.

Hope this helps
Jay

"Sam Kuehn" <sa******@hotma il.com> wrote in message
news:09******** *************** *****@phx.gbl.. .

What is the correct way to "widen" the interface of an
object? Let me explain (this is not a real example but
should illustrate my point). Say you have a person
object with some properties. And you also have a
customer object that inherits from the person object and
adds some properties. I know that you cannot directly
cast from a person to a customer. But, I am in a
situation where I need to do exactly that. I could go
and copy all of the properties of the person object into
the customer object but that doesn't seem like a very
good solution. Any ideas?