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POHow to check if IQueryable<T> has OrderBy applied before before attempting Skip() and Take()
 

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  1. POHow to check if IQueryable<T> has OrderBy applied before before attempting Skip() and Take()
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    <p>I am trying to build an extension method that will Paginated a query. But in order to avoid exception: System.NotSupportedException was unhandled by user code Message=The method 'Skip' is only supported for sorted input in LINQ to Entities. The method 'OrderBy' must be called before the method 'Skip'.</p> <p>I'd like to check if OrderBy was applied and if not just return the query... Something like this:</p> <pre><code>public static IQueryable&lt;T&gt; Paginate&lt;T&gt;(this IQueryable&lt;T&gt; query, int page, int pageSize = 50, int total = -1) { // check if OrderBy was applied // THIS DOES NOT WORK!!! //try //{ // var orderedQueryable = query as IOrderedQueryable&lt;T&gt;; //} //catch (Exception) //{ // // if the cast throws OrderBy was not applied &lt;-- DOES NOT WORK!!! // return query; //} page = (page &lt; 1) ? 1 : page; var limit = (pageSize &lt;= 0 || (total &gt;= 0 &amp;&amp; pageSize &gt; total)) ? 50 : pageSize; var skip = (page - 1)*limit; return query.Skip(skip).Take(limit); } </code></pre> <p>To make things more <em>interesting</em>, I am using Mycrosoft's <a href="https://nuget.org/packages/System.Linq.Dynamic" rel="nofollow">Dynamic Expression API</a> (aka Dynamic LINQ) so my calling code looks like</p> <pre><code>return query .OrderBy("Customer.LastName DESC, Customer.FirstName") .Paginate(1,25, 2345) .ToArray(); </code></pre> <p>or I can invoke it using strongly typed expressions like this</p> <pre><code>return query .OrderByDescending(c=&gt;c.LastName) .ThenBy(c=&gt;c.FirstName) .Paginate(1,25,2345) .ToArray(); </code></pre> <p>Is this type of checking possible? I tired using <code>IOrderableQueryable&lt;T&gt;</code> in method signature but Dynamic Linq does not return <code>IOrderableQueryable</code> when you use OrderBy(string expression) so extension would not apply...</p> <h2>UPDATE</h2> <p>Using suggestion (pointed out by @GertArnold) the only workable solution was to inspect the expression tree. However, instead of making use of entire <code>ExpressionVisitor</code> I simplified my solution by requiring that Paginate method must be called just after <code>OrderBy</code> or <code>OrderByDescending</code>. This allowed me to check only current node in expression tree instead of searching the whole tree. So this is what I did:</p> <pre><code>// snip....class level private static readonly string[] PaginationPrerequisiteMehods = new[] { "OrderBy", "OrderByDescending" }; // snip Paginate method public static IQueryable&lt;T&gt; Paginate&lt;T&gt;(this IQueryable&lt;T&gt; query, int page, int pageSize = 50, int total = -1) { // require that either OrderBy or OrderByDescending was applied just before calling Paginate.... if (query.Expression.NodeType != ExpressionType.Call) { //TODO: logging -&gt; "You have to apply OrderBy() or OrderByDescending() just before calling Paginate()" return query; } var methodName = ((MethodCallExpression) query.Expression).Method.Name; if (!Array.Exists(PaginationPrerequisiteMehods, s =&gt; s.Equals(methodName, StringComparison.InvariantCulture))) { //TODO: logging -&gt; "You have to apply OrderBy() or OrderByDescending() just before calling Paginate()" return query; } page = (page &lt; 1) ? 1 : page; var limit = (pageSize &lt;= 0 || (total &gt;= 0 &amp;&amp; pageSize &gt; total)) ? 50 : pageSize; var skip = (page - 1)*limit; return query.Skip(skip).Take(limit); } </code></pre>
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    1. COWhile not the crux of your issue, note that the expression `query as IOrderedQueryable<T>` in the commented-out section will not throw an exception if the conversion cannot be performed. It will return `null` instead. (In a failed conversion, the `as` operator evaluates to `null` while a parenthetical cast will throw an exception.) That entire block can be replaced with `if (!(query is IOrderedQueryable<T>)) { return query; }`. Not that this will solve your problem.
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    2. CO@cdhowie you are correct - it does not solve the issue. For some reason when 'as' conversion is performed I don't get null even when OrderBy is not applied. So null check does not help
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