Sequence contains no elements exception in linq without even using Single

Question

I am not using Single in LINQ below, but I am still getting a 'Sequence contains no elements' exception:

allNames = StockCollection.Where((s) => s.Name.IndexOf("A") == 0)
                          .Select((s) => s.Name)
                          .Aggregate((namesInfo, name) => namesInfo += ", " + name);

This exception comes when there is no stock starting with name 'A'.

It seems that one extension method is expecting atleast one element satisfying the condition but that's not expected.

Can you please suggest the best solution to resolve this?

Thanks in advance.

Solution 1

As Dennis Traub has pointed out, the overload of Aggregate you are using throws that exception when the source sequence is empty.

The obvious fix is to use the other overload of Aggregate that accepts an initial seed (you want string.Empty), but that would result in a leading comma in the result which you'll have to get rid of.

(EDIT: You can dodge this with .DefaultIfEmpty(string.Empty) followed by your existing Aggregate overload. This wouldn't produce a leading comma.)

In any case, using Aggregate like that to join strings isn't a great idea (produces a Schlemiel the Painter's algorithm). Here's how I would write the query:

allNames = string.Join(",", StockCollection.Select(s => s.Name)
                                           .Where(name => name.StartsWith("A"));

In .NET 3.5, you'll need a .ToArray() to materialize the Where results into an array.

Solution 2

Use with empty seed.

new string[]{}.Aggregate("", (a,b)=> a+b )

Solution 3

Using Aggregate(func) on an empty source throws an InvalidOperationException.

See documentation: http://msdn.microsoft.com/en-us/library/bb548651.aspx

Comments

[D J]

Thanks a lot for reply i am going to try this now.. But still i want to know why aggregate is throwing an exception.

[Ani]

@D J: Why? What sensible value could it return? Perhaps the default value of the return type (null in this case), but I wouldn't want that.

[D J]

Problem is like we dont know at the runtime our collection will have any object or not? so it should not throw an exception or it can just return default value.

[Myster]

+1 for DefaultIfEmpty this works well with examples more complex than strings.

[Neil Vass]

If I could, I'd give you a second upvote for for the "in .NET 3.5" tip - thanks!

[Nikita]

I think, that instead of "" should use string.Empty