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POWhy do these three pieces of LINQ code produce different (or erroneous) results?
 

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  1. POWhy do these three pieces of LINQ code produce different (or erroneous) results?
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    <p>Here is some sample data:</p> <pre><code>List&lt;Book&gt; books = new List&lt;Book&gt;() { new Book(){Title = "artemis fowl: the time paradox", Pages = 380}, new Book(){Title = "the lieutenant", Pages = 258}, new Book(){Title = "the wheel of time", Pages = 1032}, new Book(){Title = "ender's game", Pages = 404}, new Book(){Title = "the sphere", Pages = 657} }; </code></pre> <p><strong>Background:</strong></p> <p>The above uses a simplified version of a Book class. It would, of course, contain many fields. My end goal is to allow the user to perform an 'advanced' search allowing the user to specify <em>any</em> field and further allow the user to specify keywords using boolean algebra for a particular field.</p> <p>eg: In a title search text box: the + (cake | pastry) + ~demon</p> <p>The above would mean: Find all books that, in the title, have the words 'the', either of 'cake' or 'pastry', and does not have the word 'demon'.</p> <p><strong>Problem:</strong></p> <p>Baby steps will lead to the final solution. So I initially had the following code:</p> <pre><code>List&lt;Func&lt;Book, bool&gt;&gt; fs = new List&lt;Func&lt;Book, bool&gt;&gt;() { b =&gt; b.Title.Contains("me"), b =&gt; b.Title.Contains("the") }; var q2 = from b in books select b; foreach (var f in fs) q2 = q2.Where(f); foreach (Book b in q2) { Console.WriteLine("Title:\t\t{0}\nPages:\t\t{1}\n", b.Title, b.Pages); } </code></pre> <p>The above code works fine. It looks for books that contain 'the' AND 'me' in the title.</p> <p><strong>Phase 2</strong></p> <p>Now the above filter is of type Func&lt;<strong>Book</strong>, bool>. That class will be an Entity Framework generated class and I don't want to use in my UI layer where the search phrase will be input and search filters will be generated to be passed on to the BLL.</p> <p><strong>So I have the following three attempts:</strong></p> <pre><code>var q = from b in books select b; List&lt;Func&lt;string, bool&gt;&gt; filters = new List&lt;Func&lt;string, bool&gt;&gt;() { s =&gt; s.Contains("me"), s =&gt; s.Contains("the"), }; </code></pre> <hr> <pre><code>//This works... for (int i = 0; i != filters.Count; ++i) { Func&lt;string, bool&gt; daF = filters[i]; q = q.Where(b =&gt; (daF(b.Title))); } </code></pre> <hr> <pre><code> //This produces an exception... //Due to index in query? // for (int i = 0; i != filters.Count; ++i) // { // q = q.Where(b =&gt; ((filters[i])(b.Title))); // } </code></pre> <hr> <pre><code> //This runs but doesn't produce the proper output // foreach (Func&lt;string, bool&gt; filter in filters) // q = q.Where(b =&gt; filter(b.Title)); </code></pre> <hr> <pre><code>foreach (Book b in q) { Console.WriteLine("Title:\t\t{0}\nPages:\t\t{1}\n", b.Title, b.Pages); } </code></pre> <p>The first commented-out piece fires an indexer out of range exception stating that the value of i is 2.</p> <p>The second commented-out piece runs and produces output, but it prints out FOUR of the 5 books... all EXCEPT the book titled "ender's game". That's not right...</p> <p>So, reading over my post, I see that I could not reign in my bad habit of explaining every little detail...</p> <p>So there you go. Please explain why the differing outputs. And I guess you could hint on likely improvements to my current 'solution'.</p>
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