Question

Can anyone suggest some guidance for designing and implementing good Properties? I am concerned with aspects such as:

• Side-effects
• Mutual Exclusivity
• Concurrency
• Symmetry and Reversibility
• Error Handling/Exceptions

Pointers to existing write ups that answer these topics would be great. I am NOT looking for help with WPF Dependency Properties.

Answer 1

As a rule, properties shouldn’t contain almost any logic other than possibly some boundary checks and type checks (depends on language). Thus, every item on the list other than Error Handling / Exceptions should not be factors when implementing properties.

W.R.T Error handling, it is absolutely OK to throw exceptions from properties (i.e. when a calling block attempts to set a property to an invalid value). Also, using try…catch statements are appropriate when attempting to parse data.

An example of this would be in using a property to hide a request parameter in a web application:

    public int UserId
    {
          get {
               string x = Request["userid"];
                  int userid = -1;
                  if (!int.TryParse(x, out userid))
                      throw new ApplicationException("UserID must be a valid integer");

                  return userid;
              }
    }

this is a bit of a contrived and simplified example, but I hope it illustrates the point. In a real-world application, you might want to have a different method of error handling and/or parsing to check for valid ranges of ID’s and such, all depending upon your circumstances.

The wikipedia entry for properties may also be a good place for more information