指定的转换无效。
public List< PetShop.Model.ItemInfo> GetItemById(string ItemId)
   
  {

  string sql = "select item.itemid, item.name, item.listprice, product.name, item.image, product.categoryid, product.productid from item inner join product on item.productid = product.productid where item.itemid = @itemid";
  string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings["ssa"].ConnectionString;

   
  List<PetShop.Model.ItemInfo> list = new List<PetShop.Model.ItemInfo>();
  using (SqlConnection connection = new SqlConnection(connectionString))
  {
  SqlCommand command = new SqlCommand(sql, connection);
  connection.Open();

  command.Parameters.Add("@itemid", SqlDbType.NVarChar).Value = ItemId;

  SqlDataReader reader = command.ExecuteReader();

   

  if (reader.Read())
  {


  PetShop.Model.ItemInfo item = new PetShop.Model.ItemInfo()
  {
  ItemId = reader.GetString(0),
  Name = reader.GetString(1),
  Quantity = reader.GetInt32(2),
  ListPrice = reader.GetDecimal(3),
  ProductName = reader.GetString(4),
  Image = reader.GetString(5),
  CategoryId = reader.GetString(6),
  ProductId = reader.GetString(7)
  };
   
  list.Add(item);

  }
  return list;
   
  }





//////////////////////////////////////以下是报错的行/////////
  PetShop.Model.ItemInfo item = new PetShop.Model.ItemInfo()
  {
  ItemId = reader.GetString(0),
  Name = reader.GetString(1),
  Quantity = reader.GetInt32(2),
  ListPrice = reader.GetDecimal(3),
  ProductName = reader.GetString(4),
  Image = reader.GetString(5),
  CategoryId = reader.GetString(6),
  ProductId = reader.GetString(7)

------解决方案--------------------
还有可能是视图的结构与你的model结构不同 
主要原因就是数据类型错误 一般是int和字符串类型