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set { _buffer = value; } } public void Accept( IVisitor visitor) { visitor.Process( this); } } The modification to SampleClass is minimal and only requires the implementation of the IBase interface. Like the examples before, the method Accept calls the visitor.Process method. When using the Visitor pattern, the inspectee will usually only go so far, and leave the heavy lifting to the inspector (IVisitor implementation). Following is an IVisitor implementation: public class VisitorImplementation : IVisitor { private string _path; public VisitorImplementation( string path) { _path = path; } public void Process< type>(type parameter) where type: class{ if( parameter is SampleClass) { XmlSerializer s = new XmlSerializer( typeof( SampleClass ) ); TextWriter w = new StreamWriter( _path); s.Serialize( w, parameter as SampleClass ); w.Close(); } } } The class VisitorImplementation implements the IVisitor interface. The method Process<> checks the passed-in type using an if statement that tests whether the parameter is a specific type. If the type is SampleClass, then the XML serialization proceeds; otherwise nothing happens. Putting everything together, the following source code executes the Visitor pattern: VisitorImplementation visitor = new VisitorImplementation( @"c:\sampleclass.xml"); IBase @base = new SampleClass( 20, "SampleClass"); @base.Accept( visitor); VisitorImplementation and SampleClass are both instantiated, and then the method Accept is called, resulting in the creation of the XML file. The Visitor pattern implementation has the following advantages: Like the Bridge pattern, the Visitor pattern makes it possible to separate definition from implementation. In the example, this means separating the serializable class from the serialization class. Making an existing class usable as an inspectee requires a minimum of additional code.
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The Visitor pattern is a good way to modify, update, or tweak a collection of objects with a universal operation, without having to manually iterate each type. The heavy lifting is always managed by the IVisitor implementation, and not by the inspectee. The inspectee is oblivious to the entire process.
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The Visitor pattern implementation is complete, but what isn t to our liking is the problem of having to associate XML attributes with the inspectees. Frankly, the current Visitor pattern implementation is both trivial and overkill. What would be of interest is to use the Visitor pattern to serialize an already existing type, without having to use XML attributes. It s possible to do this and relatively trivial to realize. For this example, the SampleClass used is from the earlier binary serialization example (the immutable SampleClass) minus the serializable attribute. The modified IVisitor implementation is defined as follows: public class VisitorImplementation : IVisitor { private string _path; public VisitorImplementation( string path) { _path = path; } public void Process< type>(type parameter) where type: class{ if( parameter is SampleClass) { XmlTextWriter writer = new XmlTextWriter( _path, null); SampleClass cls = parameter as SampleClass; writer.WriteStartDocument(); writer.WriteStartElement( "SampleClass"); writer.WriteStartElement( "Value"); writer.WriteString( XmlConvert.ToString( cls.Value)); writer.WriteEndElement(); writer.WriteStartElement( "Buffer"); writer.WriteString( cls.Buffer); writer.WriteEndElement(); writer.WriteEndElement(); writer.WriteEndDocument(); writer.Flush(); writer.Close(); } } } In the modified implementation of the method Process, instead of instantiating XmlSerializer, XmlTextWriter is instantiated. XmlTextWriter is a useful utility class that you can use to write an XML stream. An IVisitor implementation knows the details of the type being manipulated, and hence knows which data members exist. Therefore, it isn t difficult for IVisitor to manually convert the properties into a series of XML elements. The individual XML elements are serialized using the methods WriteStartDocument, WriteStartElement, and so on.
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The advantage of this approach is that the serializable object needs to do nothing, and the IVisitor implementation does all of the heavy lifting. Thinking about this approach, it s easy to conclude that the Serializable pattern has been indirectly implemented. That thinking is correct, because in this instance the Visitor pattern directly resembles the serialization to the medium part (WriteTo) of the Serializable pattern. What needs to be understood is that the Visitor pattern only implements the browsing of an object hierarchy, and not the deserialization aspect of the Serializable pattern.
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