vb.net - Return an inherited type from a method -

vb.net - Return an inherited type from a method -

suppose have next classes defined:

public class baseclass ... end class public class derivedclass inherits baseclass ... fields, methods, etc ... end class

and then, in code, have function signature of:

public function dosomething(...) list(of baseclass)

and when seek , homecoming object of type list(of derivedclass) it, error:

value of type 'system.collections.generic.list(of baseclass)' cannot converted 'system.collections.generic.list(of derivedclass)'

i know not fields of derivedclass filled, give me needed.

is there way this, or considered bad programming practice? and, if so, right way this?

have @ this:

http://msdn.microsoft.com/en-us/library/dd799517(v=vs.110).aspx

understanding covariance , contravariance clear things bit :)

covariance

enables utilize more specific type specified. can assign instance of ienumerable (ienumerable(of derived) in visual basic) variable of type ienumerable.

example:

ienumerable<derived> d = new list<derived>(); ienumerable<base> b = d; contravariance

enables utilize more generic (less derived) type specified. can assign instance of ienumerable (ienumerable(of base) in visual basic) variable of type ienumerable.

example:

action<base> b = (target) => { console.writeline(target.gettype().name); }; action<derived> d = b; d(new derived()); invariance

means can utilize type specified; invariant generic type parameter neither covariant nor contravariant. cannot assign instance of ienumerable (ienumerable(of base) in visual basic) variable of type ienumerable or vice versa.

vb.net visual-studio