读卡器关闭时调用Read的尝试无效

本文关键字：无效 Read 调用 读卡器 | 更新日期: 2023-09-27 18:20:01

我已经编写了以下代码来从SQL Server数据库中读取数据，但它引发了一个异常：

读卡器关闭时调用ReadAsync()的尝试无效。

我已经调试了我的代码。代码中的注释。

[HttpGet]
public async Task<ActionResult> Index()
{
    List<Categorie> hfds = new List<Categorie>();
    try
    {
        using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString))
        {
            using (SqlCommand comm = new SqlCommand("SELECT id, naam, beschrijving from categorie", conn))
            {
                await conn.OpenAsync(); // connection string is ok
                using (DbDataReader reader = await comm.ExecuteReaderAsync())
                {
                    while (await reader.ReadAsync()) // on this line give compiler a 
                                                     // exception when he comes to the 
                                                     // line for the second time.
                    {
                        int id = (int)reader["ID"];
                        Categorie c = new Categorie()
                        {
                            Naam = reader["naam"].ToString(),
                            ID = id,
                            Beschrijving = reader["beschrijving"].ToString()
                        };
                        c.TopVijf = await TopicController.GeefTopVijfTopicsVanCategorie(id, conn);
                        hfds.Add(c); 
                        // after one time running the loop is everyting alright.
                    }
                }
            }
        }
    }
    catch (Exception ex)
    {
        Debug.WriteLine($"Er is een exception geworpen: {ex.Message}!", "Exception");
        return View(new HomeViewModel());
    }
    return View(new HomeViewModel() { Categorieen = hfds });
}
public static async Task<List<Topic>> GeefTopVijfTopicsVanCategorie(int catId, SqlConnection conn)
{
    if (conn.State == ConnectionState.Open)
    {
        conn.Close();
    }
    using (SqlCommand comm = new SqlCommand("declare @catid int = @id; declare @aantal int = (select top 1 MinTwee + MinEen + nul + PlusEen + PlusTwee from topic where categorieID = @catid and verwijderd = 0); if (@aantal = 0) set @aantal = 1; select top 5 id, mintwee, mineen, nul, pluseen, plustwee, naam from topic where CategorieID = @catid and verwijderd = 0 order by round(cast((mintwee * (-2) + mineen * (-1) + nul * 0 + pluseen * 1 + PlusTwee * 2) as float) / @aantal, 1) desc, creatie desc", conn))
    {
        comm.Parameters.Add(new SqlParameter("@id", catId) { SqlDbType = SqlDbType.Int });
        await conn.OpenAsync();
        List<Topic> topics = new List<Topic>();
        using (DbDataReader reader = await comm.ExecuteReaderAsync())
        {
            while (await reader.ReadAsync())
            {
                topics.Add(new Topic()
                {
                    ID = (int)reader["id"],
                    Naam = reader["naam"].ToString(),
                    MinTwee = (int)reader["mintwee"],
                    MinEen = (int)reader["mineen"],
                    Nul = (int)reader["nul"],
                    PlusEen = (int)reader["PlusEen"],
                    PlusTwee = (int)reader["PlusTwee"]
                });
            }
            reader.Close();
        }
        return topics;
        // also here must be everything good.
    }
}

另请参阅有两个DbDataReaders合一。我认为这是我的问题，但我不知道该怎么解决。你能帮我吗？

读卡器关闭时调用Read的尝试无效

我找到了！我已将此代码输出站点放置为第一个SqlCommand。

c.TopVijf = await TopicController.GeefTopVijfTopicsVanCategorie(id, conn);
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