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copied!<p>There is actually a (subtle) difference between the two. Imagine you have the following code in File1.cs:</p> <pre><code>// File1.cs using System; namespace Outer.Inner { class Foo { static void Bar() { double d = Math.PI; } } } </code></pre> <p>Now imagine that someone adds another file (File2.cs) to the project that looks like this:</p> <pre><code>// File2.cs namespace Outer { class Math { } } </code></pre> <p>The compiler searches <code>Outer</code> before looking at those <code>using</code> directives outside the namespace, so it finds <code>Outer.Math</code> instead of <code>System.Math</code>. Unfortunately (or perhaps fortunately?), <code>Outer.Math</code> has no <code>PI</code> member, so File1 is now broken.</p> <p>This changes if you put the <code>using</code> inside your namespace declaration, as follows:</p> <pre><code>// File1b.cs namespace Outer.Inner { using System; class Foo { static void Bar() { double d = Math.PI; } } } </code></pre> <p>Now the compiler searches <code>System</code> before searching <code>Outer</code>, finds <code>System.Math</code>, and all is well.</p> <p>Some would argue that <code>Math</code> might be a bad name for a user-defined class, since there's already one in <code>System</code>; the point here is just that there <em>is</em> a difference, and it affects the maintainability of your code.</p> <p>It's also interesting to note what happens if <code>Foo</code> is in namespace <code>Outer</code>, rather than <code>Outer.Inner</code>. In that case, adding <code>Outer.Math</code> in File2 breaks File1 regardless of where the <code>using</code> goes. This implies that the compiler searches the innermost enclosing namespace before it looks at any <code>using</code> directive.</p>

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