How to Navigate 3 tables in EntityFrame work WPF

Question

I am New to WPF. I has a Country class generated from database using Entity frame work. When I press pageup or pageDown keys navigate to Country class to display Country id and name next elements first to last elements. When i run the code i got Exception please tell me How to navigate 3 tables

My database class:

 

namespace LWLLC.Persistence

{

using System;

using System.Collections.Generic;

public partial class Country

{

[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]

public Country()

{

this.PurchaseOrderLineItems = new HashSet<PurchaseOrderLineItem>();

this.States = new HashSet<State>();

}

public int CountryID { get; set; }

public string Country_Name { get; set; }

public string Country_Code { get; set; }

public Nullable<System.DateTime> CreatedDate { get; set; }

public Nullable<int> CreatedBy { get; set; }

public Nullable<System.DateTime> LastUpdatedDate { get; set; }

public Nullable<int> LastUpdatedBy { get; set; }

[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]

public virtual ICollection<PurchaseOrderLineItem> PurchaseOrderLineItems { get; set; }

[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]

public virtual ICollection<State> States { get; set; }

}

}

navigate Businesslogic class is:

 

public Country NavigateCoutryCodebyID(int countryCodeID, NavigationType nav)

{

//.Include("State").Include("PurchaseOrderLineItem")

Country countryCode = null;

try

{

using (LWLLCDBEntities dbEntities = new LWLLCDBEntities())

{

//current item

switch (nav)

{

case NavigationType.First:

countryCode = dbEntities.Countries.FirstOrDefault();

break;

case NavigationType.Last:

countryCode = dbEntities.Countries.OrderByDescending(t => t.CountryID).FirstOrDefault();

break;

case NavigationType.Current:

countryCode = dbEntities.Countries.Where(i => i.CountryID == countryCodeID).FirstOrDefault();

break;

case NavigationType.Next:

countryCode = dbEntities.Countries.Where(i => i.CountryID > countryCodeID).FirstOrDefault();

if (countryCode == null)

countryCode = dbEntities.Countries.FirstOrDefault();

break;

case NavigationType.Prev:

countryCode = dbEntities.Countries.Where(i => i.CountryID < countryCodeID).FirstOrDefault();

if (countryCode == null)

countryCode = dbEntities.Countries.OrderByDescending(t => t.CountryID).FirstOrDefault();

break;

}

}

}

catch (Exception ex)

{

if (log.IsErrorEnabled)

{

log.Error("BusinessLogic.NavigateCountryCodebyID in Business Logic threw exception: ", ex);

}

}

return countryCode;

}