XmlMarkupBuilder.cs

public class XmlMarkupBuilder : DynamicObject
{
    public override bool TryInvokeMember(InvokeMemberBinder binder, object[] args, out object result)
    {
        XElement xml = new XElement(binder.Name);

        var attrCount = binder.CallInfo.ArgumentNames.Count;
        var elementCount = args.Length - attrCount;

        for (int i = 0; i < elementCount; i++)
        {
            xml.Add(args[i]);
        }

        for (var i = 0; i < attrCount; i++)
        {
            var attrName = binder.CallInfo.ArgumentNames[i];
            if (attrName[0] == '@') attrName = attrName.Substring(1);

            xml.Add(new XAttribute(attrName, args[i + elementCount]));
        }

        result = xml;
        return true;
    }
}

class Program
{
    static void Main(string[] args)
    {
        dynamic b = new XmlMarkupBuilder();
        XElement xml =
            b.Persons(
                b.Person("Tom", age: 10),
                b.Person("Jerry", age: 8));

        Console.WriteLine(xml);

        // output:
        // <Persons>
        //   <Person age="10">Tom</Person>
        //   <Person age="8">Jerry</Person>
        // </Persons>

        Console.WriteLine("Press enter to exit...");
        Console.ReadLine();
    }
}