How to read Elastic Beanstalk Environment Properties in .net?

It looks like this behavior has changed in Elastic Beanstalk. The docs now say

Settings applied in the AWS Management Console override the same settings in configuration files, if they exist. This lets you have default settings in configuration files, and override them with environment specific settings in the console.

So you can now use the same config names in your web.config and in the Elastic Beanstalk config, and the Elastic Beanstalk values will override any in your web.config. It looks like EB simply adds new entries to the web.config file, so there will be two entries for any values defined in both places. Since the EB-added entries are later in the file they take precedence.

In your .ebextensions/myoptions.config file:

option_settings:
  - option_name: MyServiceUrl
    value: change me

This will add the "MyServiceUrl" option in your EB Environment Properties section (as you're seeing already). When deployed, this will add the following to your Web.Config file:

<appSettings>
  <add key="MyServiceUrl" value="change me" />
</appSettings>

If you RDP into your EC2 instance, you'll see this.

When you change the property using the EB console, the setting will be modified in your Web.Config file.

So you access this property using the standard AppSettings method:

string value = ConfigurationManager.AppSettings["MyServiceUrl"];

The Catch:

You need to ensure that your Web.Config file does not contain this setting, otherwise EB does not replace it. If your Visual Studio deployment package includes this setting, then EB will not replace it and you will always receive the deployed value when you access the property via your code.

The Solution:

In you Web.Release.config file, have the setting removed during Visual Studio deployment:

<appSettings>
  <add key="MyServiceUrl" xdt:Transform="Remove" xdt:Locator="Match(key)" />
</appSettings>

This will remove the setting from Web.Config during Visual Studio deployment and will allow EB to add the value into the file during EB deployment.