c# - Cast expression failure on Generic -

apologize in advance, such trivial question confused

i have class hierarchy follows

namespace mynamespace {     public class classa {}     public class typea<a> : classa     {         public p1 { get; set; }     }     public class subtypea<a> : typea<a> : classa     {         public typea<a> p2 { get; set; }          public void foo()         {             var x = new typea<classa>();             var y = (typea<classa>) p2;         }     } } 

why can't c# cast p2 typea<classa> while p2 instance of typea<a> a of type classa?

thanks

you're looking covariance , isn't supported on classes, in interfaces , delegates.

design interface follows:

public interface itypea<out a>     : classa {     p1     {         get;     } }  public class classa { }  public class typea<a> : itypea<a> : classa {     public p1     {         get;         set;     } }  public class subtypea<a> : typea<a> : classa {     public typea<a> p2     {         get;         set;     }      public void foo()     {         var x = new typea<classa>();         itypea<classa> y = p2;     } } 

the drawback here property of covariant generic type isn't valid if can set.

type inference madness

i want suggest you shouldn't use type inference on explicit typing everywhere.

see following sentence in question:

var y = (typea<classa>)p2; 

if able whole cast (as test isn't possible without covariance), you're using explicit cast provide type of variable, while same can better expressed follows:

// implicit cast typea<classa> y = p2; 

implicit typing/type inference isn't more readable , shouldn't used specify type, because c# can explicitly define type given reference declaration...

that's reason behind answer doesn't use explicit cast:

itypea<classa> y = p2;