c# - Extension methods with base and sub-classes -

update

requesting re-open because other answers don't have solution, 1 of comments question has solution want accept works scenario.

original question

i having trouble writing extension methods non-abstract base classes , sub-classes select appropriate extension method.

example code

i have simple example below (abstracted bigger project) uses extension method "run". expected output listed in comment next each class.

public class parent { }; // should output "parent" public class childa : parent { }; // should output "child a" public class childb : parent { }; // should output "parent"  // expected output: childa, parent, parent public class program {     public static void main()     {         var commands = new list<parent>() { new childa(), new childb(), new parent() };         console.writeline(string.join(", ", commands.select(c => c.run())));     } } 

here attempts far, there has cleaner way this:

1. no type-checking - results in parent extension method being used exclusively (parent, parent, parent)
2. explicit type checking - right output, have explicitly check types each extension possibility (childa, parent, parent)
3. attempted convert.changetype dynamic type - run-time exception since extension won't catch dynamic type (no output)
4. attempted generic cast reflection - not quite operational yet, not sure if approach valid

list of attempts

public static class extensions {     public static string run(this childa model)     {         return "childa";     }     public static string run(this parent model)     {         return model.run1(); // change test different approaches     }     public static string run1(this parent model) // no type-checking     {         return "parent";     }     public static string run2(this parent model) // explicitly check sub-types     {         if (model childa)             return ((childa)model).run();         else             return "parent";     }     public static string run3(this parent model) // attempted dynamic type conversion     {         if (model.gettype().basetype == typeof(parent))         {             dynamic changedobj = convert.changetype(model, model.gettype());             return changedobj.run();         }         else             return "parent";     }     public static string run4(this parent model) // attempted reflected generic type conversion     {         if (model.gettype().basetype == typeof(parent))         {             var method = typeof(extensions).getmethod("cast");             var generic = method.makegenericmethod(new[] { model.gettype() });             //var generic = generic.invoke(new object(), null);             //return generic.run();             return "not working yet";         }         else             return "parent";     }     public static t cast<t>(this object input)     {         return (t) input;        }  } 

creating 2 extension methods parent , childa, can move association runtime using dynamic.

console.writeline(string.join(", ", commands.select(c => extensions.run(c dynamic))));