Hi,
This is strange. We develop here with Windows7 x64 with the latest patches, and haven't seen any issue.WindowsBase is indeed only used in xlsx, but we haven't seen problems here. And I can't think on any bug loading WindowsBase.dll, since we don't load it ourselves, it is just a referenced assembly like all the others.
Could you have some corrupted windowsbase, or one with the wrong permissions?
I have tried it here with the latest patches setting FlexCel to anycpu, x86 or x64 and it always worked fine. (of course it didn't work if you tried to call an x86 assembly from an x64 app or viceversa, but that is expected, you can't call 64bit code from 32 or 32 from 64).
In any case, there is no reason to compile FlexCel in anything else than AnyCpu, since it is a fully managed assembly and it will run in 64bits when called from 64bits, and in 32bits when called form 32 bits. And you can't mix 64bit with 32 anyway.
(If I understood correctly and you were compiling FlexCel in x86). If it is your own application which you set to AnyCpu, that is not recommended by Microsoft, and that's why it defaults to x86 for applications you create. AnyCpu used in apps can bring trouble since some of your users will be using a 32bit app and some others a 64 bit app doubling your possible bugs.
Back to the problem, are you seeing this in more than 1 machine? As said, we didn't see it here and we haven't got any report from any other customer about an issue like this. Couldn't this be some permissions you changed in your machine?
Also is this .NET 3.5 or 4?
Regards,
Adrian.