Summary of the inverse and covariance of generics

Generalization of the inverse and covariance of generics C # generic Inverse covariant

• The concept of change
• Covariant (foo< parent class > = foo< subclass >)
• Contravariance (foo< Subclass > = foo< parent >)
• The interaction between contravariance and covariance

The concept of change

2. //Parent class = Subclass
4. string str = "string";
6. object obj = str; //Changed

Covariant (foo< parent class > = foo< subclass >)

2. //Generic delegate:
4. public delegate T myfunca<t> (); //does not support contravariance and covariance  
6. Public Delegate T myfuncb< out t> (); //Support co-change
10. myfunca<Object> funcaobject = null;
12. myfunca<string> funcastring = null;
14. myfuncb<Object> funcbobject = null;
16. myfuncb<string> funcbstring = null;
20. Funcaobject = funcastring; //Compile failed, Myfunca does not support contravariance and covariance
22. Funcbobject = funcbstring; //changed, covariant

Contravariance (foo< Subclass > = foo< parent >)

2. public delegate void myactiona<t > (T param); //does not support contravariance and covariance  
4. Public Delegate void myactionb< in t> (T param); //Support inverter
8. myactiona<Object> actionaobject = null;
10. myactiona<string> actionastring = null;
12. myactionb<Object> actionbobject = null;
14. myactionb<string> actionbstring = null;
18. actionastring = Actionaobject; //myactiona does not support contravariance and covariance, compilation fails
20. actionbstring = Actionbobject; //changed, invert

If the In keyword is added as a generic modifier on a generic parameter, then that generic parameter can only be used as the input parameter of the method, or as a parameter of a write-only property, not as a method return value, etc., in short, can only be "in", not out. The Out keyword instead.

The interaction between contravariance and covariance

2. Public Interface IBar< in T> {}
4. //should be in
6. Public Interface IFoo< in T>
8. {
10. void Test(ibar<t> bar);
12. }
14. //or out
16. Public Interface IFoo< out T>
18. {
20. void Test(ibar<t> bar);
22. }

IFoo with covariant capability requires that its generic parameters (IBar) have the inverse capability

Summary of the inverse and covariance of generics