C# Language Specification

Variance conversion

The purpose of variance annotations is to provide for more lenient (but still type safe) conversions to interface and delegate types. To this end the definitions of implicit (§6.1) and explicit conversions (§6.2) make use of the notion of variance-convertibility, which is defined as follows:

A type T1, …, A_n> is variance-convertible to a type T1, …, B_n> if T is either an interface or a delegate type declared with the variant type parameters T1, …, X_n>, and for each variant type parameter X_i one of the following holds:

• 
  X_i is covariant and an implicit reference or identity conversion exists from A_i to B_i
  
• 
  X_i is contravariant and an implicit reference or identity conversion exists from B_i to A_i
  
• 
  X_i is invariant and an identity conversion exists from A_i to B_i
  

4.