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POHow to check for the presence of an OrderBy in a ObjectQuery<T> expression tree
 

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  1. POHow to check for the presence of an OrderBy in a ObjectQuery<T> expression tree
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    <p>I'm using T4 for generating repositories for LINQ to Entities entities. </p> <p>The repository contains (amongst other things) a List method suitable for paging. The documentation for <a href="http://msdn.microsoft.com/en-us/library/bb738474.aspx" rel="nofollow noreferrer">Supported and Unsupported Methods</a> does not mention it, but you can't "call" <code>Skip</code> on a unordered <code>IQueryable</code>. It will raise the following exception:</p> <blockquote> <p>System.NotSupportedException: The method 'Skip' is only supported for sorted input in LINQ to Entities. The method 'OrderBy' must be called before the method 'Skip'..</p> </blockquote> <p>I solved it by allowing to define a default sorting via a partial method. But I'm having problems checking if the expression tree indeed contains an <code>OrderBy</code>.</p> <p>I've reduced the problem to as less code as possible:</p> <pre><code>public partial class Repository { partial void ProvideDefaultSorting(ref IQueryable&lt;Category&gt; currentQuery); public IQueryable&lt;Category&gt; List(int startIndex, int count) { IQueryable&lt;Category&gt; query = List(); ProvideDefaultSorting(ref query); if (!IsSorted(query)) { query = query.OrderBy(c =&gt; c.CategoryID); } return query.Skip(startIndex).Take(count); } public IQueryable&lt;Category&gt; List(string sortExpression, int startIndex, int count) { return List(sortExpression).Skip(startIndex).Take(count); } public IQueryable&lt;Category&gt; List(string sortExpression) { return AddSortingToTheExpressionTree(List(), sortExpression); } public IQueryable&lt;Category&gt; List() { NorthwindEntities ent = new NorthwindEntities(); return ent.Categories; } private Boolean IsSorted(IQueryable&lt;Category&gt; query) { return query is IOrderedQueryable&lt;Category&gt;; } } public partial class Repository { partial void ProvideDefaultSorting(ref IQueryable&lt;Category&gt; currentQuery) { currentQuery = currentQuery.Where(c =&gt; c.CategoryName.Contains(" ")); // no sorting.. } } </code></pre> <p>This is not my real implementation!</p> <p>But my <strong>question</strong> is, how could I implement the <code>IsSorted</code> method? The problem is that LINQ to Entities query's are always of the type <code>ObjectQuery</code>, which implements <code>IOrderedQueryable</code>.</p> <p>So how should I make sure an <code>OrderBy</code> method is present in the expression tree? Is the only option to parse the tree?</p> <p><strong>Update</strong><br> I've added two other overloads to make clear that it's not about how to add sorting support to the repository, but how to check if the <code>ProvideDefaultSorting</code> partial method has indeed added an <code>OrderBy</code> to the expression tree.</p> <p>The problem is, the first partial class is generate by a template and the implementation of the second part of the partial class is made by a team member at another time. You can compare it with the way the .NET Entity Framework generates the EntityContext, it allows extension points for other developers. So I want to try to make it robust and not crash when the <code>ProvideDefaultSorting</code> is not implemented correctly.</p> <p>So maybe the question is more, how can I confirm that the <code>ProvideDefaultSorting</code> did indeed add sorting to the expression tree.</p> <p><strong>Update 2</strong><br> The new question was answered, and accepted, I think I should change the title to match the question more. Or should I leave the current title because it will lead people with the same problem to this solution?</p>
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