Permitted user-defined conversions

C# permits only certain user-defined conversions to be declared. In particular, it is not possible to redefine an already existing implicit or explicit conversion. A class or struct is permitted to declare a conversion from a source type S to a target type T only if all of the following are true: S and T are different types. Either S or T is the class or struct type in which the operator declaration takes place. Neither S nor T is object or an interface-type. T is not a base class of S, and S is not a base class of T.
The restrictions that apply to user-defined conversions are discussed further in 17.9.3.