How to find path of active app.config file?

Try this

AppDomain.CurrentDomain.SetupInformation.ConfigurationFile

If you mean you are only getting a null return when you use NUnit, then you probably need to copy the ConnectionString value the your app.config of your application to the app.config of your test library.

When it is run by the test loader, the test assembly is loaded at runtime and will look in its own app.config (renamed to testAssembly.dll.config at compile time) rather then your applications config file.

To get the location of the assembly you're running, try

System.Reflection.Assembly.GetExecutingAssembly().Location

Strictly speaking, there is no single configuration file. Excluding ASP.NET¹ there can be three configuration files using the inbuilt (System.Configuration) support. In addition to the machine config: app.exe.config, user roaming, and user local.

To get the "global" configuration (exe.config):

ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None)
                    .FilePath

Use different ConfigurationUserLevel values for per-use roaming and non-roaming configuration files.

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¹ Which has a completely different model where the content of a child folders (IIS-virtual or file system) web.config can (depending on the setting) add to or override the parent's web.config.