title: Is it a common practice to copy code from the .net libraries?
body: <p>I once saw a project in which at the root element the programmer had folders like <code>MS.Internal</code> or <code>Microsoft.*</code>. He had copied the decompiled source from the .net classes in classes not marked as sealed to modify them or to be able to access them or modifying them in another way. </p>
  
  <p>I wondered wheter this is the common way if there is no workaround for such situations or whether this is even legal? </p>
  
  <p>If marking the classes as sealed and internal should protect us; then why are classes and Methods like <code>BooleanBoxes</code> or <code>DoubleUtil.IsNaN(double)</code> internal? They are good for optimizations.</p>