C# Multi level HTML TagBuilder object - Design pattern Review needed

Question

I find the TagBuilder written in C# little poor without the ability to "host" inside it sub tags (as objects not as strings).

So I wrote a simple class that inherits TagBuilder and allow to save sub tags. I can navigate inside it and add things to it while I move across the application. And when I call the Top Tag Object .ToString, it will render the entire html

public class MultiLevelHtmlTag : TagBuilder
{
 public MultiLevelHtmlTag(string tagName) : base(tagName) { }
 public List<MultiLevelHtmlTag> InnerTags = new List<MultiLevelHtmlTag>();
 public override string ToString()
 {
 if (InnerTags.Count > 0)
 {
 foreach (MultiLevelHtmlTag tag in InnerTags)
 {
 this.InnerHtml += tag.ToString();
 }
 }
 return base.ToString();
 }
}

this is how I use it:

MultiLevelHtmlTag top = new MultiLevelHtmlTag("div");
top.InnerTags.Add(new MultiLevelHtmlTag("span"));
log.Debug(top.ToString());

output:

<div><span></span></div>

What do you think about it? is that a goot pattern?

• Andrei Tanasescu: I would create a separate class for all valid HTML tags, and apply the Composite pattern.
• Jeff Mercado: You have some fundamental issues in your code. Your ToString() method has some major problems. See what happens if you call it multiple times. It should only return a string representation of the object. It shouldn't be modifying it.

Answer 1

I would make sure the list is not exposed publicly - the caller can then do whatever they want with that list, even assign null to it. So I've refactored it a little bit and implemented Add() (you may also want to implement Remove(), etc.):

/// <summary>
/// Simple class that inherits TagBuilder and allow to save sub tags.
/// </summary>
public class MultiLevelHtmlTag : TagBuilder
{
 /// <summary>
 /// List of inner tags.
 /// </summary>
 private readonly IList<MultiLevelHtmlTag> innerTags = new List<MultiLevelHtmlTag>();
 /// <summary>
 /// Initializes a new instance of the <see cref="MultiLevelHtmlTag"/> class.
 /// </summary>
 /// <param name="tagName">The name of the tag.</param>
 public MultiLevelHtmlTag(string tagName) : base(tagName)
 {
 }
 /// <summary>
 /// Gets the inner tag list.
 /// </summary>
 /// <value>The inner tag list.</value>
 public IEnumerable<MultiLevelHtmlTag> InnerTags
 {
 get
 {
 return new ReadOnlyCollection<MultiLevelHtmlTag>(this.innerTags);
 }
 }
 /// <summary>
 /// Adds the specified tag to the inner tag list.
 /// </summary>
 /// <param name="tag">The tag to add.</param>
 public void Add(MultiLevelHtmlTag tag)
 {
 if (tag == null)
 {
 throw new ArgumentNullException("tag");
 }
 this.innerTags.Add(tag);
 }
 /// <summary>
 /// Returns a <see cref="System.String"/> that represents this instance.
 /// </summary>
 /// <returns>
 /// A <see cref="System.String"/> that represents this instance.
 /// </returns>
 public override string ToString()
 {
 var sb = new StringBuilder();
 foreach (var tag in this.innerTags)
 {
 sb.Append(tag.ToString());
 }
 this.InnerHtml = sb.ToString();
 return base.ToString();
 }
}

• Marcel Popescu: Great response; in my project I've just changed the inner tags to be TagBuilder (so that I can add "regular" TagBuilder objects to it); there's no actual need to have them as MultiLevelHtmlTags.
• Leonardo Herrera: @ANeves - you need to use concat your results to the existing inner text or else you will end up with all empty tags.