C# Language Specification

Variance safety

The occurrence of variance annotations in the type parameter list of a type restricts the places where types can occur within the type declaration.

A type T is output-unsafe if one of the following holds:

• 
  T is a contravariant type parameter
  
• 
  T is an array type with an output-unsafe element type
  
• 
  T is an interface or delegate type S1,… A_K> constructed from a generic type S1, .. X_K> where for at least one A_i one of the following holds:
  

• 
  X_i is covariant or invariant and A_i is output-unsafe.
  
• 
  X_i is contravariant or invariant and A_i is input-safe.
  

A type T is input-unsafe if one of the following holds:

• 
  T is a covariant type parameter
  
• 
  T is an array type with an input-unsafe element type
  
• 
  T is an interface or delegate type S1,… A_K> constructed from a generic type S1, .. X_K> where for at least one A_i one of the following holds:
  

• 
  X_i is covariant or invariant and A_i is input-unsafe.
  
• 
  X_i is contravariant or invariant and A_i is output-unsafe.
  

Intuitively, an output-unsafe type is prohibited in an output position, and an input-unsafe type is prohibited in an input position.

A type is output-safe if it is not output-unsafe, and input-safe if it is not input-unsafe.

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