Get web/app.config setting and expand environment variables when developing locally

NameValueCollectionExtensions.cs

using System;
using System.Collections.Specialized;
using JetBrains.Annotations;

namespace MyExtensions
{
    public static class NameValueCollectionExtensions
    {
        [CanBeNull]
        public static string GetWithEnv([NotNull] this NameValueCollection nvc, [NotNull] string name)
        {
            if (nvc == null) throw new ArgumentNullException(nameof(nvc));
            if (name == null) throw new ArgumentNullException(nameof(name));

            string value = nvc.Get(name);
            if (value != null)
                value = Environment.ExpandEnvironmentVariables(value);
            return value;
        }
    }
}

Web.config

<?xml version="1.0" encoding="utf-8"?>

<configuration>
  <appSettings>
    <add key="SomeApiUserName" value="%myapp_someapi_username%" />
    <add key="SomeApiPassword" value="%myapp_someapi_password%" />
  </appsettings>
</configuration>