Mixing C# & VB In The Same Project

No, you can't. An assembly/project (each project compiles to 1 assembly usually) has to be one language. However, you can use multiple assemblies, and each can be coded in a different language because they are all compiled to CIL.

It compiled fine and didn't complain because a VB.NET project will only actually compile the .vb files and a C# project will only actually compile the .cs files. It was ignoring the other ones, therefore you did not receive errors.

Edit: If you add a .vb file to a C# project, select the file in the Solution Explorer panel and then look at the Properties panel, you'll notice that the Build Action is 'Content', not 'Compile'. It is treated as a simple text file and doesn't even get embedded in the compiled assembly as a binary resource.

Edit: With asp.net websites you may add c# web user control to vb.net website

Well, actually I inherited a project some years ago from a colleague who had decided to mix VB and C# webforms within the same project. That worked but is far from fun to maintain.

I decided that new code should be C# classes and to get them to work I had to add a subnode to the compilation part of web.config

        <codeSubDirectories>
            <add directoryName="VB"/>
            <add directoryName="CS"/>
        </codeSubDirectories>

The all VB code goes into a subfolder in the App_Code called VB and the C# code into the CS subfolder. This will produce two .dll files. It works, but code is compiled in the same order as listed in "codeSubDirectories" and therefore i.e Interfaces should be in the VB folder if used in both C# and VB.

I have both a reference to a VB and a C# compiler in

<system.codedom>
    <compilers>

The project is currently updated to framework 3.5 and it still works (but still no fun to maintain..)

You can not mix vb and c# within the same project - if you notice in visual studio the project files are either .vbproj or .csproj. You can within a solution - have 1 proj in vb and 1 in c#.

Looks like according to this you can potentially use them both in a web project in the App_Code directory:

http://pietschsoft.com/post/2006/03/30/ASPNET-20-Use-VBNET-and-C-within-the-App_Code-folder.aspx