LINQ to return null if an array is empty

问题

public class Stuff
{
  public int x;
  // ... other stuff
}

I have a IEnumerable<Stuff> and I want to build a int[] of all of the x properties of all the Stuff objects in the collection.

I do:

IEnumerable<Stuff> coll;
// ...
var data = coll.Select(s => s.x).ToArray();

What I want is a null array rather than a int[0] if the collection is empty. In other words, if !coll.Any(), then I want data = null. (My actual need is that coll is an intermediate result of a complex LINQ expression, and I would like to do this with a LINQ operation on the expression chain, rather than saving the intermediate result)

I know that int[0] is more desirable than null in many contexts, but I am storing many of these results and would prefer to pass around nulls than empty arrays.

So my current solution is something like:

var tmp = coll.Select(s => s.x).ToArray();
int[] data = tmp.Any() ? tmp : null;

Any way to do this without storing tmp?

EDIT: The main question is how to do this without storing intermediate results. Something like NULLIF() from T-SQL where you get back what you passed in if the condition is false, and NULL if the condition is true.

回答1:

If you're doing this a lot, you could write an extension method:

public static class IEnumerableExt
{
    public static T[] ToArrayOrNull<T>(this IEnumerable<T> seq)
    {
        var result = seq.ToArray();

        if (result.Length == 0)
            return null;

        return result;
    }
}

Then your calling code would be:

var data = coll.Select(s => s.x).ToArrayOrNull();

回答2:

Create the array only if coll is not empty, so the other way round:

int[] data = null;
if(coll.Any()) data = coll.Select(s => s.x).ToArray();

回答3:

There's not a way to get Select to return null, but if you don't want to create an additional array you could do:

var tmp = coll.Select(s => s.x);
int[] data = tmp.Any() ? tmp.ToArray() : null;

来源：https://stackoverflow.com/questions/28611083/linq-to-return-null-if-an-array-is-empty

标签

linq

linq-to-objects