Problem with List<IEnumerable<string>>

firicddb · November 2012

Hi,

Why do I get a wrong result for this simple code in LINQPad?


List<int> toSearch= new List<int>{100,199,95};
List<string> assets = new List<string>{"95873","95950","95935","95942","105019","95938","199999","97490","100442","100167","100167","95873"};

List<IEnumerable<string>> list = new List<IEnumerable<string>>();

foreach ( var n in toSearch)
{
	IEnumerable<string> query = (from asset in assets
									where asset.Contains(n.ToString())
									select asset);
	list.Add(query);	
}

foreach (var v in list)
	Console.WriteLine(v.Count());

Output:
6
6
6

The correct output would be:
3
1
6

LINQPad v.4.42.01
.Net 4.0

Thanks,

sgmoore · November 2012

This is called 'Closing over the loop variable'

I leave it to an expert to explain

http://blogs.msdn.com/b/ericlippert/archive/2009/11/12/closing-over-the-loop-variable-considered-harmful.aspx

You can fix it by taking a copy of the variable n and using that variable in your query


   foreach ( var n in toSearch)
   {
      var copyOfN = n;
      IEnumerable<string> query = (from asset in assets
                                     where asset.Contains(copyOfN.ToString())
                                    select asset);
      list.Add(query);	
   }

firicddb · November 2012

Thank you! That clears it up.

JoeAlbahari · November 2012

The good news is that this misfeature of C# has been fixed. In C# 5.0, you no longer need to copy the iteration variable.

Problem with List<IEnumerable<string>>

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