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POEntity Framework - expression on similar properties
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  1. POEntity Framework - expression on similar properties
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    copied!<p>I'm still fairly new to Entity Framework (currently developing using v4.1), so I may be going about this in completely the wrong way, so I'd welcome any guidance. </p> <p>I currently have two classes: <strong>Item</strong> which just has a few simple fields, and <strong>FavouriteItem</strong> which holds a reference to an item and a user (but the user relationship isn't relevant to the discussion). The item has a nullable <strong>DateVisible</strong> field which I'm using as a flag to indicate if an item should be displayed or not.</p> <p>In order to get a list of visible items, I'm using a simple query:</p> <pre><code>var items = itemRepository .QueryAll() .Where(i =&gt; i.DateVisible &lt;= DateTime.Now) </code></pre> <p>Where obviously QueryAll returns an IQueryable. This is a straightforward example, as the conditions are a little more complex, but it demonstrates my issue.</p> <p>Next, if I want to return a list of favourites for items which are visible, then I can do a similar query:</p> <pre><code>var favourites= favouriteRepository .QueryAll() .Where(f =&gt; f.Item.DateVisible &lt;= DateTime.Now) </code></pre> <p>This works fine. However, the code is duplicated. So what I'd like to do is somehow make this code more central - as I'm about to add a <strong>FollowingItem</strong> class which again has an Item property, and so I'd have to repeat the code again - not good.</p> <p>I started to create an expression for the list of items, which works well:</p> <pre><code>public static Expression&lt;Func&lt;Item, bool&gt;&gt; IsVisibleExpression&lt;Item&gt;() { return item =&gt; item.DateVisible != null &amp;&amp; item.DateVisible &lt;= DateTime.Now; } public static IQueryable&lt;Item&gt; WhereVisible&lt;Item&gt;(this IQueryable&lt;Item&gt; queryable) { return queryable.Where(Item.IsVisibleExpression()); } var items = itemRepository .QueryAll() .WhereVisible() </code></pre> <p>And again, I can create these expressions for each class:</p> <pre><code>public static Expression&lt;Func&lt;FavouriteItem, bool&gt;&gt; IsVisibleExpression&lt;FavouriteItem&gt;() { return favourite =&gt; favourite.Item.DateVisible != null &amp;&amp; favourite.Item.DateVisible &lt;= DateTime.Now; } </code></pre> <p>But this again just duplicates the code. Is there any way the same code be used between them. I know I could do a join on the <code>itemRepository.QueryAll().WhereVisible()</code>, but is there not a way to do this that doesn't involve doing this everywhere?</p> <p><strong>Update</strong>: I tried creating an interface for the classes:</p> <pre><code>public interface IEntityWithItem { Item Item { get; set; } } </code></pre> <p>And creating an expression for it:</p> <pre><code> public static IQueryable&lt;TEntity&gt; WhereItemVisible&lt;TEntity&gt;(this IQueryable&lt;TEntity&gt; queryable) where TEntity : IEntityWithItem { return queryable.Where(ui =&gt; ui.Item.DateVisibleFrom != null &amp;&amp; ui.Item.DateVisibleFrom.Value &lt;= DateTime.Now); } </code></pre> <p>Which I've called from:</p> <pre><code>// Get the favourites var favourites = favouriteItemRepository.QueryAll() .WhereItemVisible() .Where(favourite =&gt; favourite.UserId == user.Id); </code></pre> <p>But it gives the error: "<em>Unable to cast the type 'FavouriteItem' to type 'IEntityWithItem'. LINQ to Entities only supports casting Entity Data Model primitive types.</em>"</p>
 

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