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2013-08-13T03:18:52.913
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2013-08-13T05:43:45.980
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I think the order of elements retrieved by a <code>LINQ</code> is preserved, at least for <code>LINQ to Object</code>, for <code>LINQ to SQL or Entity</code>, it may depend on the order of the records in the table. For <code>LINQ to Object</code>, I'll try explaining why it preserves the order. In fact when the <code>LINQ</code> query is executed, the <code>IEnumerable</code> source will call to <code>GetEnumerator()</code> to start looping with a <code>while loop</code> and get the next element using <code>MoveNext()</code>. This is how a <code>foreach</code> works on the <code>IEnumerable</code> source. We all know that a <code>foreach</code> will preserve the order of the elements in a list/collection. Digging more deeply into the <code>MoveNext()</code>, I think it just has some <code>Position</code> to save the current <code>Index</code> and <code>MoveNext()</code> just increase the <code>Position</code> and <code>yield</code> the corresponding element (at the new position). That's why it should preserve the order, all the code changing the original order is redundant or by explicitly calling to <code>OrderBy</code> or <code>OrderByDescending</code>. If you think this <pre><code>int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 }; foreach(var i in numbers) if(i < 5) Console.Write(i + " "); </code></pre> prints out <code>4 1 3 2 0</code> you should think this <pre><code>int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 }; IEnumerator ie = numbers.GetEnumerator(); while(ie.MoveNext()){ if((int)ie.Current < 5) Console.Write(ie.Current + " "); } </code></pre> also prints out <code>4 1 3 2 0</code>. Hence this <code>LINQ</code> query <pre><code>var lowNums = from n in numbers where n < 5 select n; foreach (var i in lowNums) { Console.Write(i + " "); } </code></pre> should also print out <code>4 1 3 2 0</code>. Conclusion: The order of elements in <code>LINQ</code> depends on how <code>MoveNext()</code> of an <code>IEnumerator</code> obtained from an <code>IEnumerable</code> is implemented. However, it's for sure that the order of elements in <code>LINQ</code> result will be the same order a <code>foreach</code> loop works on the elements.
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1. POWill the result of a LINQ query always be guaranteed to be in the correct order?
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