Adding a default string if list empty

问题:

I'm writing out an aggregated list of statuses. It works fine except for the situation where there are none. At the moment, null is rendered and the position is empty.

item.Stuff.Where(e => Condition(e))
  .Select(f => f.Status)
  .Aggregate(String.Empty, (a, b) => a + b)

Now, I'd like to populate the table element with "---" in case the list is filtered down to an empty one by Condition but I can't decide on a method.

What would be a smooth way to approach it?

I've tried something like the atrocity below but it looks, well..., atrociously and it doesn't render right, neither. I get to see the actual source code line (preceded by False or True) instead of the values.

item.Stuff.Where(e => Condition(e)).Count() < 1
  ? "---"
  : item.Stuff.Where(e => Condition(e))
    .Select(f => f.Status)
    .Aggregate(String.Empty, (a, b) => a + b)

回答1:

You could do something like this. If the status list is reasonably small (otherwise one should use StringBuilder anyway and not string concatenation).

item.Stuff.Where(e => Condition(e))
    .Select(f => f.Status)
    .Aggregate("---", (a, b) => (a == "---") ? b : (a + b));

It checks if the default text was replaced and, if it was, it concatenates the next status element to the already existing text mass.

This will return "---" if and only if it's never evaluated, i.e. if the list is empty. Otherwise one would get the same result as previously.

---

If Status is an enum, and you only need the distinct statuses, you can use the behavior of the [Flags] attribute.

If you define the enum like this

[Flags]
enum Status
{
    None = 0,
    Active = 1,
    Inactive = 2,
    Pending = 4,
    Deleted = 8
    ...
}

you can just do:

item.Stuff.Where(e => Condition(e))
  .Aggregate(Status.None, (a, b) => a | b)

The result is a collection of all statuses that are present in the list, and the output is nicely formatted list (Active, Inactive, Pending) or None if it's never run.

回答2:

You can use DefaultIfEmpty(string.Empty).First(); MSDN

Example

var item =  item.Stuff.Where(e => Condition(e))
  .Select(f => f.Status)
  .Aggregate(String.Empty, (a, b) => a + b);

var newitem = item.DefaultIfEmpty(new Item() { i = 1, Status = "---" }).First();