C# Language Specification

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.

For a given source type S and target type T, if S or T are nullable types, let S₀ and T₀ refer to their underlying types, otherwise S₀ and T₀ are equal to S and T respectively. 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:

• 
  S₀ and T₀ are different types.
  
• 
  Either S₀ or T₀ is the class or struct type in which the operator declaration takes place.
  
• 
  Neither S₀ nor T₀ is an interface-type.
  
• 
  Excluding user-defined conversions, a conversion does not exist from S to T or from T to S.
  

The restrictions that apply to user-defined conversions are discussed further in §10.10.3.

2.