Here's a snippet of code I have that does similar:

var contentCache = context.ContentCache;
var services = ApplicationContext.Current.Services;
var domainService = services.DomainService;

// Get homepages.
return domainService
    // First, get each content node with a domain set.
    .GetAll(true)
    .Select(x => new
    {
        x.DomainName,
        x.RootContentId,
        Node = contentCache.GetById(x.RootContentId.Value)
    })
    // Avoid null exceptions.
    .Where(x => x.Node != null);

I'm sure you can figure out the rest from that.