use namespace="System.Collections"
use namespace="System.Xml.Linq"

@{
    var prefix = "NUGET_VOLATILE_FEED_";

    var feeds = Environment.GetEnvironmentVariables()
        .Cast<DictionaryEntry>()
        .Where(entry => ((string)entry.Key).StartsWith("NUGET_VOLATILE_FEED_"))
        .Select(entry => new KeyValuePair<string, string>(((string)entry.Key).Substring(prefix.Length), (string)entry.Value))
        .ToList();

    if (feeds.Any())
    {
        var nugetConfig = XDocument.Load("NuGet.config");
        var packageSources = nugetConfig.Element("configuration").Element("packageSources");
        var addElements = packageSources.Elements("add").ToList();
        foreach (var feed in feeds)
        {
            var valueToUpdate = addElements.FirstOrDefault(f => string.Equals(f.Attribute("key").Value, feed.Key, StringComparison.OrdinalIgnoreCase));
            if (valueToUpdate == null)
            {
                packageSources.Add(new XElement("add", new XAttribute("key", feed.Key), new XAttribute("value", feed.Value)));
            }
            else
            {
                valueToUpdate.Attribute("value").Value = feed.Value;
            }
        }

        nugetConfig.Save("NuGet.config");
    }
}