Month: July 2009

This is essentially a redo of an earlier blog post of mine on how to create the illusion of a strongly typed xml in Boo using Duck Typing, except this time we’re using the dynamic keyword in C#.

http://cid-dfcd2d88d3fe101c.skydrive.live.com/embedrowdetail.aspx/blog/justnbusiness/XmlBuilder.cs

Here is an example of how it might look to use such a thing.

dynamic builder = new XmlBuilder();
builder.Main.One = "Hello";
builder.Main.Two = "World";

Console.WriteLine(builder.ToString());

Console.WriteLine(
    "{0} {1}!",
    builder.Main.One,
    builder.Main.Two);

builder = new XmlBuilder();
builder.Main["One"] = "Hello";
builder.Main["Two"] = "World";

Console.WriteLine(builder.ToString());

Console.WriteLine(
    "{0} {1}!",
    builder.Main["One"],
    builder.Main["Two"]);

XmlBuilder is a class I created myself, it inherits from DynamicObject and it intercepts TryGetMember, TrySetMember, TryGetIndex and TrySetIndex in order to allow you to navigate an internal XmlDocument object. Here is the output when the above is run.

Notice the use of properties corresponds to drilling into elements and the use of indexers corresponds to drilling into Attributes. This is just a proof of concept but you could see how this sort of thing would be useful.

Here is the complete XmlBuilder class that makes it all happen.

public class XmlBuilder : DynamicObject
{
    private Func<string, XmlElement> createElement;
    private Func<string, XmlAttribute> createAttribute;
    private XmlNode node;

    public XmlBuilder()
    {
        this.node = new XmlDocument();
        this.createElement = s => ((XmlDocument)this.node).CreateElement(s);
        this.createAttribute = s => ((XmlDocument)this.node).CreateAttribute(s);
    }

    private XmlBuilder(
        XmlNode node, 
        Func<string, XmlElement> createElement, 
        Func<string, XmlAttribute> createAttribute)
    {
        this.node = node;
        this.createElement = createElement;
        this.createAttribute = createAttribute;
    }

    public override bool  TryGetMember(GetMemberBinder binder, out object result)
    {
        XmlNode found = this.node[binder.Name];
        if (found == null)
        {
            found = this.createElement(binder.Name);
            this.node.AppendChild(found);
        }

        result = new XmlBuilder(found, this.createElement, this.createAttribute);
        return true;
    }

    public override bool TrySetMember(SetMemberBinder binder, object value)
    {
        XmlNode found = this.node[binder.Name];
        if (found == null)
        {
            found = this.createElement(binder.Name);
            this.node.AppendChild(found);
        }

        found.InnerText = value.ToString();
        return true;
    }

    public override bool TryGetIndex(GetIndexBinder binder, object[] indexes, out object result)
    {
        if (indexes.Length != 1)
        {
            throw new InvalidOperationException("You may only specify a single index.");
        }

        XmlNode found = null;
        if (indexes[0] is string)
        {
            string name = (string)indexes[0];
            found = this.node.Attributes[name];
            if (found == null)
            {
                found = this.createElement(name);
                this.node.AppendChild(found);
            }
        }
        else if (indexes[0] is int)
        {
            int index = (int)indexes[0];
            found = this.node.Attributes[index];
        }

        if (found != null)
        {
            result = new XmlBuilder(found, this.createElement, this.createAttribute);
            return true;
        }
        else
        {
            result = null;
            return false;
        }
    }

    public override bool TrySetIndex(SetIndexBinder binder, object[] indexes, object value)
    {
        if (indexes.Length != 1)
        {
            throw new InvalidOperationException("You may only specify a single index.");
        }

        XmlAttribute found = null;
        if (indexes[0] is string)
        {
            string name = (string)indexes[0];
            found = this.node.Attributes[name];
            if (found == null)
            {
                found = this.createAttribute(name);
                this.node.Attributes.Append(found);
            }
        }
        else if (indexes[0] is int)
        {
            int index = (int)indexes[0];
            found = this.node.Attributes[index];
        }

        if (found != null)
        {
            found.InnerText = value.ToString();
            return true;
        }
        else
        {
            return false;
        }
    }

    public override string ToString()
    {
        return this.node.InnerXml;
    }
}