Building a 3-Tier App with Silverlight 3, .NET RIA Services, and Azure Table Storage

// AzureContext.cs
//

using System;
using System.Collections.Generic;
using System.Data.Services.Client;
using System.Web.DomainServices;
using Microsoft.Azure.Linq;

namespace Microsoft.Azure.DomainServices {

    [MetadataProvider(typeof(AzureTypeDescriptionProvider))]
    public abstract class AzureDomainService<TDataContext> : DomainService where TDataContext : DataContext, new() {

        private TDataContext _dataContext;
        private Dictionary<object, string> _etagMap;

        protected TDataContext DataContext {
            get {
                if (_dataContext == null) {
                    _dataContext = CreateDataContext();
                    _dataContext.PartitionKey = GetPartitionKey();
                }

                return _dataContext;
            }
        }

        public override bool UsesUtcDateTimes {
            get {
                return true;
            }
        }

        protected virtual TDataContext CreateDataContext() {
            return new TDataContext();
        }

        protected override void PersistChangeSet(ChangeSet changeSet) {
            _dataContext.SubmitChanges();
            InitializeETagMap();
        }

        protected string GetETag(object entity) {
            if (entity == null) {
                throw new ArgumentNullException("entity");
            }

            string etag = null;
            if ((_etagMap != null) && _etagMap.TryGetValue(entity, out etag)) {
                return etag;
            }

            return null;
        }

        protected virtual string GetPartitionKey() {
            return null;
        }

        private void InitializeETagMap() {
            foreach (EntityDescriptor entityDescriptor in DataContext.DataServiceContext.Entities) {
                _etagMap[entityDescriptor.Entity] = entityDescriptor.ETag;
            }
        }

        protected override void OnDeserializingObject(object obj, IDictionary<string, object> state) {
            if (_etagMap == null) {
                _etagMap = new Dictionary<object, string>();
            }

            object etagValue = null;
            if (state.TryGetValue("_ETag", out etagValue)) {
                string etag = etagValue as string;
                if (String.IsNullOrEmpty(etag) == false) {
                    _etagMap[obj] = etag;
                }
            }

            base.OnDeserializingObject(obj, state);
        }

        protected override void OnSerializingObject(object obj, IDictionary<string, object> state) {
            base.OnSerializingObject(obj, state);

            if (_etagMap == null) {
                _etagMap = new Dictionary<object, string>();
                InitializeETagMap();
            }

            string etag = null;
            if (_etagMap.TryGetValue(obj, out etag)) {
                state["_ETag"] = etag;
            }
        }
    }
}