157 recommendations for writing high-quality code to improve C # programs--recommendation 43: Make generic parameters in interfaces support covariant

Recommendation 43: Make the generic parameter in the interface support covariant

In addition to the use of generic parameter-compatible interfaces, as mentioned in the previous recommendation, there is another way to support covariance by adding the Out keyword to the generic declarations in the interface, as follows:

    Interface isalary< out t>    // use out keyword     {        void pay ();    }

        Static void Main (string[] args)        {            isalarynew basesalarycounter<programmer>();            IsalaryNew basesalarycounter<manager>();            Printsalary (s);            Printsalary (t);        }         Static void Printsalary (isalary<employee> s)    // usage correct         {            s.pay ();        }

This code cannot be compiled before FCL4.0, because ienumerable<t> this interface is not declared as Ienumerable<out t> in the FCL;:

 static  void   Main () {IList  <Programmer> programmers = new  list< ;            Programmer> (); IList  <Manager> managers = new  list<manager>();            Printpersonname (programmers);        Printpersonname (managers);  static  void  Printpersonname ( Ienumerable<employee> persons) { foreach  (Employee person in   persons) {Console.WriteLine (person.            Name); }        }

FCL4.0 has modified many interfaces to support covariance, such as Ienumerable<out t>, Ienumerator<out t>, Iquerable<out t>, and so on. Because Ienumerable<out t> now supports covariance, the previous code works well in FCL4.0.

In our own code, if you are writing a generic interface, it is recommended that you add the Out keyword unless you determine that the generic parameter in the interface does not involve a variant. Covariance increases the scope of use of the interface and has little side effects.

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157 recommendations for writing high-quality code to improve C # programs--recommendation 43: Make generic parameters in interfaces support covariant