Functors when should I use them whats their intended use [closed]

Answer 1

Interesting with terminology; my spontaneous interpretation of the term "Functor" was that it referred to anonymous methods. So that will be my take on it.

These are some of my typical uses:

Comparisons (usually for sorting a list):

List ints = new List();
ints.AddRange(new int[] { 9, 5, 7, 4, 3, 5, 3 });
ints.Sort(new Comparison(delegate(int x, int y)
    {
        return x.CompareTo(y);
    }));
// yes I am aware the ints.Sort() would yield the same result, but hey, it's just
// a conceptual code sample ;o)

// and the shorter .NET 3.5 version:
ints.Sort((x, y) =>
{
    return x.CompareTo(y);
});

I will use this approach for comparisons, rather than having it in its own method an using a delegate for that method, in the cases where this particular sort happens in one place only. If it is likely that I will want to use the same comparison somewhere else, it gets to live in its own, reusable method.

Another of my fairly common uses is in unit testing, when the test relies on some event being raised. I have found that to be essential when unit testing workflows in Workflow Foundation:

WorkflowRuntime runtime = WorkflowHost.Runtime;  
WorkflowInstance instance = runtime.CreateWorkflow(typeof(CreateFile)); 
EventHandler WorkflowIdledHandler = delegate(object sender, WorkflowEventArgs e)
{
    // get the ICreateFileService instance from the runtime  
    ISomeWorkflowService service = WorkflowHost.Runtime.GetService();

    // set the desired file content  
    service.DoSomeWork(instance.InstanceId, inputData);
};  
// attach event handler
runtime.WorkflowIdled += WorkflowIdledHandler;  

instance.Start();  
// perform the test, and then detach the event handler
runtime.WorkflowIdled -= WorkflowIdledHandler; 

In this case it is simpler to have the event handler declared as anonymous methods since it uses the instance variable that is defined in the unit test method scope. Had I intstead opted to implelment the event handler as its own separate method I would also need to figure out a way for it to pick up instance, probably by introducing a class level member, which would not seem like a perfect design in a unit test class.

There are more cases where I find this in my code, but they usually have one or two things in common:

• I have no interest in referencing that piece of code from anywhere else than in that particular place.
• The method needs access to data that would be out of the scope for a regular method