Hi Sumeet,
Source code in the App_Code folder is compiled into a single assembly, all the files in the App_Code folder must be in the same programming language. It can't have different languages at root level or even at child level folders.
To overcome this problem you have to create separate folders for C# and for VB.
Store class files separately in those folders and configure the folder hierarchy in the web.config file.
<compilation>
<codeSubDirectories>
<add directoryName="VBCode" />
<add directoryName="CSCode" />
</codeSubDirectories>
</compilation>
For more details refer below link.
https://msdn.microsoft.com/en-us/library/ms366714.aspx
https://msdn.microsoft.com/en-us/library/t990ks23.aspx
https://www.codeproject.com/Articles/31557/A-Beginner-s-Guide-to-ASP-NET-Application-Folders
