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    <h3>If you're expecting a Single record, it's always good to be explicit in your code.</h3> <p>I know others have written why you use one or the other, but I thought I'd illustrate why you should NOT use one, when you <strong>mean</strong> the other.</p> <p><em>Note: In my code, I will typically use <code>FirstOrDefault()</code> and <code>SingleOrDefault()</code> but that's a different question.</em></p> <p>Take, for example, a table that stores <code>Customers</code> in different languages using a Composite Key ( <code>ID</code>, <code>Lang</code> ): </p> <pre><code>DBContext db = new DBContext(); Customer customer = db.Customers.Where( c=&gt; c.ID == 5 ).First(); </code></pre> <p>This code above introduces a possible logic error ( difficult to trace ). It will return more than one record ( assuming you have the customer record in multiple languages ) but it will always return only the first one... which may work sometimes... but not others. It's unpredictable.</p> <p>Since your intent is to return a Single <code>Customer</code> use <code>Single()</code>;</p> <p>The following would throw an exception ( which is what you want in this case ):</p> <pre><code>DBContext db = new DBContext(); Customer customer = db.Customers.Where( c=&gt; c.ID == 5 ).Single(); </code></pre> <p>Then, you simply hit yourself on the forehead and say to yourself... OOPS! I forgot the language field! Following is the correct version:</p> <pre><code>DBContext db = new DBContext(); Customer customer = db.Customers.Where( c=&gt; c.ID == 5 &amp;&amp; c.Lang == "en" ).Single(); </code></pre> <p><code>First()</code> is useful in the following scenario:</p> <pre><code>DBContext db = new DBContext(); NewsItem newsitem = db.NewsItems.OrderByDescending( n =&gt; n.AddedDate ).First(); </code></pre> <p>It will return ONE object, and since you're using sorting, it will be the most recent record that is returned.</p> <p>Using <code>Single()</code> when you feel it should explicitly always return 1 record will help you avoid logic errors.</p>
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    1. POLINQ Single vs First
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    1. COBoth the Single and First methods can take an expression parameter for filtering, so the Where function is not necessary. Example: Customer customer = db.Customers.Single( c=> c.ID == 5 );
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      1. USJosh Noe
    2. CO@JoshNoe - I'm curious, is there a difference between `customers.Where(predicate).Single()` `customers.Single(predicate)`?
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    3. CO@drzaus - Logically, no, they both filter the values to be returned based on the predicate. However, I checked out the disassembly, and Where(predicate).Single() has three extra instructions in the simple case I made. So, while I'm no IL expert, but it appears that customers.Single(predicate) should be more efficient.
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