ASP.net where to add assembly to web.config while only compiling dll?

I need to compile a class library (DLL) from an ASP.NET source. Now when I try to build it says this:

The type 'System.ComponentModel.ISupportInitialize' is defined in an asse开发者_如何学JAVAmbly that is not referenced. You must add a reference to assembly 'System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.

Now I know that I have to put it in the web.config file. But there is no web.config file it's just the source of the dll that needs to be compiled.

How can I do this?

You should follow the directions in the error message and add a reference to the System.dll assembly in your class library project.

Right-click on your project, select Add Reference... in the context menu, then choose the System entry in the .NET tab.