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POLINQ to SQL bug (or very strange feature) when using IQueryable, foreach, and multiple Where
 

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  1. POLINQ to SQL bug (or very strange feature) when using IQueryable, foreach, and multiple Where
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    <p>I ran into a scenario where LINQ to SQL acts very strangely. I would like to know if I'm doing something wrong. But I think there is a real possibility that it's a bug.</p> <p>The code pasted below isn't my real code. It is a simplified version I created for this post, using the Northwind database.</p> <p>A little background: I have a method that takes an <code>IQueryable</code> of <code>Product</code> and a "filter object" (which I will describe in a minute). It should run some "Where" extension methods on the <code>IQueryable</code>, based on the "filter object", and then return the <code>IQueryable</code>.</p> <p>The so-called "filter object" is a <code>System.Collections.Generic.List</code> of an anonymous type of this structure: <code>{ column = fieldEnum, id = int }</code></p> <p>The fieldEnum is an enum of the different columns of the <code>Products</code> table that I would possibly like to use for the filtering.</p> <p>Instead of explaining further how my code works, it's easier if you just take a look at it. It's simple to follow.</p> <pre><code>enum filterType { supplier = 1, category } public IQueryable&lt;Product&gt; getIQueryableProducts() { NorthwindDataClassesDataContext db = new NorthwindDataClassesDataContext(); IQueryable&lt;Product&gt; query = db.Products.AsQueryable(); //this section is just for the example. It creates a Generic List of an Anonymous Type //with two objects. In real life I get the same kind of collection, but it isn't hard coded like here var filter1 = new { column = filterType.supplier, id = 7 }; var filter2 = new { column = filterType.category, id = 3 }; var filterList = (new[] { filter1 }).ToList(); filterList.Add(filter2); foreach(var oFilter in filterList) { switch (oFilter.column) { case filterType.supplier: query = query.Where(p =&gt; p.SupplierID == oFilter.id); break; case filterType.category: query = query.Where(p =&gt; p.CategoryID == oFilter.id); break; default: break; } } return query; } </code></pre> <p>So here is an example. Let's say the List contains two items of this anonymous type, <code>{ column = fieldEnum.Supplier, id = 7 }</code> and <code>{ column = fieldEnum.Category, id = 3}</code>.</p> <p>After running the code above, the underlying SQL query of the <code>IQueryable</code> object should contain:</p> <pre><code>WHERE SupplierID = 7 AND CategoryID = 3 </code></pre> <p>But in reality, after the code runs the SQL that gets executed is</p> <pre><code>WHERE SupplierID = 3 AND CategoryID = 3 </code></pre> <p>I tried defining <code>query</code> as a property and setting a breakpoint on the setter, thinking I could catch what's changing it when it shouldn't be. But everything was supposedly fine. So instead I just checked the underlying SQL after every command. I realized that the first <code>Where</code> runs fine, and <code>query</code> stays fine (meaning <code>SupplierID = 7</code>) until right after the <code>foreach</code> loop runs the second time. Right after <code>oFilter</code> becomes the second anonymous type item, and not the first, the 'query' SQL changes to <code>Supplier = 3</code>. So what must be happening here under-the-hood is that instead of just remembering that <code>Supplier</code> should equal 7, LINQ to SQL remembers that Supplier should equal <code>oFilter.id</code>. But <code>oFilter</code> is a name of a single item of a <code>foreach</code> loop, and it means something different after it iterates.</p>
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      1. POLINQ to SQL bug (or very strange feature) when using IQueryable, foreach, and multiple Where
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    1. COBrian's got the bug nailed (it's about the evaluation time of expressions), but just a quick note that the ".AsQueryable()" on the end of your initial query is unnecessary, since db.Products is Table<Product>, which is IQueryable<Product> already.
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    2. COpossible duplicate of [C# Captured Variable In Loop](http://stackoverflow.com/questions/271440/c-sharp-captured-variable-in-loop)
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