ln.templates/ln.templates/elements/Element.cs

using System;
using System.IO;
using System.Collections.Generic;

namespace ln.templates.elements
{
    public abstract class Element
    {
        public ContainerElement Container { get; internal set; }

        protected Element(ContainerElement container)
        {
            Container = container;
            if (Container != null)
                Container.children.Add(this);
        }

        public abstract void Generate(TextWriter writer, Template.Context context);
    }

    public class ContainerElement : Element
    {
        internal List<Element> children { get; } = new List<Element>();

        public ContainerElement(ContainerElement container)
            :base(container)
        {
        }

        public void GenerateChildren(TextWriter writer, Template.Context context)
        {
            foreach (Element c in children)
                c.Generate(writer, context);
        }

        public override void Generate(TextWriter writer, Template.Context context)
        {
            GenerateChildren(writer, context);
        }

        public Element[] Children
        {
            get => children.ToArray();
        }
    }

    public class TextElement : Element
    {
        public String Text { get; }
        public TextElement(ContainerElement container,String text)
            :base(container)
        {
            Text = text;
        }

        public override void Generate(TextWriter writer, Template.Context context)
        {
            writer.Write(Text);
        }
    }

    public class ExpressionElement : Element
    {
        public Expression Expression { get; }

        public ExpressionElement(ContainerElement container,Expression expression)
            :base(container)
        {
            Expression = expression;
        }

        public override void Generate(TextWriter writer, Template.Context context)
        {
            object v = Expression.Evaluate(context.ExpressionContext);
            if (v != null)
                writer.Write(v.ToString());
        }
    }
    public class SetElement : Element
    {
        public String SetName { get; }
        public Expression Expression { get; }

        public SetElement(ContainerElement container, String setName,Expression expression)
            : base(container)
        {
            SetName = setName;
            Expression = expression;
        }

        public override void Generate(TextWriter writer, Template.Context context)
        {
            object v = Expression.Evaluate(context.ExpressionContext);
            context.ExpressionContext.SetMappedValue(SetName, v);
        }
    }
}