Interface cannot contain data fields - they can contain only abstract properties.

Interfaces consist of methods, properties, events, indexers, or any combination of those four member types. An interface cannot contain constants, fields, operators, instance constructors, destructors, or types. It cannot contain static members. Interfaces members are automatically public, and they cannot include any access modifiers.


http://msdn.microsoft.com/en-us/library/ms173156.aspx