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  • Covariance and Contra-variance in C#

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    Relation between generic type parameter and generic type definition is known as variance.

    Covariance and contravariance are extensions for generic type parameters for arrays, delegates and interfaces.

    1.COVARIANCE-  In Covariance 'out' keyword is used to declare input parameter. It keeps the work compatible. In covariance, where IEnumerable<object> is expected  we can use IEnumerable<string> in place of that.

     

    e.g of covariance:-

     

    public interface IEnumerable<out T> 
    { 
    IEnumerator<T> GetEnumerator(); 
    } 
    public interface IEnumerator<out T>
     { 
       T Current { get; } 
       bool MoveNext(); 
    }

     

    2.CONTRAVARIANCE:- The functions of contravariance are just different from covariance. Input parameters are passed by using 'In' keyword.

     

    e.g of contravariance:-

     

    void DoSomethingToAFrog(Action<Frog> action, Frog frog) 
    { 
    action(frog); }
     ... Action<Animal> feed = animal=>{animal.Feed();} 
     DoSomethingToAFrog(feed, new Frog());

     

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