How to retrieve related entity data in Dynamics CRM 2011

This sample shows how to retrieve multiple entities using the RetrieveMultiple method with QueryExpression along with their related entity columns. The code returns columns from the primary account record as well as the firstname and lastname of the primary contacts associated with the account. It is a simple code where you need to specify the relationship name between two entities:

//Create multiple accounts with primary contacts
    Entity contact = new Entity("contact");
    contact.Attributes["firstname"] = "ContactFirstName";
    contact.Attributes["lastname"] = "ContactLastName";
    Guid contactId = _orgService.Create(contact, null);

    Entity account = new Entity("account");
    account["name"] = "Test Account1";
    EntityReference primaryContactId = new EntityReference("contact", contactId);
    account["primarycontactid"] = primaryContactId;

    Guid accountId1 = _orgService.Create(account, null);
    account["name"] = "Test Account2";
    Guid accountId2 = _orgService.Create(account, null);
    account["name"] = "Test Account3";
    Guid accountId3 = _orgService.Create(account, null);

    //Create a query expression specifying the link entity alias and the columns of the link entity that you want to return
    QueryExpression qe = new QueryExpression();
    qe.EntityName = "account";
    qe.ColumnSet = new ColumnSet();
    qe.ColumnSet.Columns.Add("name");

    qe.LinkEntities.Add(new LinkEntity("account", "contact", "primarycontactid", "contactid", JoinOperator.Inner));
    qe.LinkEntities[0].Columns.AddColumns("firstname", "lastname");
    qe.LinkEntities[0].EntityAlias = "primarycontact";

    EntityCollection ec = _orgService.RetrieveMultiple(qe);

    Console.WriteLine("Retrieved {0} entities", ec.Entities.Count);
    foreach (Entity act in ec.Entities)
    {
       Console.WriteLine("account name:" + act["name"]);
       Console.WriteLine("primary contact first name:" + act["primarycontact.firstname"]);
       Console.WriteLine("primary contact last name:" + act["primarycontact.lastname"]);
    }