if( whatever is IDisposable) {
  (whatever as IDisposable).Dispose();
}

if( whatever is IDisposable) { //check
  ((IDisposable)whatever).Dispose(); //cast - won't fail
}

IDisposable whateverDisposable = whatever as IDisposable;
if( whateverDisposable != null ) {
   whateverDisposable.Dispose();
}

if(whatever is IDisposable)  //check
{
  ((IDisposable)whatever).Dispose(); //cast - won't fail
}

if(whatever is IDisposable)
{
  (whatever as IDisposable).Dispose();
}

IDisposable whateverDisposable = whatever as IDisposable;
if(whateverDisposable != null )
{
    whateverDisposable.Dispose();
}

if (whatever is IDisposable) { //check
    // <-- here, some other thread changes the value of whatever
    ((IDisposable)whatever).Dispose(); // could fail
}

IDisposable whateverDisposable = whatever as IDisposable;
if( whateverDisposable != null)
{
   whateverDisposable.Dispose();
}

public static void DisposeIfNecessary(this object obj)
{
   if (obj != null && obj is IDisposable)
      ((IDisposable)obj).Dispose();
}