C# – Sequence contains no elements exception in linq without even using Single

clinqlinq-to-objectssequencesstring

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.

Best Answer

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.

Related Solutions

C# – Using LINQ to concatenate strings

This answer shows usage of LINQ (Aggregate) as requested in the question and is not intended for everyday use. Because this does not use a StringBuilder it will have horrible performance for very long sequences. For regular code use String.Join as shown in the other answer

Use aggregate queries like this:

string[] words = { "one", "two", "three" };
var res = words.Aggregate(
   "", // start with empty string to handle empty list case.
   (current, next) => current + ", " + next);
Console.WriteLine(res);

This outputs:

, one, two, three

An aggregate is a function that takes a collection of values and returns a scalar value. Examples from T-SQL include min, max, and sum. Both VB and C# have support for aggregates. Both VB and C# support aggregates as extension methods. Using the dot-notation, one simply calls a method on an IEnumerable object.

Remember that aggregate queries are executed immediately.

More information - MSDN: Aggregate Queries

If you really want to use Aggregate use variant using StringBuilder proposed in comment by CodeMonkeyKing which would be about the same code as regular String.Join including good performance for large number of objects:

 var res = words.Aggregate(
     new StringBuilder(), 
     (current, next) => current.Append(current.Length == 0? "" : ", ").Append(next))
     .ToString();

C# – Concat all strings inside a List using LINQ

String.Join(delimiter, list);

is sufficient.

Best Answer

Related Solutions

C# – Using LINQ to concatenate strings

C# – Concat all strings inside a List using LINQ

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