The power of FUNC<T>. Not a new George Clinton song.

by Anthony 9. March 2014 09:32

So this post has been a long time coming.  I have been busy with life in general so it has been a while since I posted.  But, this post in particular was something I have wanted to get around to for about 6 years now.  So why so long?  I'm not really sure; I have been using this particular technique for a long time and I just never got around to writing about it.  Now I know many people reading this already know the benefits of Func<T> and Action<T>, but I wanted to show a practical example of its use.  

So recently at work I was presented with a task to create a server side BB Code parser, the why is not important.  I found one that was helpful, but it was a bit too rigid for me.  So I decided to modify it with a factory pattern.  Everything was going great and for the most part it was an easy translation until two tags completely wrecked my world.  OK, maybe that is a bit of an exaggeration, but they did pose a problem.

Most tags in BBCode map directly to HTML ([b]=<b>, [u] = <u>, [url=www.google.com] = <a href="www.google.com">, etc...) however the [list] and [size] tags do not. To specify an ordered uppercase roman numeral list in BbCode it is [list=I] in HTML it is <ol style="list-style-type:upper-roman;">.  So what to do?  Func<T> to the rescue.  Func<T> allowed me to include the functionality of my translation with out sacrificing encapsulation.  So lets look at the code.

My first implementation of my Tag class for BB Code was pretty simple:

/// <summary>
/// Name of the BB Code tag
/// </summary>
public string BbTagName { get; set; }
/// <summary>
/// Name of the corresponding Html Tag
/// </summary>
public string HtmlTagName { get; set; }
/// <summary>
/// The property/style attrib the BB Code links to 
/// </summary>
public string HtmlProperty { get; set; }
/// <summary>
/// If true links to a HTML style attribute not a property  
/// </summary>
public bool IsStyleReplacement { get; set; }
/// <summary>
/// Initializes the formatting tag /// </summary> public void Init(); /// <summary> /// formats the tag /// </summary> /// <param name="Data">The data that will be converted to HTML</param> /// <returns>The converted string</returns> public string Format(string Data)

This allowed me to create a factory like this:

public class BbCodeParser
{
    private List<Tag> _tags = new List<Tag>
    {
        new Tag
        {
            BbTagName = "list",
            HtmlTagName = "ol",
        },

        new Tag
        {
			//li maps directly to html li no translation needed
            BbTagName = "li",
        },
        
        new Tag
        {
            BbTagName = "size",
            HtmlProperty = "font-size",
            HtmlTagName = "span",
            IsStyleReplacement = true,
        },
        new Tag
        {
            BbTagName = "font",
            HtmlProperty = "font-family",
            HtmlTagName = "span",
            IsStyleReplacement = true,
        },
    };

    public void Init()
    {
        //init all tags
        _tags.ForEach(t => t.Init());
    }

    public string Format(string data)
    {
        //runformat on all tags
        _tags.ForEach(t => data = t.Format(data));
        return data;
    }
}

 

Easy Peasy. Next came the dreaded regex for BBCode (if you would like to see it explained go here it is an awesome regex site):

//{0} will be replace with the BBCode

private const string BbCodeRegEx = "\\[(?:\\b{0}\\b)(?:\\s*)(=?)((?:.|\\n)*?)(?:\\s*)\\]((?:.|\\n)*?)\\[\\/\\b{0}\\b(?:\\s*)\\]";

This was a generic regex that captures pretty much all BB code tags (my full implementation required some additional chicanery.)   So now for the format class:

return _regReplace.Replace(Data,htmlReplacement);

Everything was happy then came my [list=[a,i,I,A,1]] problem.  I had a few options:

  1. Allow the factory creator to use their own regex.  (I know as a library consumer I would hate this option).
  2. Create a special tag for list. (only if there was no other option).
  3. Tell my boss we can't do it :) (not really an option)
  4. Allow the factory creator to implement their own translation 

In .Net 2.0 the last option would require a delegate that pointed to a method in a separate class, not very fun or supportable.  But since 3.5 introduced Func<T> and Action<T>, it is actually a pretty easy change:

Firstly I added the property:

/// <summary>
/// Allow the consumer to provide a custom translation
/// </summary>
public Func<string, string> PropertyConversion { get; set; } 

Then I modified the RegEx replacement to support the new custom code:

/// <summary>
/// formats the tag
/// </summary>
/// <param name="Data">The data that will be converted to HTML</param>
/// <returns>The converted string</returns>
public string Format(string Data)
{
	return _regReplace.Replace(Data,MatchReplacer);
}

private string MatchReplacer(Match m)
{
	string htmlReplacement;
	//this will be the value after the '=' in bbcode
	string convertedValue = m.Groups[2].Value;
	
	//if a conversion exists use it
	if (PropertyConversion != null)
	{
		convertedValue = this.PropertyConversion(convertedValue);
	}

	//in indexed groups index 0 is the entire find so we need to start at 1
	if (m.Groups[1].Value == "=")
	{
		htmlReplacement = IsStyleReplacement ? _htmlReplacementForStyle : _htmlReplacementForAttrib;
		htmlReplacement = Regex.Replace(htmlReplacement, "\\$2", convertedValue);

	}
	else
	{
		htmlReplacement = _htmlReplacementBasic;
	}
	htmlReplacement = Regex.Replace(htmlReplacement, "\\$3", m.Groups[3].Value);
	return htmlReplacement;
}

I highlighted the important code.  And that was it.  Now the consumer can do all kinds of crazy stuff.

So my new list tag implementation looks like this:

new Tag
{
	BbTagName = "list",
	HtmlProperty = "list-style-type",
	HtmlTagName = "ol",
	IsStyleReplacement = true,
	PropertyConversion = s =>
	{
		switch (s)
		{
			case "1":
				return "decimal";
			case "A":
				return "upper-alpha";
			case "a":
				return "lower-alpha";
			case "i":
				return "lower-roman";
			case "I":
				return "upper-roman";
			default:
				return "decimal";
		}
	},
},

So no matter the translation I pretty much have it covered.  This has been a useful technique for me for years, I just always forget to post it.  Hopefully it helps someone.

I have attached the full code below:

BbCodeToHtml.zip (4.39 mb)

Happy Coding. 

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