Queries the Umbraco database and returns a list of either all domains, or all domains associated with a particular node ID.

Domain.cs

/// <summary>
/// Represents a domain
/// </summary>
public class Domain
{
    /// <summary>
    /// Gets the unique domain id
    /// </summary>
    public int DomainId { get; set; }

    /// <summary>
    /// Gets the name of the Domain
    /// </summary>
    public string Name { get; set; }

    /// <summary>
    /// Gets the root node id where this domain is applied
    /// </summary>
    public int RootNodeId { get; set; }
}

Domains.cs

using System.Collections.Generic;
using umbraco.BusinessLogic;

// Original: https://gist.github.com/florisrobbemont/5155690#file-domain-cs-L10
public class Domains : Queries
{
    private static Domain PrepareDomain(umbraco.cms.businesslogic.web.Domain umbracoDomain)
    {
        return new Domain
        {
            DomainId = umbracoDomain.Id,
            Name = umbracoDomain.Name,
            RootNodeId = umbracoDomain.RootNodeId
        };
    }

    private static List<Domain> PrepareDomains(List<umbraco.cms.businesslogic.web.Domain> umbracoDomains)
    {
        var domains = new List<Domain>();

        foreach (var domain in umbracoDomains)
        {
            domains.Add(PrepareDomain(domain));
        }

        return domains;
    }

    public static List<Domain> GetUmbracoDomains()
    {
        var sqlHelper = Application.SqlHelper;
        var result = new List<umbraco.cms.businesslogic.web.Domain>();
        using (var dr = sqlHelper.ExecuteReader("SELECT id, domainName, domainRootStructureID as RootNodeId from umbracoDomains"))
        {
            while (dr.Read())
            {
                var domainId = dr.GetInt("id");

                result.Add(new umbraco.cms.businesslogic.web.Domain(domainId));
            }
        }

        return PrepareDomains(result);
    }

    public static List<Domain> GetUmbracoDomainsById(int id)
    {
        var sqlHelper = Application.SqlHelper;
        var result = new List<umbraco.cms.businesslogic.web.Domain>();
        using (var dr = sqlHelper.ExecuteReader("SELECT id, domainName, domainRootStructureID as RootNodeId from umbracoDomains WHERE domainRootStructureID = " + id))
        {
            while (dr.Read())
            {
                var domainId = dr.GetInt("id");

                result.Add(new umbraco.cms.businesslogic.web.Domain(domainId));
            }
        }

        return PrepareDomains(result);
    }
}