Today I have recieved a comment on my article on Optimizing Rendered Page Size, that it is not possible to override the LoadPageStateFromPersistenceMediummethod of a Page class because it is marked as internal in ASP.NET 2.0. Strange thing was that I have made just that in one of the projects I have been working on with ASP.NET 2.0! So I went for some searching.
First thing I have checked the code to see if it really compiles with the override - it did. There is even a support from Intelisense for overriding this method. Next I want to see if the is LoadPageStateFromPersistenceMediummethod really internal. Using reflector I have been able to confirm this. It really is "protected internal virtual". So the question is: how am I able to override it?
The reason we are able to override such a method is that "protected internal" is an OR relationship which means that such a member is accessible in the types that derive from our type AND in all types defined in the assembly. This should make it clear why we are able to override this method.
What may be less clear is why overriding "protected internal" requires sometimes "protected override" instead of "protected internal override". If you read once more the definition, it clearly states that protected internal is accessible to sub-types and the assembly. So if you want to override a method in the same assembly you would use "protected internal override". If on the other hand you want to override this method in other assemblies you must not use the "internal" access modifier because it would make the method visible to every type in your assembly - which would in fact change the visibiliti of the method. This would of course result in an error from the compiler. In this case you would get the CS0507 error.
I hope this sheds some light on the issue.