Common LINQ mistakes

1. Never use

   enumerable.Count() > 0

   
   Instead use

   enumerable.Any()

   
   Count() will enumerate entire collection, Any() will stop after first item.
   
2. Never ever use
   

   from x in enumerable
   orderby x.Item1
   orderby x.Item2
   select x;
   

   Your collection will be ordered by x.Item2, not by x.Item1 then by x.Item2. Instead use:
   

   from x in enumerable
   orderby x.Item1, x.Item2
   select x;
   

   The same is applicable for extensions methods.
   Wrong:
   

   var ordered = enumerable.OrderBy(x=>x.Item1).OrderBy(x=>x.Item2);
   

   Correct:
   

   var ordered = enumerable.OrderBy(x=>x.Item1).ThenBy(x=>x.Item2);
   

3. Do not forget about closures scope.
   

   foreach (var x in enumerable)
   {
    var some = enumerable2.Where(y => y == x);
    list.Add(some);
   }
   

   As far as x is closed, you probably assume that 'some' will depend on each item from 'enumerable'. But actually, 'x' is defined outside of foreach (due to C# implementation), so all 'some's will be calculated using the last value. To prevent this, either fold all enumerables (in general - all lazy calculations) inside blocks, or use locally scoped variables.
   

   foreach (var x in enumerable)
   {
    var local = x; 
    var some = enumerable2.Where(y => y == local);
    list.Add(some);
   }
   

   Fortunately, Resharper will not let you forget about this.