this won't work because there is no implicit conversion between
ienumerable<notifypropertychanging>. you need to call:
the reason behind this is type safety. in c#4, co-/contravariance was added to the language, which generally helps making these type of conversions valid. there are some limitations though, since the type involved needs to be declared co-/contravariant, and it only applies to interfaces and delegates.
the reason why you can't implicitly convert
list<tbase> is, that it would make this code valid:
list<t> values = new list<t>(); // add some values... list<tbase> bases = values; bases.add(new tbase()); // woops. we broke type safety by adding a tbase to a list of t's
now, if you tried to do this when
values was of type
ienumerable<t>, it would be valid in c#4. this is because you can only get values out of
ienumerable<t>, so we don't need to worry about lesser types being added. therefore
ienumerable<t> is covariant, and is declared as:
out means "covariant". (the keywords is actually the best way to remember which way values may go in a variant type.
co-/contravariance is a rather large subject, but this article does a good job of explaining the most important parts. and if you want to learn more, eric lippert has an 11-part blog post series about the feature. eric is one of the language designers.
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