Question

In another question on SO I answered with code like the one below and got a comment that the LINQ-query probably was evaluated in every iteration of the for/each. Is that true?

I know that LINQ-querys does not executes before its items is evaluated so it seems possible that this way to iterate the result can make it run on every iteration?

Dim d = New Dictionary(Of String, String)()     d.Add('Teststring', 'Hello')     d.Add('1TestString1', 'World')     d.Add('2TestString2', 'Test')      For Each i As String In From e In d Where e.Value.StartsWith('W') Select e.Key     MsgBox('This key has a matching value:' & i)     Next 

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Answer 1

NO… in a foreach, the ‘GetEnumerator’ is called only once (ever), and that is used going forward.

EDIT: I put a statement here about the result set being stored temporarily… that’s only true for some cases… not this one, so I took it out.

EDIT: Please forgive this for being overly verbose… but I wanted to show you what is happening… so here’s a Console app 🙂

using System; using System.Collections.Generic; using System.Linq;  namespace ConsoleApplication1 {     class Program     {         static void Main(string[] args)         {             foreach (string item in MyCustomEnumerator()                 .Where(item => item.StartsWith('abc')))             {             Console.WriteLine(item);             }         }          static IEnumerable<string> MyCustomEnumerator()         {             Console.WriteLine(DateTime.Now);              yield return 'abc1';              Console.WriteLine(DateTime.Now);              yield return 'abc2';              Console.WriteLine(DateTime.Now);              yield return 'abc3';              Console.WriteLine(DateTime.Now);              yield return 'xxx';         }     } } 

EDIT: This will result in a DateTime, then abc1, then a DateTime, then abc2, then a DateTime, then abc3, then a DateTime.