Need to overwrite XMLWriter's method

Question

I need to overwrite the XMLWriter\'s method \"WriteElementString\" to not write the element if the value is empty, the code bellow didn\'t work, tried override

Answer 1

You need to create an object that decorates XmlWriter to achieve what you are trying to do. More on the Decorator Pattern

public class MyXmlWriter : XmlWriter
{
    private readonly XmlWriter writer;

    public MyXmlWriter(XmlWriter writer)
    {
        if (writer == null) throw new ArgumentNullException("writer");
        this.writer = writer;
    }

    // This will not be a polymorphic call
    public new void WriteElementString(string localName, string value)
    {
        if (string.IsNullOrWhiteSpace(value)) return;

        this.writer.WriteElementString(localName, value);
    }

    // the rest of the XmlWriter methods will need to be implemented using Decorator Pattern
    // i.e.
    public override void Close()
    {
        this.writer.Close();
    }
    ...
}

Using the above object in LinqPad

var xmlBuilder = new StringBuilder();
var xmlSettings = new XmlWriterSettings
{
    Indent = true
};

using (var writer = XmlWriter.Create(xmlBuilder, xmlSettings))
using (var myWriter = new MyXmlWriter(writer))
{
    // must use myWriter here in order for the desired implementation to be called
    // if you pass myWriter to another method must pass it as MyXmlWriter
    //    in order for the desired implementation to be called
    myWriter.WriteStartElement("Root"); 
    myWriter.WriteElementString("Included", "Hey I made it");
    myWriter.WriteElementString("NotIncluded", "");
}

xmlBuilder.ToString().Dump();

Output:

  Hey I made it

What you are trying to do, is to override a method using an extension method which is not what they are intended to do. See the Binding Extension Methods at Compile Time section on the Extension Methods MSDN Page The compiler will always resolve WriteElementString to the instance implemented by XmlWriter. You would need to manually call your extension method XmlWriterExtensions.WriteElementString(writer, localName, value); in order for your code to execute as you have it.