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POLINQ-SQL reuse - CompiledQuery.Compile
 

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    <p>I have been playing about with LINQ-SQL, trying to get re-usable chunks of expressions that I can hot plug into other queries. So, I started with something like this:</p> <pre><code>Func&lt;TaskFile, double&gt; TimeSpent = (t =&gt; t.TimeEntries.Sum(te =&gt; (te.DateEnded - te.DateStarted).TotalHours)); </code></pre> <p>Then, we can use the above in a LINQ query like the below (LINQPad example):</p> <pre><code>TaskFiles.Select(t =&gt; new { t.TaskId, TimeSpent = TimeSpent(t), }) </code></pre> <p>This produces the expected output, <em>except, a query per row is generated for the plugged expression</em>. This is visible within LINQPad. Not good.</p> <p>Anyway, I noticed the <code>CompiledQuery.Compile</code> method. Although this takes a <code>DataContext</code> as a parameter, I thought I would include ignore it, and try the same <code>Func</code>. So I ended up with the following:</p> <pre><code>static Func&lt;UserQuery, TaskFile, double&gt; TimeSpent = CompiledQuery.Compile&lt;UserQuery, TaskFile, double&gt;( (UserQuery db, TaskFile t) =&gt; t.TimeEntries.Sum(te =&gt; (te.DateEnded - te.DateStarted).TotalHours)); </code></pre> <p>Notice here, that I am not using the <code>db</code> parameter. However, now when we use this updated parameter, only <strong>1</strong> SQL query is generated. The Expression is successfully translated to SQL and included within the original query.</p> <p>So my ultimate question is, what makes <code>CompiledQuery.Compile</code> so special? It seems that the <code>DataContext</code> parameter isn't needed at all, and at this point i am thinking it is more a convenience parameter to generate full queries.</p> <p>Would it be considered a good idea to use the <code>CompiledQuery.Compile</code> method like this? It seems like a big hack, but it seems like the only viable route for LINQ re-use.</p> <p><strong>UPDATE</strong></p> <p>Using the first <code>Func</code> within a <code>Where</code> statment, we see the following exception as below:</p> <pre><code>NotSupportedException: Method 'System.Object DynamicInvoke(System.Object[])' has no supported translation to SQL. </code></pre> <p>Like the following:</p> <pre><code>.Where(t =&gt; TimeSpent(t) &gt; 2) </code></pre> <p>However, when we use the <code>Func</code> generated by <code>CompiledQuery.Compile</code>, the query is successfully executed and the correct SQL is generated.</p> <p>I know this is not the ideal way to re-use <code>Where</code> statements, but it shows a little how the Expression Tree is generated.</p>
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