Compiler Error CS0023

Operator 'operator' cannot be applied to operand of type 'type'

An attempt was made to apply an operator to a variable whose type was not designed to work with the operator. For more information, see Types (C# Programming Guide) and C# Operators.

The following sample generates CS0023:

// CS0023.cs
namespace x
{
   public class a
   {
      public static void Main()
      {
         string s = "hello";
         s = -s;   // CS0023, minus operator not allowed on strings
      }
   }
}