External AppSettings File NOT merging with web.config

You can access multiple config files by using WebConfigurationmanager method. add namespace:

using System.Web.Configuration;

So, to access the appSettings of

../SomeProjectFolder/Environment/Web.config, you can do:

var config = WebConfigurationManager.OpenWebConfiguration("~/SomeProjectFolder/Environment/");
string username = config.AppSettings.Settings["username"].Value;

Hope this helps.

I was able to confirm that the external app.config works with a simple project.

app.Config (in same directory as web.config)

<appSettings>
  <add key="testAppConfigString" value="APP string exists!"/>
  <add key="testOverrideString" value="!!OVERRIDE string exists in app.config!"/>
</appSettings>

web.config

...
  <appSettings file="app.config">
    <add key="testWebConfigString" value="web config string exists!"/>
    <add key="testOverrideString" value="OVERRIDE string exists in web.config!"/>
  </appSettings>
...

Default.aspx

...
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">

    web: <asp:Label runat="server" ID="lblWeb" /><br/>
    app: <asp:Label runat="server" ID="lblApp" /><br/>
    override: <asp:Label runat="server" ID="lblOverride" /><br/>

</asp:Content>
... 

Inside the Default.aspx.cs

    protected void Page_Load(object sender, EventArgs e)
    {
        lblWeb.Text = ConfigurationManager.AppSettings["testWebConfigString"];
        lblApp.Text = ConfigurationManager.AppSettings["testAppConfigString"];
        lblOverride.Text = ConfigurationManager.AppSettings["testOverrideString"];
    }

The resulting page should have the following text:

web: web config string exists!
app: APP string exists!
override: !!OVERRIDE string exists in app.config!

I just had the same issue (configs not merging as expected), but after explicitly deleting the /bin and /obj directories from the solution-folder and performing a rebuild, everything worked as expected again, so I would suggest you try that and see how it works...

PS: Also make sure you set the file properties of the external config to 'Copy Always'. Otherwise it won't exist in the bin-directory where your running application lives.