Wp7 数据连接异常

我写了一个代码来映射一个表emp包含字段empId empName Salary。但它在到达 for 循环时在单击按钮 1 时引发异常无效的应用程序会话 ID。需要帮助来纠正它.我想查看一个 EmpName = ss 的 empId

私有常量字符串 ConnectionString = "isostore：/Emp1.sdf";

    [Table(Name = "Emp")]
    public class Emp2
    {
        [Column(IsPrimaryKey = true, IsDbGenerated = true)]
        public int EmpId
        {
            get;
            set;
        }
        [Column]
        public string EmpName
        {
            get;
            set;
        }
        [Column]
        public int Salary
        {
            get;
            set;
        }
    }

    public MainPage()
    {
        InitializeComponent();
        using (CountryDataContext context = new CountryDataContext(ConnectionString))
        {
            if (!context.DatabaseExists())
            {
                context.CreateDatabase();
            }
            else
            {
                MessageBox.Show("Employee table exist already");
            }
        }


    }

    private IList<Emp2> getcountry()
    {
        IList<Emp2> countryList = null;
        using (CountryDataContext context = new CountryDataContext(ConnectionString))
        {
            IQueryable<Emp2> query = from c in context.Emp where c.EmpName=="ss"   select c;
            countryList = query.ToList();
        }
        return countryList;
    }

    public class CountryDataContext : DataContext
    {
        public CountryDataContext(string connectionString)
            : base(connectionString)
        {
        }
        public Table<Emp2> Emp
        {
            get
            {
                return this.GetTable<Emp2>();
            }
        }

    }
    private void button1_Click(object sender, RoutedEventArgs e)
    {
        IList<Emp2> emp= this.getcountry();
        try
        {
            foreach (Emp2 a in emp)
            {
                MessageBox.Show(a.EmpId.ToString());
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }

    }