Decorator pattern - adding values dynamically

[Doug]

I am looking at using the decorator pattern to create a rudimentary
stored proc generator but am unsure about something. For each class
that identifies a part of the stored proc, what if I want to add a
value dynamically. I'm including some code to show what I mean. This
is real basic on what I want to do:

using System;

namespace ClassLibrary1
{
public interface Value
{
string GetValue();
}

public abstract class Decorator : Value
{
protected Value _nextValue;

public Decorator(Value theValue)
{
_nextValue = theValue;
}
public virtual string GetValue()
{
return _nextValue.GetV alue();
}
}

public class Header : Value
{
private string _description = "************** *************** ****";

public Header()
{}
public string GetValue()
{
return _description;
}
}

public class Name : Decorator
{
private string _description = "Name:";

public Name(Value component) : base (component)
{
}
public override string GetValue()
{
return _nextValue.GetV alue() + "\r\n" + _description;
}
}

public class Date : Decorator
{
private string _description = "Date:";

public Date(Value component) : base (component)
{
}
public override string GetValue()
{
return _nextValue.GetV alue() + "\r\n" + _description;
}
}

public class Developer : Decorator
{
private string _description = "Developer: ";

public Developer(Value component) : base (component)
{
}
public override string GetValue()
{
return _nextValue.GetV alue() + "\r\n" + _description;
}
}

}

I've tested this and called it like so:

ClassLibrary1.V alue StoredProc = new ClassLibrary1.D eveloper(new
ClassLibrary1.D ate(new ClassLibrary1.N ame(new
ClassLibrary1.H eader())));
MessageBox.Show (StoredProc.Get Value());

My question would be if I wanted to pass an actual name into the Name
class or a date into the Date class, etc. I'm assuming I do that in
the constructor of each class, but wouldn't that make calling each
class messy? Does that perhaps negate the using of the Decorator
pattern here?

[Greg Young]

I think part of your confusion here is that you are not using the decorator
pattern here (this is basic inheritance?). The decorator pattern would state
that you override to add functionality but you still call the subject's
method (unless you are wanting to work with an instance decorator). consider
the following code.

public class Foo {
public virtual void Bar() {
Console.WriteLi ne("Foo.Bar");
}
}

now there is a requirement that it must also print "hello" we create a
decorator (keep in mind we are only dealing with one ..

public class FooHelloDecorat or {
Foo m_Subject;
public FooHelloDecorat or(Foo _Subject) {
m_Subject = _Subject;
}

public Foo Unwrap() {
return m_Subject;
}
public overrides void Bar() {
Console.WriteLi ne("hello");
m_Subject.Bar() ;
}
}

we apply our decorator by saying

Foo f = new Foo();
f = new FooHelloDecorat or(f);
..
..
..
later we can say
Foo OriginalFoo = f.Unwrap(); //takes the decorator off
As for your question .. this is a general problem with inheritance ..
generics can help you to a great degree with this problem as you can make
the base generic then derive the base to a closed type.

Cheers,

Greg
"Doug" <dn******@dtgne t.comwrote in message
news:11******** **************@ i3g2000cwc.goog legroups.com...

>I am looking at using the decorator pattern to create a rudimentary
stored proc generator but am unsure about something. For each class
that identifies a part of the stored proc, what if I want to add a
value dynamically. I'm including some code to show what I mean. This
is real basic on what I want to do:

using System;

namespace ClassLibrary1
{
public interface Value
{
string GetValue();
}

public abstract class Decorator : Value
{
protected Value _nextValue;

public Decorator(Value theValue)
{
_nextValue = theValue;
}
public virtual string GetValue()
{
return _nextValue.GetV alue();
}
}

public class Header : Value
{
private string _description = "************** *************** ****";

public Header()
{}
public string GetValue()
{
return _description;
}
}

public class Name : Decorator
{
private string _description = "Name:";

public Name(Value component) : base (component)
{
}
public override string GetValue()
{
return _nextValue.GetV alue() + "\r\n" + _description;
}
}

public class Date : Decorator
{
private string _description = "Date:";

public Date(Value component) : base (component)
{
}
public override string GetValue()
{
return _nextValue.GetV alue() + "\r\n" + _description;
}
}

public class Developer : Decorator
{
private string _description = "Developer: ";

public Developer(Value component) : base (component)
{
}
public override string GetValue()
{
return _nextValue.GetV alue() + "\r\n" + _description;
}
}

}

I've tested this and called it like so:

ClassLibrary1.V alue StoredProc = new ClassLibrary1.D eveloper(new
ClassLibrary1.D ate(new ClassLibrary1.N ame(new
ClassLibrary1.H eader())));
MessageBox.Show (StoredProc.Get Value());

My question would be if I wanted to pass an actual name into the Name
class or a date into the Date class, etc. I'm assuming I do that in
the constructor of each class, but wouldn't that make calling each
class messy? Does that perhaps negate the using of the Decorator
pattern here?

[Greg Young]

Sorry didn't type everything :)

correction*

(keep in mind we are only dealing with one method but a decorator would
override every method in the object passing through the calls to its
subject, it has to or else the calls would not work)

Cheers,

Greg
"Greg Young" <dr************ *******@hotmail .comwrote in message
news:uT******** ******@TK2MSFTN GP04.phx.gbl...

>I think part of your confusion here is that you are not using the decorator
pattern here (this is basic inheritance?). The decorator pattern would
state that you override to add functionality but you still call the
subject's method (unless you are wanting to work with an instance
decorator). consider the following code.

public class Foo {
public virtual void Bar() {
Console.WriteLi ne("Foo.Bar");
}
}

now there is a requirement that it must also print "hello" we create a
decorator (keep in mind we are only dealing with one ..

public class FooHelloDecorat or {
Foo m_Subject;
public FooHelloDecorat or(Foo _Subject) {
m_Subject = _Subject;
}

public Foo Unwrap() {
return m_Subject;
}
public overrides void Bar() {
Console.WriteLi ne("hello");
m_Subject.Bar() ;
}
}

we apply our decorator by saying

Foo f = new Foo();
f = new FooHelloDecorat or(f);
.
.
.
later we can say
Foo OriginalFoo = f.Unwrap(); //takes the decorator off
As for your question .. this is a general problem with inheritance ..
generics can help you to a great degree with this problem as you can make
the base generic then derive the base to a closed type.

Cheers,

Greg
"Doug" <dn******@dtgne t.comwrote in message
news:11******** **************@ i3g2000cwc.goog legroups.com...

>>I am looking at using the decorator pattern to create a rudimentary
stored proc generator but am unsure about something. For each class
that identifies a part of the stored proc, what if I want to add a
value dynamically. I'm including some code to show what I mean. This
is real basic on what I want to do:

using System;

namespace ClassLibrary1
{
public interface Value
{
string GetValue();
}

public abstract class Decorator : Value
{
protected Value _nextValue;

public Decorator(Value theValue)
{
_nextValue = theValue;
}
public virtual string GetValue()
{
return _nextValue.GetV alue();
}
}

public class Header : Value
{
private string _description = "************** *************** ****";

public Header()
{}
public string GetValue()
{
return _description;
}
}

public class Name : Decorator
{
private string _description = "Name:";

public Name(Value component) : base (component)
{
}
public override string GetValue()
{
return _nextValue.GetV alue() + "\r\n" + _description;
}
}

public class Date : Decorator
{
private string _description = "Date:";

public Date(Value component) : base (component)
{
}
public override string GetValue()
{
return _nextValue.GetV alue() + "\r\n" + _description;
}
}

public class Developer : Decorator
{
private string _description = "Developer: ";

public Developer(Value component) : base (component)
{
}
public override string GetValue()
{
return _nextValue.GetV alue() + "\r\n" + _description;
}
}

}

I've tested this and called it like so:

ClassLibrary1. Value StoredProc = new ClassLibrary1.D eveloper(new
ClassLibrary1. Date(new ClassLibrary1.N ame(new
ClassLibrary1. Header())));
MessageBox.Sho w(StoredProc.Ge tValue());

My question would be if I wanted to pass an actual name into the Name
class or a date into the Date class, etc. I'm assuming I do that in
the constructor of each class, but wouldn't that make calling each
class messy? Does that perhaps negate the using of the Decorator
pattern here?

[Doug]

Hi Greg,

Are you sure I'm not using the decorator pattern in my example? I
mimicked the pattern from the book Foundations of Object-Oriented
Programming Using .NET 2.0 Patterns
by Christian Gross. In Chapter 5 it uses an example of making a
hamburger and describing how the decorator pattern can be applied to
stack the ingredients of the hamburger. I believe I followed that
pattern pretty closely when I wrote my sample code (although I could be
wrong).

[Greg Young]

Well my decorator pattern is how they list it in GoF which is considerred
the major reference.

There are some decorator patterns similar to what you have but they are
called "instance decorators" in that pattern you do not encapsulate the
subject but you still _always_ call the base. The intent of a decorator
pattern is to add functionality, not to change it.

What you are doing there is overriding and changing the behavior of the
original object this is basic inheritance.

I don't have a copy of the reference you mention but will try to find a
copy.

Cheers,

Greg Young
MVP - C#
http://codebetter.com/blogs/gregyoung
"Doug" <dn******@dtgne t.comwrote in message
news:11******** **************@ p79g2000cwp.goo glegroups.com.. .

Hi Greg,

Are you sure I'm not using the decorator pattern in my example? I
mimicked the pattern from the book Foundations of Object-Oriented
Programming Using .NET 2.0 Patterns
by Christian Gross. In Chapter 5 it uses an example of making a
hamburger and describing how the decorator pattern can be applied to
stack the ingredients of the hamburger. I believe I followed that
pattern pretty closely when I wrote my sample code (although I could be
wrong).

[Greg Young]

http://research.umbc.edu/~tarr/dp/le...orator-2pp.pdf

is a nice little presentation on the decorator (in java but code should be
fairly readable and examples still apply).

Cheers,

Greg
"Greg Young" <dr************ *******@hotmail .comwrote in message
news:et******** ******@TK2MSFTN GP04.phx.gbl...

Well my decorator pattern is how they list it in GoF which is considerred
the major reference.

There are some decorator patterns similar to what you have but they are
called "instance decorators" in that pattern you do not encapsulate the
subject but you still _always_ call the base. The intent of a decorator
pattern is to add functionality, not to change it.

What you are doing there is overriding and changing the behavior of the
original object this is basic inheritance.

I don't have a copy of the reference you mention but will try to find a
copy.

Cheers,

Greg Young
MVP - C#
http://codebetter.com/blogs/gregyoung
"Doug" <dn******@dtgne t.comwrote in message
news:11******** **************@ p79g2000cwp.goo glegroups.com.. .

>Hi Greg,

Are you sure I'm not using the decorator pattern in my example? I
mimicked the pattern from the book Foundations of Object-Oriented
Programming Using .NET 2.0 Patterns
by Christian Gross. In Chapter 5 it uses an example of making a
hamburger and describing how the decorator pattern can be applied to
stack the ingredients of the hamburger. I believe I followed that
pattern pretty closely when I wrote my sample code (although I could be
wrong).