一个实体对象不能由多个 IEntityChangeTracker 实例引用

直接来解决办法：

public partial class BaseDAL<T> where  T:class
{
        GzItcastEntities db = new GzItcastEntities();
    
         /// <summary>
        /// 新增
        /// </summary>
        /// <param name="model"></param>
        /// <returns></returns>
        public virtual T Add(T model)
        {
            db.Set<T>().Add(model);
            db.SaveChanges();
            db.Entry<T>(model).State = EntityState.Detached;　　//增加这句可以解决问题
            return model;
        } 


        /// <summary>
        /// 根据条件查询一个实体对象
        /// </summary>
        /// <param name="where"></param>
        /// <returns></returns>
        public virtual T GetModel(Expression<Func<T, bool>> where)
        {
            T model = db.Set<T>().Where(where).ToList().FirstOrDefault();
            db.Entry(model).State = EntityState.Detached;　　//增加这句可以解决问题
            return model;
        } 
}

说明：
vs2010才会抛出这个异常，2012不会

在vs2012 GzItcastEntities 默认继承于DbContext

在vs2010 GzItcastEntities 默认继承于ObjectContext  

如果继承于 ObjectContext  请调用Detach()方法（未实验）

补充：又发现个新办法，将DbContext 存入Session中，

GzItcastEntities = DbContextHelper.GetDbContext() as GzItcastEntities ;

 public static class DbContextHelper
    {
        private static DbContext db = null;
        private const string SessionKey_DbContext = "DbContext";
        

        /// <summary>
        /// 获得DbContext
        /// </summary>
        /// <returns></returns>
        public static DbContext GetDbContext()
        {
            db = HttpContext.Current.Session[SessionKey_DbContext] as DbContext;
            if (db == null)
            {
                db = new GZRA_ADSP_DBEntities();
                HttpContext.Current.Session[SessionKey_DbContext] = db;
            }
            return db;
        }
    }