I would like to clarify that at this point we are not planning to convent the Exchange provider in VB. A valid workaround would be to to create two sub-folders in the App_Code and separate your C# and VB files in each folder and declare those folders in the web.config files are shown in this help article
Alternative valid approach will be to create a class library project that contains the Exchange provider classes. You can refer this library to your web site. This will include the class library dll files in your web site Bin folder and you are ready to use the Exchange provider in your VB web site. Of course you are absolutely able to add vb files to your web site's App_Code folder and they will be compiled without any issues.
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