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POWhy does this LINQ expression break my loop & conversion logic?
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  1. POWhy does this LINQ expression break my loop & conversion logic?
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    copied!<h3>Background</h3> <p><code>ArticleService</code> is a class that provides methods for front-end layers to facilitate business with the back-end.</p> <p>Two of its base responsibilities are to convert ViewModels (<code>ArticleViewModel</code>) to the appropriate Models (<code>Article</code>) when persisting data, and the reverse, convert Models to ViewModels when fetching data... so often that I created a private method building ViewModel objects:</p> <pre><code>private ArticleViewModel BuildViewModel(Article a) { return new ArticleViewModel { Title = a.Title /* all properties */ } } </code></pre> <p>Moving along, the <code>ArticleService</code> provides a method to fetch all articles from the data store, returning them as ViewModels: <code>public IEnumerable&lt;ArticleViewModel&gt; All()</code></p> <p>A calling class uses it like this: <code>var articleViewModels = _articleService.All();</code></p> <p>Simple, right?</p> <h3>Problem:</h3> <p>I originally wrote <code>All()</code> lazily with a classic <code>foreach</code> loop:</p> <pre><code>private IEnumerable&lt;ArticleViewModel&gt; All() { var viewModels = new List&lt;ArticleViewModel&gt;(); foreach (var article in _db.Articles) viewModels.Add(BuildViewModel(article)); return viewModels; } </code></pre> <p>It worked fine - <code>articleViewModels</code> was an instantiate list of all view models.</p> <p>Next, I used ReSharper to convert this loop to a LINQ statement for performance and prettiness, and then combined the assignment statement with the return statement. Result:</p> <pre><code>private IEnumerable&lt;ArticleViewModel&gt; All() { return _db.Articles.Select(article =&gt; BuildViewModel(article)).ToList(); } </code></pre> <p>I debugged the LINQ statement and the beast awakened: </p> <blockquote> <p>LINQ to Entities does not recognize the method 'ArticleViewModel BuildViewModel(Article)' and this method cannot be translated into a store expression. </p> </blockquote> <h3>Question - Why does this LINQ statement break my code?</h3> <p><em>Note: Stepping back to an explicit declaration, assignment, return worked with the LINQ statement, so I'm almost certain it's something to do with the lambda logic.</em></p>
 

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