C# – Should a Computed Value Be Exposed as a Property or Method?

cclass-designdesign-patternsmethodsproperties

I have a C# class that represents a content type in a web content management system.

We have a field that allows a web content editor to enter an HTML template for how the object is displayed. It basically uses the handlebars syntax for substituting object property values into the HTML string:

<h1>{{Title}}</h1><p>{{Message}}</p>

From a class design perspective, should I expose the formatted HTML string (with substitution) as a property or method?

Example as property: 

public class Example
{
  private string _template;
  public string Title { get; set; }
  public string Message { get; set; }
  public string Html 
  {
    get
    {
      return this.ToHtml();
    }
    protected set { }
  }

  public Example(Content content)
  {
    this.Title = content.GetValue("title") as string;
    this.Message = content.GetValue("message") as string;
    _template = content.GetValue("template") as string;
  }

  private string ToHtml()
  {
    // Perform substitution and return formatted string.
  }  
}

Example as method:

public class Example
{
  private string _template;
  public string Title { get; set; }
  public string Message { get; set; }

  public Example(Content content)
  {
    this.Title = content.GetValue("title") as string;
    this.Message = content.GetValue("message") as string;
    _template = content.GetValue("template") as string;
  }

  public string ToHtml()
  {
    // Perform substitution and return formatted string.
  }  
}

I'm not sure from a design standpoint does it make a difference or are there reasons why one approach is better than the other?

Best Answer

UPDATE: This question was the subject of my blog in May 2014. Thanks for the great question!

To add to Robert Harvey's answer: a property should be:

  • logically a property of the class, the way that say its color or year or model are the properties of a car.

  • not more than, let's say, ten times slower to compute than fetching from a field.

  • something you don't mind being computed while debugging. The VS debugger automatically computes properties.

  • unable to fail. Getters should always return a value no matter what the state of the object is.

I don't think your proposed Html property hits any of those. Don't make it a property unless it hits all of them.

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