public class ComboData
{

 public int Id { get; set;}

 public string Value { get; set;}

 public string description { get; set;}

}

List<ComboData> ListData = new List<ComboData>();
ListData.Add(new ComboData { Id = "1", Value = "One" });
ListData.Add(new ComboData { Id = "2", Value = "Two" });
ListData.Add(new ComboData { Id = "3", Value = "Three" });
ListData.Add(new ComboData { Id = "4", Value = "Four" });
ListData.Add(new ComboData { Id = "5", Value = "Five" });

cbotest.ItemsSource = ListData;
cbotest.DisplayMemberPath = "Value";
cbotest.SelectedValuePath = "Id";

cbotest.SelectedValue = "2";

public class DAL
{
    Dbcontext db = new Dbcontext();

    public List<ComboData> GetAll()
    {
        var sp = db.ComboDatas.ToList();
        return sp;
    }
}