A class which when added to a web application exposes the provides an OData service on top of CRM / xRM

Crm.svc.cs

using System;
using System.Collections.Generic;
using System.Data.Services;
using System.Data.Services.Common;
using System.Linq;
using System.Web;
using System.ServiceModel.Web;
using Microsoft.Xrm.Client;
using log4net;

namespace CrmOData
{

    /// <summary>
    /// Exposes an OData service providing access to CRM
    ///               
    /// Examples of how to use service.
    ///               
    /// URI     : http://myWebServer/CrmOData/Crm.svc
    /// Purpose : Demonstrates exposed endpoints
    ///               
    /// URI     : http://myWebServer/CrmOData/Crm.svc/myCustomer
    /// Purpose : Demonstrates how to retrieve all customers
    ///               
    /// URI     : http://myWebServer/CrmOData/Crm.svc/myCustomer?$filter=lastName eq 'Reilly'
    /// Purpose : Demonstrates how to retrieve all customers with the Surname "Reilly"
    ///               
    /// URI     : http://myWebServer/CrmOData/Crm.svc/myCustomer?$select=firstName,lastName
    /// Output  : Does not work.
    ///                
    /// "$select statements are not supported. This problem is being discussed 
    ///  here http://social.msdn.microsoft.com/Forums/en/adodotnetdataservices/thread/366086ee-dcef-496a-ad15-f461788ae678 
    ///  and is caused by the fact that CrmDataContext implements the IExpandProvider interface which in turn causes 
    ///  the DataService to lose support for $select projections"
    ///                
    ///  See http://social.microsoft.com/Forums/en/crmdevelopment/thread/31daedb4-3d75-483a-8d7f-269af3375d74 for original post discussing this
    ///               
    ///  URI     : http://myWebServer/CrmOData/Crm.svc/myCustomer(guid'783323a1-b1f1-4910-b5be-a2f37e62d0ba')/currentBalance 
    ///  Purpose : Retrieves the current balance of the customers account
    ///               
    ///  URI     : http://myWebServer/CrmOData/Crm.svc/myCustomer(guid'783323a1-b1f1-4910-b5be-a2f37e62d0ba')/currentBalance/$value
    ///  Output  : 321186905.8600
    ///  Purpose : The raw value
    ///               
    ///  URI     : http://myWebServer/CrmOData/Crm.svc/myCustomer(guid'783323a1-b1f1-4910-b5be-a2f37e62d0ba')?$expand=transactions
    ///  Purpose : Retrieves a customer by their guid'783323a1-b1f1-4910-b5be-a2f37e62d0ba', with the transactions property expanded (the equivalent of Include in Entity Framework I guess)
    /// </summary>
    public class Crm : DataService< Xrm.DataContext >
    {
        private static ILog _log;

        /// <summary>
        /// Initialise the service (this method is called only once to initialize service-wide policies.)
        /// </summary>
        /// <param name="config"></param>
        public static void InitializeService(DataServiceConfiguration config)
        {
            //Allows access to everything
            config.SetEntitySetAccessRule("*", EntitySetRights.AllRead);
            config.SetEntitySetPageSize("*", 10); //Only allow access to 10 items at a time - don't want to bring down CRM
            config.SetServiceOperationAccessRule("*", ServiceOperationRights.AllRead);

            config.DataServiceBehavior.MaxProtocolVersion = DataServiceProtocolVersion.V2;

            // set cache policy to this page 
            HttpContext context = HttpContext.Current;    
            HttpCachePolicy cachePolicy = HttpContext.Current.Response.Cache;

            // server&private: server and client side cache only - not at proxy servers
            cachePolicy.SetCacheability(HttpCacheability.ServerAndPrivate);

            // default cache expire: 60 seconds
            cachePolicy.SetExpires(HttpContext.Current.Timestamp.AddSeconds(60));
            
            // cached output depends on: accept, charset, encoding, and all parameters (like $filter, etc) 
            cachePolicy.VaryByHeaders["Accept"] = true;
            cachePolicy.VaryByHeaders["Accept-Charset"] = true;
            cachePolicy.VaryByHeaders["Accept-Encoding"] = true;
            cachePolicy.VaryByParams["*"] = true;

            //allow client to send Cache-Control: nocache headers to invalidate cache
            cachePolicy.SetValidUntilExpires(false); 

            //Log service startup initialisation
            _log = log4net.LogManager.GetLogger("Crm.svc");
            _log.Info("Crm.svc initialising...");
        }

        /// <summary>
        /// Allows the user to get the id of a specific CrmEntity given a supplied entity name 
        /// and a supplied predicate which consists of a propertyName and a string propertyValue (eg "112001-S").  
        ///               
        /// If there is a need for a predicate with different type of value (eg int / datetime / decimal)
        /// then it could be introduced
        ///               
        /// Example URI : http://myWebServer/CrmOData/Crm.svc/GetId?entityName='myCustomer'&propertyName='customerNumber'&propertyValue='23456KL-P'
        /// </summary>
        /// <param name="entityName">eg "myCustomer"</param>
        /// <param name="propertyName">eg "customerNumber"</param>
        /// <param name="propertyValue">eg "23456KL-P"</param>
        /// <returns></returns>
        [WebGet]
        public Guid? GetEntityId(string entityName, string propertyName, string propertyValue)
        {
            var entities = CurrentDataSource.GetEntities(entityName);

            var entitiesWhere = entities.Where(x => (x.GetPropertyValue(propertyName) as string) == propertyValue);

            var guid = entitiesWhere.Select(x => x.Id)
                                    .SingleOrDefault();

            return guid;
        }

        /// <summary>
        /// Handle exceptions
        /// </summary>
        /// <param name="args"></param>
        protected override void HandleException(HandleExceptionArgs args)
        {
            base.HandleException(args);

            //Log all exceptions
            _log.Error(string.Format("\r\nResponseContentType: {0}\r\nResponseStatusCode: {1}\r\nResponseWritten: {2}\r\nUser: {3}{4}",
                args.ResponseContentType, args.ResponseStatusCode, args.ResponseWritten, HttpContext.Current.User.Identity.Name, args.Exception.GetExceptionDetails()),
                args.Exception);
        }
    }
}