Do not write code like:

      if (x.ToLower() == y.ToLower()) { } // Compare two strings case insensitive

or

      Hashtable x = new Hashtable();
      if (x.ContainsKey(y.ToUpper()) { }  // Compare Hasttable key case insensive.

Using ToLower() or ToUpper() create a new string which is not required, just use:

     if (String.Compare(x, y, true) == 0) { }

or

     Hashtable x = new Hashtable(CaseInsensitiveCompare.DefaultInvariant); // Use case insensive IEqualityComparer 
     if (x.ContainsKey(y)) { }

Note: for .NET 1.1 and older use Hashtable constructor with IHashCodeProvider and IComparer.