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Created attachment 4563 [details]
Source code presenting the problem.
Description of Problem:
According to the first comment from http://connect.microsoft.com/VisualStudio/feedback/details/521722/system-activator-createinstance-throws-system-missingmethodexception-with-constructor-containing-an-optional-parameter the attached code should invoke the constructor of A. It works on .NET (tested 3.5, 4.0, 4.5), but on Mono it throws MissingMethodException, due to the default parameter.
Steps to reproduce the problem:
1. Compile the code attached.
MissingMethodException is thrown.
How often does this happen?
There are some differences in behaviour of Type.Missing in different .NET versions. Refer to the question I've asked on StackOverflow for details: http://stackoverflow.com/questions/18105825/bindingflags-optionalparameterbinding-varying-behaviour-in-different-frameworks
According to Hans Passant's comment in the aforementioned stackoverflow question, this might be an implementation detail, while this usage of BindingFlags.OptionalParameterBinding is illegal (as stated in documentation). Tomorrow I'll try to verify a proper example.
Fixed in master