How can I avoid code duplication

Question

I have the follwoing code and I would like to write it in a way that I have minimum lines of code and the work is done the same way. How can I do that?

List<Category> categoryList = new List<Category>();
categoryList = Category.LoadForProject(project.ID).ToList();
List<string> categories = new List<string>(Categories);
IList<Category> currentCategories = Category.LoadForProject(project.ID).ToList();
if (currentCategories != null)
{
    foreach (var existingCategories in currentCategories)
    {
        if (categories.Contains(existingCategories.Name))
           categories.Remove(existingCategories.Name);
        else
            existingCategories.Delete(Services.UserServices.User);
    }
    foreach (string item in categories)
    {
        Category category = new Category(project, item.ToString());
        category.Project = project;
        category.Save();
   }
}

---

List<string> priorities = new List<string>(Priorities);
IList<Priority> currentPriorities = Priority.LoadForProject(project.ID).ToList();
if (currentPriorities != null)
{
   foreach (var existingPriorities in currentPriorities)
   {
       if (priorities.Contains(existingPriorities.Name))
           priorities.Remove(existingPriorities.Name);
       else
           existingPriorities.Delete(Services.UserServices.User);
   }
   foreach (string item in priorities)
   {
       Priority priority = new Priority(project, item.ToString());
       priority.Project = project;
       priority.Save();
   }
}

Answer 1

Something like this should do it:

public IList<T> DoYourThing<T>(IList<T> items, IList<T> currentItems, Project project) where T : CommonBaseType
{
  if (currentItems != null)
  {
    foreach (var existingItem in currentItems)
    {
      if (items.Contains(existingItem.Name))
        items.Remove(existingItem.Name);
      else
        existingItems.Delete(Services.UserServices.User);
    }
    foreach (string item in items)
    {
      T newItem = Activator.CreateInstance(typeof(T), new object[] {project, item.ToString()}) as T;
      newItem.Project = project;
      newItem.Save();
    }
  }

  return currentItems;
}

Then you can call it like this:

var currentCategories = DoYourThing(Categories.ToList(), Category.LoadForProject(project.ID).ToList());
var currentProjects = DoYourThing(Priorities.ToList(), Priority.LoadForProject(project.ID).ToList());

Finally, you should note two things in particular: First, there is a generic condition on the function where T : CommonBaseType. I am assuming that Category and Project have a common base type or interface that includes Name. If not, you should get rid of the condition and use Dynamic to get at Name.

Second, I am using Activator.Create to create the class for you. This is the tricky part that makes it difficult to figure out, if you don't know that trick

Good luck!