1. A computer-readable storage medium containing computer-executable instructions implementing a framework for declarative expression of data processing, the framework comprising: an interface that is accessible by an application to access data from multiple data stores that store data in different formats; a metadata-driven infrastructure comprising declarative representations of metadata for the data supported by the framework, the declarative representations of metadata specifying classes of data with properties, class validation rules and property validation rules, and a mapping of classes to data stores; and a data store adapter for each of the multiple data stores that provides access to data of the data store by performing conversions between data of a class mapped to the data store and data of the data store; and a framework engine being operable to process a call from the application via the interface to access data of the multiple data stores based on the classes specified in the declarative representations by utilizing the metadata-driven infrastructure to validate data of a class and data of the properties of the class based on the declarative representation of metadata specifying class validation rules and property validation rules, to identify using the mapping a data store adapter for accessing data of the class, and to invoke the identified data store adapter to access data of the class as represented by the call from the application wherein the validation rules are stored in a declarative manner and the validation process is decoupled from the application.