ConstraintExpressionBase is the abstract base class for the generated ConstraintExpression class, which represents a compound constraint in the process of being constructed from a series of syntactic elements. NOTE: ConstraintExpressionBase is aware of some of its derived classes, which is an apparent violation of encapsulation. Ideally, these classes would be a single class, but they must be separated in order to allow parts to be generated under .NET 1.x and to provide proper user feedback in syntactically aware IDEs.