XML Serialization in ASP.NET - Part 2 (Page 3 of 4 )
In Part 1 we examined how to dehydrate (serialize) an object instance into an XML file. In this part we'll look at reversing the process - turning an XML file back into an object instance.
Just Add Water
Now let's look at this from the opposite angle. We've just seen how to serialize an object into an XML file and save it to disk, but now suppose we already had an XML file saved and wanted to use it to instantiate an object. In the downloadable code found at the end of this article, you will find a file called, ned.xml. We’re going to use that XML file to create a Person object. Its contents look like this:
You'll notice that this XML document has exactly the same structure as the XML file that we wrote to disk a moment ago but the data it contains is, of course, different. Now put on your wicked mad scientist grins and look at the code required to bring this beast of an object to life:
dim oNed as Person dim oStmR as StreamReader
'Pull in contents of an object serialized into an XML file 'and deserialize it into an object oStmR = new StreamReader(Server.MapPath("ned.xml")) oNed = oXS.Deserialize(oStmR) oStmR.Close()
Before anything else, we declare a Person object and StreamReader object. Next, we create an instance of the StreamReader object and feed it the stored XML file. Then we instantiate the Person object by calling the Deserialize() method of the XMLSerializer object. This method uses the StreamReader object to read the contents of the XML file and then instantiates an object whose state matches that described in the XML file. Finally, we close up the StreamReader object and then output the results of the newly created object's Hello() and Goodbye() methods just to prove that it was successfully created. It's just like that instant oatmeal Mom used to make.
Note: Something important to remember is that when an object is instantiated through Deserialization, its constructor is not called. Just keep that in mind if you plan on doing this with any objects which are very dependent on their constructors performing some crucial function.
Do I Have To Keep My Raisins?
Perhaps you are wondering now, "Pretty cool - but what if I don't want to save my object to disk?" Another good question. There's no reason you would have to. Let's suppose that for some reason, you needed to serialize an object into an XML string to be used for some purpose and then forgotten or re-instantiated or whatever else. This can be accomplished in almost the same way that was demonstrated earlier. However, instead of using a StreamWriter object in the process, we will use a StringWriter object. See the code snippet below:
dim oDusty as new Person() dim oStrW as new StringWriter() dim sXML as string
'Serialize object into an XML string oXS.Serialize(oStrW, oDusty) sXML = oStrW.ToString() oStrW.Close()
As you can see, we instantiate a new Person object and StringWriter object and then assign values to the Name and Age properties of the Person object. We then call the Serialize() method of the XMLSerializer object and the Person object is serialized into an XML document and placed in the StringWriter object.
Before we move on, it is important to understand some things about the StringWriter and StreamWriter objects and Inheritance. The Serialize() method of the XMLSerializer object is an overloaded method and one of its signatures is: Overloads Public Sub Serialize(TextWriter, Object). This means we must send it a TextWriter object and some other object.
"Wait a minute!" I hear you shouting, "If it needs to be sent a TextWriter object, why are we sending it StringWriters and StreamWriters?" That's because of Inheritance.
In object oriented development, objects can be derived from other objects, inheriting some or all of the original object's characteristics. This is where StringWriter and StreamWriter come from. They are "descendants" of TextWriter. Think of it this way: A man named Fritz Meyer has two children, Hansel and Gretel. Hansel is not Fritz, but he is a Meyer as is Gretel and when they have a Meyer family reunion, Fritz, Hansel, and Gretel can all get in the door because they are all Meyers. Similarly, because StreamWriter and StringWriter are both descended from TextWriter, they can be used with this call to Serialize(). Unfortunately, StreamWriter doesn't have a way to present its contents as a string data type, but StringWriter does and we are interested, at this point, in getting the XML string rather than saving it to a file.
That is why, in the code snippet above, we send a StringWriter to Serialize() instead of a StreamWriter. (For more information on inheritence and how it is used in .NET, be sure to read: Using Object-Orientation in ASP.NET: Inheritance.)
After the serialization takes place, we capture the XML string by calling the ToString() method of the StringWriter object and placing the results in a string variable. Next, we close the StringWriter object because we no longer need it. We now have our hands on the XML string and can do with it what we please. In the downloadable example code, all we do with it is output it to the browser.