jump to navigation

XML Serializing between WebService proxy classes and Entity classes July 8, 2008

Posted by codinglifestyle in ASP.NET, C#.
Tags: , , , , ,
trackback
If you are looking at this page I would suggest you ask yourself the following questions:
  • Is the architecture of my software broken?  Why am a referencing a class on either side of a webservice?
  • Are web services really necessary in this situation?
My answers are yes, it is broken, and yes, there is an egrigious use of web services throughout.  Unfortunately, this product is many years old and I have to do what the customer asks.  So here I have an entity class which is not a simple container for data.  It contains additional methods and logic.  This entity is populated by the database and then forked over by webservices.  However, the UI layer also needs access to the additional methods and logic in the entity.  The WSDL generated proxy class does not contain these methods or logic; just a simple representation of the public properties of the class with straight gets and sets.  
 
The challenge then, is to serialize the web service proxy class back to the entity class.  Once this is done, I will have access to the additional methods and logic I need.  We can do this by using what web services use, XML serialization.  Its a fast way of getting our data from the proxy class back to XML.  Then deserialize the XML back to the entity class.
 
        ///
        /// Serialize the data from an object of Type_A to a new object of Type_B
        ///
        ///The Type_A object containing the data to serialize
        ///The Type_B to be created and deserialized with the Type_A object
        ///An object of Type_B
        static object SerializeObjects(object oSource, Type tDestination)
        {
            XmlNoNamespaceWriter xw = null;
            Object oDestination = null;
 
            try
            {
                //Prepare to convert XML entity to XmlNoNamespaceWriter
                MemoryStream ms = new MemoryStream();
                xw = new XmlNoNamespaceWriter(ms, Encoding.UTF8);
                XmlSerializer xsA = new XmlSerializer((oSource.GetType()));
 
                //Serialize web service xml data to xml stream
                xsA.Serialize(xw, oSource);
 
                //Deserialize stream to entity actual
                XmlSerializer xsB = new XmlSerializer(tDestination);
                xw.BaseStream.Position = 0;
                oDestination = (WSEntityTest.Entity)xsB.Deserialize(xw.BaseStream);
            }
            catch
            { }
            finally
            {
                //Close the stream
                if (xw != null)
                    xw.Close();
            }
 
            return oDestination;
        }
 
So what we are doing is using a memory stream and XmlTextWriter to contain the XML.  The XmlSerializer serializes the proxy class to the stream.  If you wanted to look at the resulting XML, you could use an XmlDocument to load the stream and take a look.  If we do that, we will notice a small problem.  Every child element will contain a namespace attribute (xmlns). This will throw a spanner in the works when deserializing the data to our entity class. Basically what the namespace is telling the XmlSerializer is the data doesn’t belong in our entity class so it will skip it. It’s just doing its job because technically we shouldn’t be serializing data from the class of type A to a class of type B. However, this is exactly what we need to do. To work around this, I looked high and low for a simple way of turning off the namespace (in .NET v2.0).  Nothing simple turned up, hence the use of XmlNoNamespaceWriter which is derived from XmlTextWriter.   The implementation of this class is shown below:
 
    public class XmlNoNamespaceWriter : System.Xml.XmlTextWriter
    {
        bool m_bSkipAttribute = true;
 
        public XmlNoNamespaceWriter(System.IO.Stream writer, System.Text.Encoding encoding) : base(writer, encoding)
        {
        }
 
        public override void WriteStartElement(string sPrefix, string sLocalName, string sNS)
        {
            base.WriteStartElement(null, sLocalName, null);
        }
 
 
        public override void WriteStartAttribute(string sPrefix, string sLocalName, string sNS)
        {
            //If the sPrefix or localname are “xmlns”, don’t write it.
            if (sPrefix.CompareTo(“xmlns”) == 0 || sLocalName.CompareTo(“xmlns”)==0)
            {
                m_bSkipAttribute = true;               
            }
            else
            {
                base.WriteStartAttribute(null, sLocalName, null);
            }
        }
 
        public override void WriteString(string sText)
        {
            //If we are writing an attribute, the sText for the xmlns
            //or xmlns:sPrefix declaration would occur here. Skip
            //it if this is the case.
            if(!m_bSkipAttribute)
            {
                base.WriteString(sText);
            }
        }
 
        public override void WriteEndAttribute()
        {
            //If we skipped the WriteStartAttribute call, we have to
            //skip the WriteEndAttribute call as well or else the XmlWriter
            //will have an invalid state.
            if(!m_bSkipAttribute)
            {
                base.WriteEndAttribute();
            }
            //reset the boolean for the next attribute.
            m_bSkipAttribute = false;
        }
 
 
        public override void WriteQualifiedName(string sLocalName, string sNS)
        {
            //Always write the qualified name using only the
            //localname.
            base.WriteQualifiedName(sLocalName,null);
        }
    }
 
I found this class here, and modified it slightly to use a stream rather than a file. So now we get past our namespace issue and the serializer won’t be confused by the namespaces. Now all we need is a second XmlSerializer created with our entity type.  This will deserialize the stream in to a new instance of our entity class.  The usage of the method is shown below:
 
//Retreive the WebService proxy class
localhost.Service1 svc       = new Client.localhost.Service1();
localhost.Entity xmlEntity   = svc.HelloWorld(); 
 
//Serialize the proxy class to a new entity class
Entity entity = (Entity) Program.SerializeObjects(xmlEntity, typeof(Entity));
 
Advertisements

Comments»

1. Kirk Evans - May 2, 2009

Great change to the XmlNoNamespaceWriter code, taking a stream in the ctor. Amazing how I continue to find people who found that post useful, thanks for the reference link. Always good to see that a post actually helped someone. And thanks for helping keep Hungarian notation alive 🙂


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: