How to set/get value to the custom field of a case in MS dynamics CRM using C#

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C#

ASP.NET

CRM

I have a requirement that i need to provide two contact fields for each case in MS Dynamics CRM.So i created a custom field " SecondaryContact "with dataType lookup(contact)for case(incident) entity in MS Dynamics CRM online.

now i want to pass value(contact id)from my appilication(in c#) to the custom field and also i want retrive all the cases based on custom field(contact Id) 

My code at present is to retrive all the cases based on PrimaryContactId
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string primaryContactId = Session["ContactId"].ToString().Trim();
var context = new ForessCRMServiceContext(FssServiceProxy.orgService);
 var result = from r in context.IncidentSet where r.PrimaryContactId.Equals(primaryContactId) orderby r.CreatedOn descending select r;
foreach (Incident incident in result)
  {
	dr["TICKETID"] = incident.TicketNumber;
	dr["SUBJECT"] = incident.Title;
	dr["STATUS"] = incident.StatusCode.Value;
	dr["ROWNUMBER"] = iRowNumber;
	dr["INCIDENTID"] = incident.IncidentId;
	dt.Rows.Add(dr);
  }
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and iam creating case like this
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string primaryContactId = Session["ContactId"].ToString();          
IOrganizationService _service = FssServiceProxy.orgService;
Entity incident = new Entity();
incident.LogicalName = "incident";

incident["title"] = txtSubject.Text.Trim();
incident["description"] = txtDescription.Text.Trim();
string AccountId = ConfigurationManager.AppSettings["AccountId"] == null || ConfigurationManager.AppSettings["AccountId"].ToString().Trim().Equals("") ? "" : ConfigurationManager.AppSettings["AccountId"].ToString().Trim();
Guid customerid = new Guid(AccountId);

EntityReference CustomerId = new EntityReference("account", customerid);
incident["customerid"] = CustomerId;
Guid contactid = new Guid(primaryContactId);
EntityReference EprimaryContactId = new EntityReference("contact", contactid);
incident["primarycontactid"] = EprimaryContactId;
Guid TicketID = _service.Create(incident);
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Kindly advise me to achive this

What I have tried:

i tried like below to retrieve the cases but iam getting error that " 'Incident' entity doesn't contain attribute with Name = 'SecondaryContact'."

QueryExpression query = new QueryExpression();
query.EntityName = "incident";
ColumnSet cols = new ColumnSet();
cols.AddColumns(new string[] { "ticketnumber", "title", "statecode", "incidentid", "SecondaryContact"});
query.ColumnSet = cols;
RetrieveMultipleResponse retrived = new RetrieveMultipleResponse();

RetrieveMultipleRequest retrive = new RetrieveMultipleRequest();
retrive.Query = query;
retrived = (RetrieveMultipleResponse)FssServiceProxy.orgService.Execute(retrive);

var result = retrived.Results;