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POVB.NET linq group by with anonymous types not working as expected
 

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  1. POVB.NET linq group by with anonymous types not working as expected
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    <p>I was toying around with some of the linq samples that come with LINQPad. In the "C# 3.0 in a Nutshell" folder, under Chater 9 - Grouping, there is a sample query called "Grouping by Multiple Keys". It contains the following query:</p> <pre><code>from n in new[] { "Tom", "Dick", "Harry", "Mary", "Jay" }.AsQueryable() group n by new { FirstLetter = n[0], Length = n.Length } </code></pre> <p>I added the string "Jon" to the end of the array to get an actual grouping, and came up with the following result:</p> <p><img src="https://i.stack.imgur.com/DL4qP.png" alt="C# LINQPad result"></p> <p>This was exactly what I was expecting. Then, in LINQPad, I went to the VB.NET version of the same query:</p> <pre><code>' Manually added "Jon" from n in new string() { "Tom", "Dick", "Harry", "Mary", "Jay", "Jon" }.AsQueryable() _ group by ng = new with _ { _ .FirstLetter = n(0), _ .Length = n.Length _ } into group </code></pre> <p>The result does not properly group Jay/Jon together.</p> <p><img src="https://i.stack.imgur.com/GhycT.png" alt="VB.NET LINQPad result"></p> <p>After pulling my hair out for a bit, I discovered <a href="http://msdn.microsoft.com/en-us/library/bb384767.aspx" rel="nofollow noreferrer">this MSDN article</a> discussing VB.NET anonymous types. In VB.NET they are mutable by default as opposed to C# where they are immutable. In VB, you need to add the <code>Key</code> keyword to make them immutable. So, I changed the query to this (notice the addition of <code>Key</code>):</p> <pre><code>from n in new string() { "Tom", "Dick", "Harry", "Mary", "Jay", "Jon" }.AsQueryable() _ group by ng = new with _ { _ Key .FirstLetter = n(0), _ Key .Length = n.Length _ } into group </code></pre> <p>This gave me the correct result:</p> <p><img src="https://i.stack.imgur.com/QAI9t.png" alt="enter image description here"></p> <p>So my question is this:</p> <ol> <li>Why does mutability/immutability of anonymous types matter when linq does an equality comparison? Notably, in Linq-to-SQL it doesn't matter at all, which is likely just a product of the translation to SQL. But in Linq-to-objects it apparently makes all the difference.</li> <li>Why would MS have chosen to make VB's anonymous types mutable. I see no real advantage, and after mucking around with this issue I see some very real disadvantages. Namely that your linq queries can have subtle bugs.</li> </ol> <p>-- EDIT --</p> <p>Just an interesting extra piece of info... Apparently this is keyed property issue is widely known. I just didn't know what to Google for. It's been discussed <a href="https://stackoverflow.com/questions/6556077/distinct-in-linq-with-anonymous-types-in-vb-net">here</a> and <a href="https://stackoverflow.com/questions/626935/why-group-by-key-of-anonymous-objects-does-not-behave-the-way-expected">here</a> on stackoverflow. Here's another example of the issue using anonymous types and Distinct:</p> <pre><code>Dim items = New String() {"a", "b", "b", "c", "c", "c"} Dim result = items.Select(Function(x) New With {.MyValue = x}).Distinct() Dim result2 = items.Select(Function(x) New With {Key .MyValue = x}).Distinct() 'Debug.Assert(result.Count() = 3) ' Nope... it's 6! Debug.Assert(result2.Count() = 3) </code></pre>
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    1. COThank you! I've been working around this problem for over a year now, and all my Grouping code is bastardized to fix this weirdness. I just knocked up a sample to demonstrate this problem and try to find a cure, but this appears to be the solution. Adding the 'Key' keyword is the solution. I've put the code on pastebin here: http://pastebin.com/pa7KjZv2
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