Basically, the way method invocation works in C# is that the compiler looks at the most derived class first, and sees whether any newly declared methods (not including overrides) are applicable for the arguments for the call. If there's at least one applicable method, overload resolution works out which is the best one. If there isn't, it tries the base class, and so on.
I agree this is surprising - it's an attempt to counter the "brittle base class" issue, but I would personally prefer that any overridden methods were included in the candidate set.
Method invocation is described in section 7.6.5.1 of the C# 5 specification. The relevant parts here is:
- The set of candidate methods is reduced to contain only methods from the most derived types: For each method
C.F
in the set, where C
is the type in which the method F
is declared, all methods declared in a base type of C
are removed from the set. Furthermore, if C
is a class type other than object, all methods declared in an interface type are removed from the set. (This latter rule only has affect when the method group was the result of a member lookup on a type parameter having an effective base class other than object and a non-empty effective interface set.)
And in the member lookup part of 7.4, override
methods are explicitly removed:
Members that include an override
modifier are excluded from the set.
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