SQLCommand和SqlDataReader错误

Question

没有数据时无效尝试读取。

此行发生此错误

string ingredientName = reader2.GetString(0);

我的代码：

var ingredientList = new List<Ingredient>();

SqlCommand staffCommand = new SqlCommand();

string conString = EventsUnlimited.Properties.Settings.Default.DatabaseEventsUnlimitedConnectionString;

using (SqlConnection connection = new SqlConnection(conString))
{
    connection.Open();

    using (SqlCommand command = new SqlCommand("SELECT IngredientID, Quantity FROM CourseIngredients WHERE CourseID =" + courseID, connection))
    {
        using (SqlDataReader reader = command.ExecuteReader())
        {
            while (reader.Read())
            {
                // Obtains the columns from customer
                int ingredientID = reader.GetInt32(0);
                decimal quantity = reader.GetDecimal(1);

                string ingredientNameConstruct;

                SqlCommand ingredientCommand = new SqlCommand();
                string conString2 = EventsUnlimited.Properties.Settings.Default.DatabaseEventsUnlimitedConnectionString;

                using (SqlConnection connection2 = new SqlConnection(conString2))
                {
                    connection2.Open();

                    using (SqlCommand command2 = new SqlCommand ("SELECT IngredientName FROM Stock WHERE IngredientID =" + ingredientID, connection2))
                    {
                        using (SqlDataReader reader2 = command2.ExecuteReader())
                        {
                            string ingredientName = reader2.GetString(0);
                            ingredientNameConstruct = ingredientName;
                        }  
                    }
                }

                ingredientList.Add(new Ingredient(courseID, ingredientNameConstruct, ingredientID, quantity));
            }
        }
    }

    return ingredientList;
}

我不确定导致这个问题的原因。表中有数据和我试图读取的行。

Answer 1

你没有为你的Read()对象调用reader2方法，它实际上是根据返回的结果读取一行，在实际读取列值之前添加它：

using (SqlDataReader reader2 = command2.ExecuteReader())
{
      if(reader2.Read()) // this is needed
      {
          string ingredientName = reader2.GetString(0);
          ingredientNameConstruct = ingredientName;
      }
}  

如果你期望多行然后使用while循环，并且如果它总是单行期望结果它们你也可以使用reader2.ExecuteScalar作为评论中提到的@ bradbury9：

string ingredientName = command2.ExecuteScalar()?.ToString();
ingredientNameConstruct = ingredientName;

Answer 2

在DataReader.Read DataReader中调用reader2方法以获得结果。