Using a common static value to programatically identify classes

Question

In a reporting application, I have a number of different types of graphs, each of which is output with code in a specific class. Eg. clsPieChart, clsBarChart, etc. Each of those is derived from a base class, clsChartBase.

Now, the report can be put together dynamically, so there is a string ID code in the database for each type of chart, eg. "pie", "bar", etc.

My solution has been to use a common static variable in each class to match it up with the database code, eg:

Class clsPieChart
  Inherits clsChartBase
  Public ReadOnly Shared _ChartID As String = "pie" ' Each class has one of these.

This is how I can identify which class needs to be instantiated to handle that part of the report. Is this a reasonable approach, or is there a better way to do this sort of thing?

dotNET · Accepted Answer · 2013-10-09 02:59:25Z

1

I would suggest having a Dictionary(Of String, Type) instead - that keeps all the information in a single place, and means you can perform the mapping programmatically. You could make another mapping in the other direction as well, of course - and that can be autogenerated from the first (e.g. using ToDictionary() in LINQ).

I'd also strongly advise you to get rid of the cls prefix for your class names - it's worth reading Microsoft's naming conventions so that your code is idiomatic.

edited Oct 9, 2013 at 2:59

dotNET

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answered Oct 8, 2013 at 7:15

Jon Skeet

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5 Comments

ingredient_15939 Over a year ago

Thanks. Yeah The Hungarian is just a byproduct of my working with javascript, php, etc in text editors. Tough habit to break.

Jon Skeet Over a year ago

@ingredient_15939: It's well worth trying to break it. Following the conventions of the language you're working in is crucial to writing code which looks and feels right to others.

ingredient_15939 Over a year ago

True. :) Even with a dictionary, the class still needs the static string though, yes? Or are you saying I don't need it with that method?

Jon Skeet Over a year ago

@ingredient_15939: No, there's no need for a static string variable here. What benefit do you think it would have?

ingredient_15939 Over a year ago

Sorry, I'm being a dill.. the string in the dictionary is all I need. :) Thanks again!

dotNET · Accepted Answer · 2013-10-08 07:25:58Z

0

You could do TypeOf obj Is YourClass to identify the class of your object. You will get rid of keeping a shared variable within the class.

answered Oct 8, 2013 at 7:25

dotNET

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3 Comments

ingredient_15939 Over a year ago

Thanks, but there's no object at that point, there's only the text field from the database, which I need to compare with something before instantiating the corresponding class.

dotNET Over a year ago

I'm not sure I understand that correctly. At some point you must traverse through a decision tree to create an object of the corresponding class. If you have a db field that lets you decide the chart class, then Jon's method of creating a Dictionary(Of String, Type) is the best for you. That way, you can create a one-liner that will give you the corresponding type when you pass the class identifier (e.g. "pie") as key to it.

ingredient_15939 Over a year ago

Yep that's correct. Jon's solution is neat and easily understandable to another reader what's going on when they see the assignments in one place.

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