.NET FOLLOWER

    Starting development for Microsoft Dynamics CRM 2011, the first challenge we face is how to connect to the CRM. Communication with Microsoft Dynamics CRM 2011 through the code mostly comes to interaction with two Web Services supplied by the CRM: so-called Discovery (IDiscoveryService) and Organization (IOrganizationService) Web Services. For a certain Microsoft Dynamics CRM installation the URL to the IDiscoveryService is unique. To be more precise, it’s unique for an on-premise installation while for Microsoft Dynamics CRM Online it might be one of a few variants depending on the world region. Read more about possible URLs to the Discovery Web Service here. In the same time, a single Microsoft Dynamics CRM installation can host multiple business organizations on multiple servers. Accessing data for a particular organization through the IOrganizationService, we have to use the URL specified for that organization.

Most of the time we work with data (entity records) and metadata (entities, attributes, relationships, etc.) of our organization(s), so the IOrganizationService is a primary Web Service to deal with. The IDiscoveryService, in turn, is generally used to get the right URL to an Organization Web Service. So, the initial connection process could be divided into two stages:

• a client application grabs CRM‘s server name, protocol (http or https) and port (if applicable), and user credentials (login and password), then sends request to the IDiscoveryService. The IDiscoveryService Web Service determines the organizations that the user is a member of and sends the information about found organizations back to the client. Among other things the information includes actual URLs to Organization Web Services. The client application chooses one organization to interact with and fetches out the URL to the corresponding IOrganizationService;
• the client application connects to IOrganizationService using the URL and manages data and metadata of the organization;

Often after the initial connection the received URL to IOrganizationService is stored somewhere to be used next times as it allows omitting the interaction with Discovery Web Service. However, we have to bear in mind that, in boundaries of a particular installation of Microsoft Dynamics CRM or Microsoft Dynamics CRM Online, the location of servers and organizations may change due to some datacenter management or load balancing. Therefore, we should use IDiscoveryService from time to time since it provides the actual URL to the IOrganizationService that serves our organization at a given moment. For example, we could try refreshing our connection string for IOrganizationService when we fail to access organization using the URL received previously.

To develop for Microsoft Dynamics CRM 2011, Microsoft provides with the Software Development Kit (SDK), which contains a good set of samples to start playing with CRM. After the SDK has been installed, the samples are usually available at SDK\SampleCode\CS. The most of the sample solutions connect to CRM by means of the special class called ServerConnection and defined in the SDK\SampleCode\CS\helpercode\CrmServiceHelpers.cs. The ServerConnection implements all typical steps necessary to connect to CRM: obtains user credentials, communicates with Discovery Web Service to get list of organizations accessible for user, allows user to pick an organization, creates an IOrganizationService proxy and refreshes the WCF connection security token from time to time. The code for connecting to CRM through the ServerConnection class usually looks like the following:

...
using Microsoft.Crm.Sdk.Samples;
using Microsoft.Xrm.Sdk.Client;
...
// Obtain the user logon credentials,  send request to Discovery Web Service, 
// pick out an organization and then get the target organization's Web address
ServerConnection serverConnect = new ServerConnection();
ServerConnection.Configuration config = serverConnect.GetServerConfiguration();

// Connect to the Organization service. 
using (OrganizationServiceProxy serviceProxy = ServerConnection.GetOrganizationProxy(config))
{
	// Enable early-bound type support.
	serviceProxy.EnableProxyTypes();

	// do something useful
}

The ServerConnection uses the DeviceIdManager class inside. The class defined in the SDK\SampleCode\CS\helpercode\DeviceIdManager.cs is intended to register a computing device with Microsoft account (Windows Live ID) and needed only for authentication in Microsoft Dynamics CRM Online. So, to use the ServerConnection we have to add both CrmServiceHelpers.cs and DeviceIdManager.cs files to a project.

The ServerConnection.Configuration class is to encapsulate all information needed for setting up a connection to Discovery Web Service. Based on the object of ServerConnection.Configuration, ServerConnection creates a proxy for IOrganizationService.

Note, however, since ServerConnection is designed to work within sample console-based applications, it has some “features” that are not applicable for other types of projects. For example, the following could be placed among such “features”: prompting user to type credentials in the console window; persisting CRM server connection information to a file C:\Users\<username>\AppData\Roaming\CrmServer\Credentials.xml for later reuse; storing user password in the Windows Credential Manager. Fortunately, all of the mentioned points might be overcome by creating a class inherited from the ServerConnection and by implementing our own mechanism of reading parameters required for establishing a connection to CRM. The custom class shown below exposes the methods that parse the xml string containing connection parameters and return a proper instance of ServerConnection.Configuration. That allows keeping parameters wherever we want, we are not bound to the file system or Windows Credential Manager anymore. That’s quite important if we plan to use the code in Windows Azure, for example.

using System;
using System.Linq;
using System.ServiceModel.Description;
using System.Xml.Linq;
using Microsoft.Crm.Sdk.Samples;
using Microsoft.Crm.Services.Utility;
using Microsoft.Xrm.Sdk.Client;
using Microsoft.Xrm.Sdk.Discovery;

namespace dotNetFollower
{
  public class ServerConnectionEx : ServerConnection
  {
    /// <summary>
    /// Parses xml string and builds a Configuration object with connection parameters
    /// </summary>
    public Configuration Parse(string xml)
    {
      // read xml and fetch out parameters
      XElement configurationsFromFile = XElement.Parse(xml);
      var xmlConfig = configurationsFromFile.Nodes().First() as XElement;

      var newConfig = new Configuration();
            
      var serverAddress = xmlConfig.Element("ServerAddress");
      if (serverAddress != null && !String.IsNullOrEmpty(serverAddress.Value))
        newConfig.ServerAddress = serverAddress.Value;

      var organizationName = xmlConfig.Element("OrganizationName");
      if (organizationName != null && !String.IsNullOrEmpty(organizationName.Value))
        newConfig.OrganizationName = organizationName.Value;

      var discoveryUri = xmlConfig.Element("DiscoveryUri");
      if (discoveryUri != null && !String.IsNullOrEmpty(discoveryUri.Value))
        newConfig.DiscoveryUri = new Uri(discoveryUri.Value);

      var organizationUri = xmlConfig.Element("OrganizationUri");
      if (organizationUri != null && !String.IsNullOrEmpty(organizationUri.Value))
        newConfig.OrganizationUri = new Uri(organizationUri.Value);

      var homeRealmUri = xmlConfig.Element("HomeRealmUri");
      if (homeRealmUri != null && !String.IsNullOrEmpty(homeRealmUri.Value))
        newConfig.HomeRealmUri = new Uri(homeRealmUri.Value);

      var vendpointType = xmlConfig.Element("EndpointType");
      if (vendpointType != null)
        newConfig.EndpointType = RetrieveAuthenticationType(vendpointType.Value);

      var xElement = xmlConfig.Element("Credentials");
      if (xElement != null && xElement.HasElements)
        newConfig.Credentials = ParseInCredentials(xmlConfig.Element("Credentials"), newConfig.EndpointType);

      if (newConfig.EndpointType == AuthenticationProviderType.LiveId)
        newConfig.DeviceCredentials = DeviceIdManager.LoadOrRegisterDevice();

      var userPrincipalName = xmlConfig.Element("UserPrincipalName");
      if (userPrincipalName != null && !String.IsNullOrWhiteSpace(userPrincipalName.Value))
        newConfig.UserPrincipalName = userPrincipalName.Value;

      // save new configuration in the config property as the property 
      // is used by original ServerConnection
      config = newConfig;

      // get OrganizationUri in case it is not set
      if (newConfig.OrganizationUri == null)
      {
        if(string.IsNullOrEmpty(newConfig.OrganizationName))
          throw new ApplicationException("At least one of OrganizationUri and OrganizationName must be spesified in the xml configuration.");
        GetOrganizationInfo();
      }

      return newConfig;
    }

    ...

    /// <summary>
    /// Gets Organization Web Service url by friendly or unique name of the organization
    /// </summary>
    protected void GetOrganizationInfo()
    {
      using (DiscoveryServiceProxy serviceProxy = GetDiscoveryProxy())
      {
        // Obtain organization information from the Discovery service. 
        if (serviceProxy != null)
        {
          // Obtain information about the organizations that the system user belongs to.
          OrganizationDetailCollection orgs = DiscoverOrganizations(serviceProxy);

          if(orgs.Count == 0)
            throw new ApplicationException("User does not belong to any organizations on the specified server");

          // look for a specified organization
          foreach (var organizationDetail in orgs)
          {
            if (string.Equals(organizationDetail.UniqueName, 
                               config.OrganizationName, StringComparison.OrdinalIgnoreCase) ||
                string.Equals(organizationDetail.FriendlyName, 
                               config.OrganizationName, StringComparison.OrdinalIgnoreCase))
            {
              config.OrganizationUri  = 
                       new Uri(organizationDetail.Endpoints[EndpointType.OrganizationService]);
              config.OrganizationName = organizationDetail.FriendlyName;
              return;
            }
          }

          throw new ApplicationException("Couldn't find the " + config.OrganizationName + "organization");
        }
                
        throw new ApplicationException("An invalid server name was specified.");
      }
    }

    /// <summary>
    /// Parses xml element to get user name and password
    /// </summary>
    protected static ClientCredentials ParseInCredentials(XElement credentials, 
                             AuthenticationProviderType endpointType)
    {
      ClientCredentials result = null;

      if (credentials.HasElements)
      {
        result = new ClientCredentials();

        switch (endpointType)
        {
          case AuthenticationProviderType.ActiveDirectory:
          {
            result.Windows.ClientCredential = new System.Net.NetworkCredential()
              {
                UserName = credentials.Element("UserName").Value,
                Domain   = credentials.Element("Domain").Value,
                Password = credentials.Element("Password").Value
              };
              break;
          }
          case AuthenticationProviderType.LiveId:
          case AuthenticationProviderType.Federation:
          case AuthenticationProviderType.OnlineFederation:
          {
            result.UserName.UserName = credentials.Element("UserName").Value;
            result.UserName.Password = credentials.Element("Password").Value;
            break;
          }
        }
      }

      return result;
    }
  }
  ...
}

In case the OrganizationUri isn’t set in the xml string the Parse method tries getting the URL from Discovery Web Service using the name specified in the OrganizationName node. The OrganizationName can contain friendly or unique name of the target organization.

Note we need to slightly modify the original ServerConnection class, we need to make protected such its members as RetrieveAuthenticationType (method), GetDiscoveryProxy (method) and config (field) so that they would be accessible from within the derived ServerConnectionEx class.

Below is an example of how the custom ServerConnectionEx could be used:

// the xml string below might be stored to and read from any data source
string crmConfiguration = @"<?xml version=""1.0"" encoding=""utf-8""?>
  <Configurations>
  <Configuration>
    <ServerAddress>MyCrmOnPremise</ServerAddress>
    <OrganizationName>My Company Friendly Name</OrganizationName>
    <DiscoveryUri>http://MyCrmOnPremise/XRMServices/2011/Discovery.svc</DiscoveryUri>
    <OrganizationUri>http://MyCrmOnPremise/MyCompanyFriendlyName/XRMServices/2011/Organization.svc</OrganizationUri>
    <HomeRealmUri />                      
    <Credentials>
      <UserName>dotNetFollower</UserName>
      <Domain>hq</Domain>
      <Password>11111</Password>
    </Credentials>
    <EndpointType>ActiveDirectory</EndpointType>
    <UserPrincipalName />                                        
  </Configuration>
</Configurations>";

// Obtain the target organization's Web address and client credentials from the xml string.
ServerConnectionEx serverConnection = new ServerConnectionEx();
ServerConnection.Configuration configuration = serverConnection.Parse(crmConfiguration);

// Connect to the Organization service. 
using (OrganizationServiceProxy serviceProxy = ServerConnection.GetOrganizationProxy(configuration))
{
	// This statement is required to enable early-bound type support.
	serviceProxy.EnableProxyTypes();

	// do something useful
}

In the example we have a hardcoded xml string with connection parameters, in the reality, however, the string could be stored in whatever data store.

To make the use of ServerConnectionEx more comfortable, let’s add the following two methods to ServerConnectionEx:

public class ServerConnectionEx : ServerConnection
{
	...
	public static void DoInOrganizationServiceProxyContext(Configuration configuration,
							Action<OrganizationServiceProxy> action)
	{
		using (var proxy = GetOrganizationProxy(configuration))
		{
			proxy.EnableProxyTypes();
			action(proxy);
		}
	}

	public void DoInOrganizationServiceProxyContext(string configXml, 
							Action<OrganizationServiceProxy> action)
	{
		Configuration configuration = Parse(configXml);
		DoInOrganizationServiceProxyContext(configuration, action);
	}
	...
}

Now to deal with the OrganizationServiceProxy we can use the code like the following:

string crmConfiguration = ... // the same xml-string as used above

var serverConnection = new ServerConnectionEx();
serverConnection.DoInOrganizationServiceProxyContext(crmConfiguration, proxy =>
{
	// do something
});

Note also that to use CrmServiceHelpers.cs and DeviceIdManager.cs you have to add such references to your project as System.Runtime.Serialization, System.Security, System.ServiceModel, System.DirectoryServices.AccountManagement and Microsoft.IdentityModel. The Microsoft.IdentityModel is a part of Windows Identity Foundation, so install it if it’s not already installed. The dll itself can be found at Program Files\Reference Assemblies\Microsoft\Windows Identity Foundation\v3.5 or in GAC. And, of course, you need to add references for Microsoft.Crm.Sdk.Proxy and Microsoft.Xrm.Sdk. You can find them in SDK\BIN.

A simple demonstration project you can download here – ConnectToCrm.zip. Among others files it contains the adapted CrmServiceHelpers.cs and DeviceIdManager.cs.

    In the given blogpost I’ll describe step by step how to create a simple REST WCF Service residing in the _layouts folder. To create a demonstration project we’ll use Visual Studio 2012. I believe, however, the similar steps could be done in Visual Studio 2010 as well.

* Don’t look for a meaning in the source code of the demonstration project as its main goal is just to show how to communicate with WCF Services operating under SharePoint and how they return and accept parameters of standard and custom types.

* If you are too lazy to go through all steps, you can download the demonstration project right away and play with it 🙂

1. Create a SharePoint project in the Visual Studio 2012

* Note that if you plan to add the service to an existent project, just skip this step.

2. Add a WCF Service to the project

3. Add references to System.ServiceModel.Web and System.Web assemblies to the project

4. Modify the service’s interface

Open the ICustomRestService.cs and bring it to the following state:

using System.ServiceModel;
using System.ServiceModel.Web;

namespace CustomRestService
{
    /// <summary>
    /// Wraps the service result and provides the error message (if any)
    /// </summary>
    public class ServiceResult<T>
    {
        public bool   Success      { get; set; }
        public string ErrorMessage { get; set; }
        public T      Data         { get; set; }
    }

    /// <summary>
    /// Represents basic info about a book
    /// </summary>
    public class Book
    {
        public int    Id     { get; set; }
        public string Title  { get; set; }
        public string Author { get; set; }
    }

    /// <summary>
    /// Service's interface
    /// </summary>
    [ServiceContract]
    public interface ICustomRestService
    {
        /// <summary>
        /// Checks if book is already registered in the system
        /// </summary>
        [OperationContract]
        [WebInvoke(Method  = "POST", 
            RequestFormat  = WebMessageFormat.Json, 
            ResponseFormat = WebMessageFormat.Json, 
            BodyStyle      = WebMessageBodyStyle.WrappedRequest)]
        ServiceResult<bool> BookExists(string bookTitle);

        /// <summary>
        /// Returns book by id
        /// </summary>
        [OperationContract]
        [WebInvoke(Method  = "POST", 
            RequestFormat  = WebMessageFormat.Json, 
            ResponseFormat = WebMessageFormat.Json, 
            BodyStyle      = WebMessageBodyStyle.WrappedRequest)]
        ServiceResult<Book> GetBook(int id);

        /// <summary>
        /// Adds the passed book to the system
        /// </summary>
        [OperationContract]
        [WebInvoke(Method  = "POST",
            RequestFormat  = WebMessageFormat.Json,
            ResponseFormat = WebMessageFormat.Json,
            BodyStyle      = WebMessageBodyStyle.WrappedRequest)]
        ServiceResult<int> AddBook(Book book);
    }
}

Here the ServiceResult is a simple class that wraps the service result and provides the error message in case it occurs. The Book class provides basic info about a book and is intended to show how REST WCF Service deals with custom types.

5. Modify the service implementation

Open the CustomRestService.cs and modify it so that it looks like the following:

using System;
using System.ServiceModel.Activation;
using System.Web;
using Microsoft.SharePoint;

namespace CustomRestService
{
    [AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
    public class CustomRestService : ICustomRestService
    {
        public ServiceResult<bool> BookExists(string bookTitle)
        {
            // allow access for anonymous users

            if (string.IsNullOrEmpty(bookTitle))
                return new ServiceResult<bool>()
                { 
                    Success      = false, 
                    ErrorMessage = "Book title is undefined!" 
                };

            var result = new ServiceResult<bool>() { Success = true };

            try
            {
                SPList spList = SPContext.Current.Web.Lists["Books"];

                SPQuery spQuery = new SPQuery
                {
                    Query = "<Where><Eq><FieldRef Name='Title' /><Value Type='Text'>" + 
                        bookTitle.ToLower() + "</Value></Eq></Where>",
                    ViewAttributes = "Scope='Recursive'"
                };

                SPListItemCollection books = spList.GetItems(spQuery);
                result.Data = books.Count > 0;
            }
            catch (Exception ex)
            {
                result.Success      = false;
                result.ErrorMessage = ex.Message;
            }

            return result;
        }

        public ServiceResult<Book> GetBook(int id)
        {
            // check if user is authenticated
            if (!HttpContext.Current.Request.IsAuthenticated)
                return new ServiceResult<Book>()
                    {
                        Success      = false, 
                        ErrorMessage = "Unauthenticated access!"
                    };

            if (id <= 0)
                return new ServiceResult<Book>()
                    {
                        Success      = false, 
                        ErrorMessage = "Invalid book id!"
                    };

            var result = new ServiceResult<Book>() { Success = true };

            try
            {
                SPList spList = SPContext.Current.Web.Lists["Books"];

                SPListItem spListItem = spList.GetItemById(id);
                result.Data = new Book()
                {
                    Id     = spListItem.ID, 
                    Title  = spListItem.Title, 
                    Author = Convert.ToString(spListItem["BookAuthor"])
                };
            }
            catch (Exception ex)
            {
                result.Success      = false;
                result.ErrorMessage = ex.Message;
            }

            return result;
        }

        public ServiceResult<int> AddBook(Book book)
        {
            // check if user is authenticated
            if (!HttpContext.Current.Request.IsAuthenticated)
                return new ServiceResult<int>()
                    {
                        Success      = false, 
                        ErrorMessage = "Unauthenticated access!"
                    };

            if (book == null)
                return new ServiceResult<int>()
                    {
                        Success      = false, 
                        ErrorMessage = "Invalid book!"
                    };

            // checking if the book is already presented is omitted

            var result = new ServiceResult<int>() { Success = true };

            try
            {
                SPList spList = SPContext.Current.Web.Lists["Books"];

                SPListItem spListItem = spList.AddItem();
                spListItem["Title"]      = book.Title;
                spListItem["BookAuthor"] = book.Author;

                SPContext.Current.Web.AllowUnsafeUpdates = true;
                spListItem.Update();
                SPContext.Current.Web.AllowUnsafeUpdates = false;

                result.Data = spListItem.ID;
            }
            catch (Exception ex)
            {
                result.Success      = false;
                result.ErrorMessage = ex.Message;
            }

            return result;                
        }
    }
}

The implementation operates with the Books list and assumes that the list is available at the SharePoint Site in the boundaries of which the service is called. Note that we work with SharePoint objects in the same way as we would do this in code-behind of an .aspx-page.

The GetBook and AddBook methods require users to be authenticated before calling them, while the BookExists allows anonymous access (of course, in case it’s turned on for Web Application and Site). Since it’s a best practice, always put the checking whether user is authenticated and authorized in the beginning of every web method unless you have some contrary requirements (for example, to provide anonymous access).

6. Add Layouts mapped folder to the project

* Note that if you already have the folder, just skip this step.

Click to see how to add “Layouts” mapped folder to the project in details

    Ensure that the CustomRestService project is selected in Solution Explorer and then go to PROJECT -> Add SharePoint “Layouts” Mapped Folder. The Layouts folder along with the nested CustomRestService folder will be added to the project. For simplicity rename the nested CustomRestService to Wcf. After that the project should look as follows:

7. Add .svc file to the Layouts folder

8. Modify the .svc file

Open the CustomRestService.svc and bring it to the following:

<%@ ServiceHost Language="C#" Debug="true" 
    Service="CustomRestService.CustomRestService, $SharePoint.Project.AssemblyFullName$"
    CodeBehind="CustomRestService.cs"
    Factory="Microsoft.SharePoint.Client.Services.MultipleBaseAddressWebServiceHostFactory,
             Microsoft.SharePoint.Client.ServerRuntime, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>

The MultipleBaseAddressWebServiceHostFactory declares the REST type of the Web Service and creates proper endpoints with Web Bindings. The factory takes upon itself the configuring of the Web Service, that is why we don’t need the app.config deleted previously.

The Service attribute provides the Fully Qualified Name (including the assembly’s full name) of the class representing the service. Instead of putting in the proper full name of the assembly I used such replaceable parameter as $SharePoint.Project.AssemblyFullName$. When building the project, Visual Studio is supposed to replace the parameter with the full name of the output assembly. One nuisance here is that Visual Studio doesn’t perform replacement in .svc files by default. To make Visual Studio look into .svc, we have to manually alter the .csproj. So, open the project’s .csproj file in Notepad or other Notepad-like text editor, find the first <PropertyGroup> node, insert the following before the closing </PropertyGroup> and save the file:

<TokenReplacementFileExtensions>svc</TokenReplacementFileExtensions>

* Note that, having modified the .csproj, you likely will be prompted by Visual Studio to reload the project.

After the alteration of the project’s file it should resemble the following (some tags and attributes are skipped for simplicity):

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" DefaultTargets="Build" ...>
  <Import ... />
  <PropertyGroup>
    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
    <ProjectGuid>{16585D01-2995-48EF-8DFA-1371C9EF6EE9}</ProjectGuid>
    <OutputType>Library</OutputType>
    <AppDesignerFolder>Properties</AppDesignerFolder>
    <RootNamespace>CustomRestService</RootNamespace>
    <AssemblyName>CustomRestService</AssemblyName>
    <TargetFrameworkVersion>v3.5</TargetFrameworkVersion>
    <FileAlignment>512</FileAlignment>
    <ProjectTypeGuids>{BB1F664B-9266-4fd6-B973-E1E44974B511};...</ProjectTypeGuids>
    <SandboxedSolution>False</SandboxedSolution>
    <WcfConfigValidationEnabled>True</WcfConfigValidationEnabled>
	<TokenReplacementFileExtensions>svc</TokenReplacementFileExtensions>
  </PropertyGroup>
...
</Project>

* Note that for the project attached to the given article, the .csproj has been already modified.

Of course, instead of the $SharePoint.Project.AssemblyFullName$ you can use the real full name of your assembly. To get such name I usually use .Net Reflector. Alternatively you can make Visual Studio display the name, following instructions from the article How to: Create a Tool to Get the Full Name of an Assembly.

9. Build the project and deploy it

Right-click the CustomRestService project in Solution Explorer and then click Deploy. Behind the scenes, Visual Studio compiles the project, creates a .wsp file and deploys the package to the farm.

10. Test the service

1. First of all try to open the service in a browser. You can use for that any SharePoint Site, just add the /_layouts/wcf/customrestservice.svc at the end of the URL. For example:

   http://someservername:1200/somesite/_layouts/wcf/customrestservice.svc
   

   If you see the message “Endpoint not found“, don’t worry, that’s what we need. That means all of the steps above have been done correctly.

2. Let’s test the web methods of the service. As you remember, the service interacts with the Books list. It’s a list with two main fields: Title and BookAuthor, both are Single Line of Text (see the image below).
   
   * I do not cite the list’s schema here as you can find it (schema, instance, feature to create the instance) in the attached project.
   The test page named TestPage.aspx is added to the demonstration project as well. The page has been created as an Application Page, locates in the Layouts folder and contains one button that executes JavaScript functions to test each web method. Below is the shortened listing of the page to show how to call web methods using jQuery and Ajax:

   ...
   <%@ Page ... CodeBehind="TestPage.aspx.cs" 
            Inherits="CustomRestService.Layouts.TestPage" 
   		 DynamicMasterPageFile="~masterurl/default.master" %>
   
   <asp:Content ID="PageHead" ContentPlaceHolderID="PlaceHolderAdditionalPageHead" runat="server">
    <script src="http://code.jquery.com/jquery-latest.min.js" type="text/javascript"></script>
   
    <script type="text/javascript">
     // makes POST json-request to a web method
     var getJsonData = function (url, param, callback, errorcallback) {
       $.ajax({
   	  url        : url,
   	  dataType   : "json",
   	  type       : "POST",
   	  contentType: 'application/json; charset=utf-8',
   	  data       : JSON.stringify(param),
   	  success    : function (data) {
   		  if (callback)
   			  callback(data);
   	  },
   	  error      : function (XMLHttpRequest, textStatus, errorThrown) {
   		  if (errorcallback)
   			  errorcallback(XMLHttpRequest, textStatus, errorThrown);
   	  }
       });
     };
   
     // makes request to the BookExists method
     function testBookExists() {
       getJsonData('Wcf/CustomRestService.svc/BookExists', 
         { "bookTitle": 'the adventures of tom sawyer' },
     	  function (data) {
   		  if (data.Success)
   			  alert('Success. Result: ' + data.Data);
   		  else
   			  alert('Fail. Error: ' + data.ErrorMessage);
   	  },
   	  function(XMLHttpRequest, textStatus, errorThrown) {
   		  alert(errorThrown);
   	  });	
     }
   
     // makes request to the GetBook method
     function testGetBook() {
       getJsonData('Wcf/CustomRestService.svc/GetBook', { "id": 2 },
   	  function (data) {
   		  if (data.Success)
   			  alert('Success. Title: ' + data.Data.Title + ' Author: ' + data.Data.Author);
   		  else
   			  alert('Fail. Error: ' + data.ErrorMessage);
   	  },
   	  function (XMLHttpRequest, textStatus, errorThrown) {
   		  alert(errorThrown);
   	  });
     }
   
     // makes request to the AddBook method
     function testAddBook() {
       getJsonData('Wcf/CustomRestService.svc/AddBook', 
         { "book": { "Author": "Mark Twain", "Title": "The Adventures of Huckleberry Finn" } },
   	  function (data) {
   		  if (data.Success)
   			  alert('Success. Book Id: ' + data.Data);
   		  else
   			  alert('Fail. Error: ' + data.ErrorMessage);
   	  },
   	  function (XMLHttpRequest, textStatus, errorThrown) {
   		  alert(errorThrown);
   	  });
     }
    </script>
   </asp:Content>
   
   <asp:Content ID="Main" ContentPlaceHolderID="PlaceHolderMain" runat="server">
    <asp:Button ID="test" Text="Test Web Methods" runat="server" 
      OnClientClick="testBookExists(); testGetBook(); testAddBook(); return false;" />
   </asp:Content>
   ...
   

   Pay attention to the URLs used to send request to web methods. The given test page is located directly in _layouts, so to access the Web Service placed in the _layouts/Wcf I use the relative URL Wcf/CustomRestService.svc. If your page dealing with the Web Service resides somewhere outside of _layouts or within the folders nested in _layouts you have to provide the proper URL (relative or not) to the Web Service.
   Open the test page in a browser, the URL should be like the following

   http://someservername:1200/somesite/_layouts/TestPage.aspx
   

   Click Test Web Methods button. If everything works fine you’ll get 3 alert messages and each of them starts with the word “Success”. The order of the alerts may vary as requests are asynchronous.

The demonstration project you can download here – CustomRestService.zip. Note that the archive contains the .wsp package in the CustomRestService\bin subfolder, use it in case you are not able to compile/build the project for some reason.