Question

I have a class that follows the Command Pattern.
It has 2 methods which are Execute, and CanExecute which checks whether to invoke Execute or not (they derive from ICommand).
CanExecute invokes a few methods that check that all required services are running, the version is correct, etc.

After CanExecute is invoked, it may fail and return false and I need to know why. Is it because of a bad version, services, missing file, etc.

What is the best strategy to know what is the problem

One option is whenever a required condition fails I can throw an exception that will describe the error in the message field. However the possibility that it will fail is expected and you shouldn’t use exceptions for regular flow of control. So I’m really not sure.

Thank you.

Answer 1

You can use a collection of “reasons” that will tell the users of the class why CanExecute returned false. The reasons can be a simple IEnumerable<string>.

public bool CanExecute() {
  var messages = new List<string>();

  if (!Condition1) {
    messages.Add("Missing Condition1");
  }

  ...

  Messages = messages;
  return messages.Count == 0;
}

public IEnumerable<string> Messages { get; private set; }

Then, client code can show the collection of messages to end-users.

UPDATE:

You can also associate new commands with the messages to give the users ways to fix the problems found. In this case, instead of an IEnumerable<string>, you can create your own class that encapsulates that information:

public class Message {
  public string Text { get; set; }
  public ICommand Command { get; set; }
}

...

public bool CanExecute() {
  var messages = new List<Message>();

  if (!Condition1) {
    messages.Add(
      new Message { 
        Text = "Missing Condition1", 
        Command = new FixCondition1Command() 
      }
    );
  }

  ...

  Messages = messages;
  return messages.Count == 0;
}

public IEnumerable<Message> Messages { get; private set; }