Published: Wednesday, January 23, 2002
XML Serialization in ASP.NET
By Anthony Hart
Introduction
In the past, maintaining the state of an object in ASP often required some very inventive and
painstaking code. In the brave new world of .NET, however, Object Serialization offers us a
comparatively easy way to do just that, as well as some other useful tasks.
As a kid, I remember waking up on many a cold morning and stumbling into the kitchen with my eyes
half-closed, looking forward to whatever Mom had prepared for breakfast, only to find an anticlimactic
bowl of steaming hot just-add-boiling-water instant oatmeal waiting for me on the table. At least
I wasn't like the more unfortunate kids whose mothers force-fed them that white silt of death,
powdered milk. I am absolutely certain that something must go seriously awry in the dehydration
process of milk because upon rehydration, that stuff is just plain nasty.
Be that as it may, I think that at least one of the developers involved in creating the .NET
Framework must have been one of those abused children. I see powdered milk fingerprints all over
some of the new data management techniques in .NET. Then again, in an age of dehydrated/rehydrated
food products, what could be more logical than dehydrated/rehydrated data?
Object Serialization
The technique to which I refer is called "Object Serialization". Object Serialization is a process
through which an object's state is transformed into some serial data format, such as XML or a binary
format, in order to be stored for some later use. In other words, the object is "dehydrated" and put
away until we need to use it again. Let's look at an example to clarify this idea a little further.
Suppose we have an object defined and instantiated as shown below:
Public Class Person
private m_sName as string
private m_iAge as integer
public property Name() as string
get
return m_sName
end get
set(byval sNewName as string)
m_sName = sNewName
end set
end property
public property Age() as integer
get
return m_iAge
end get
set(byval iNewAge as integer)
m_iAge = iNewAge
end set
end property
End Class
dim oPerson as New Person()
oPerson.Name = "Powdered Toast Man"
oPerson.Age = "38"
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Let's say that for some reason, we wanted to save a copy of this object just as it is at this very
moment. We could serialize it as an XML document that would look something like this:
<Person>
<Name>Powdered Toast Man</Name>
<Age>38</Age>
</Person>
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Then, at some later time when we needed to use the object again, we could just deserialize
("rehydrate") it and have our object restored to us just as it was at the moment we serialized it.
Why All This Dried Food?
"This serialization and deserialization is all well and good," you may be saying at this point, "but
what can it be used for in real world web applications?" Very good question. What would be the sense
of dehydrating a glass of milk just to rehydrate it and then dehydrate it again without drinking any?
Never mind that it'll probably taste like the inside of your little brother's sock drawer (maybe
I should have come up with a more tasty dehydrated food for this analogy). Some good uses for
serialization/deserialization include:
- Storing user preferences in an object.
- Maintaining security information across pages and applications.
- Modification of XML documents without using the DOM.
- Passing an object from one application to another.
- Passing an object from one domain to another.
- Passing an object through a firewall as an XML string.
These are only a few of the many possibilities that serialization opens up for us.
The Dehydration Process
"Okay, okay, I get the picture," you may be muttering now, "but how do I do it?" I'm going to walk you
through a simple example of serializing an object to be saved to disk as an XML file. Keep in mind
that in .NET we can serialize objects into a binary format or SOAP format as well as into XML, but
we will focus solely on XML for this article for the sake of brevity. Also, the object in the example
is obviously very simplistic and wouldn't be practical in the real world, but it will serve as a means
to clearly illustrate how to serialize an object. The same principles used in this example can then
be applied to more complicated tasks. Note that all the code used for this example is available for download
at the end of this article; also a live demo
is available).
First of all, let's take a look at the class that is the blueprint for our object (this snippet can
be found in the file, xmlser.aspx).
<XMLRoot(ElementName:="Class_Person")> _
public class Person
private m_sName as string
private m_iAge as integer
<XMLElement(ElementName:="Property_Name")> _
public property Name() as string
get
return m_sName
end get
set(byval sNewName as string)
m_sName = sNewName
end set
end property
<XMLElement(ElementName:="Property_Age")> _
public property Age() as integer
get
return m_iAge
end get
set(byval iNewAge as integer)
m_iAge = iNewAge
end set
end property
public function Hello() as string
dim s as string
s = "Hi! My name is " & Name & " and I am " & Age & " years old."
return s
end function
public function Goodbye() as string
return "So long!"
end function
end class
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This looks pretty similar to the class we saw earlier but with a few adjustments. First of all,
notice the lines that say, <XMLRoot... and <XMLElement.... These are
.NET attributes and tell the serializer where the various members of this object will appear in the
XML document and what they will be named. Without these, serialization cannot take place. The next
difference you will notice is that there are two methods declared in the class called, Hello()
and Goodbye(). These really have no bearing on the serialization that will take place,
but we will use them later to demonstrate the state of the object at a given point in the
serialization process.
Next let's look at the code that actually instantiates the object and then serializes it.
dim oXS as XMLSerializer = new XMLSerializer(GetType(Person))
dim oLucky as new Person()
dim oStmW as StreamWriter
'Set properties
oLucky.Name = "Lucky Day"
oLucky.Age = 52
'Display property values
Response.Write("Hello() = " & oLucky.Hello() & "<br />")
Response.Write("Goodbye() = " & oLucky.Goodbye() & "<br />")
'Serialize object to XML and write it to XML file
oStmW = new StreamWriter(Server.MapPath("lucky.xml"))
oXS.Serialize(oStmW, oLucky)
oStmW.Close()
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[
View a live demo!]
First of all, we declare and instantiate an XMLSerializer object. You'll notice that we
had to tell it right from the onset, using the GetType() function, what type of object
it's going to be serializing. Next you see that we assign values to the Name and Age
properties of the Person object we instantiated. Then we output to the ASP.NET page what
the properties are set to by calling the Hello() and Goodbye() methods of
the Person object. Remember that this is only so that we can see what's happening with the
object during this process. Next comes the good stuff: We instantiate a StreamWriter
object and tell it that it will be writing to a file called, lucky.xml. We then call the
Serialize() method of the XMLSerializer object and send it our Person
object to be serialized as well as the StreamWriter object so it will write the resulting
XML to the file specified. Then we close the StreamWriter, thereby closing the file.
That's it. If everything works correctly, an XML file (lucky.xml) will be written to
disk. It should look like this:
<?xml version="1.0" encoding="utf-8"?>
<Class_Person xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<Property_Age>52</Property_Age>
<Property_Name>Lucky Day</Property_Name>
</Class_Person>
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Notice that the names of the XML elements are exactly as we specified in the <XMLRoot()>
and <XMLElement()> attributes in the class earlier.
In Part 2 we'll examine how to "rehydrate" our XML into an object
instance.
Read Part 2!
