 <p>You have to add it at the end and beginning also because <code>Join</code> will only <em>combine</em> two strings. Having said this when combining <code>"1"</code> and <code>"2"</code> with a speaterator of <code>','</code> you get <code>"1','2"</code> where the seperator is just added <em>between</em> the two elements.</p> <p>As of <a href="https://msdn.microsoft.com/library/57a79xd0%28v=vs.110%29.aspx" rel="nofollow">MSDN</a>:</p> <blockquote> <p>Concatenates all the elements of a string array, using the specified separator <strong>between each element</strong>.</p> </blockquote> <p>In the end the following solution seems to be best in terms of memory and speed because you have least number of string-concatenations (two for start and end and one for every element --> 2 + n). </p> <pre><code>var daysConverted = "'" + string.Join("','", days) + "'"; </code></pre>