使用.net core mvc 将空值插入对象列表时出错？

Question

我在将 select 查询结果保存到模型 class 时遇到问题。

我正在尝试将取自 excel sheet 的值与本地数据库中的值相匹配。我一直收到错误 "Data is Null. This method or property cannot be called on Null values " 或类似的错误。我不认为我可以传递空值，因为我运行一个 SQL 查询用 0 替换所有空值。我也不关心是否将空值传递给这些属性，所以要么字段可为空或删除空值会起作用...我认为？

以下是我的代码：

 //initialize spreadsheet package,
        //load excel file, create dictionary of
        //naic/industry key/value pairs
        SpreadsheetInfo.SetLicense("FREE-LIMITED-KEY");
        ExcelFile ef = ExcelFile.Load("C:/Data Sources/NAICS sheets/3-Digit-NAICS-Codes.xls");
        Dictionary<object, object> naics = new Dictionary<object, object>();

        //Connection string for local MSSQL database
        SqlConnection con = new SqlConnection("Data Source=.\SQLEXPRESS;Initial Catalog=gov_world;Integrated Security=True;Connect Timeout=15;Encrypt=False;TrustServerCertificate=True;ApplicationIntent=ReadWrite;MultiSubnetFailover=False");


        //Create array of client objects returned from query
        Client[] allRecords = null;
        string sql = "SELECT v_company, client_pk, v_naic_1, v_naic_2, v_naic_3, v_naic_4, v_naic_5 FROM  dbo._clients";

        //Iterate through elements of excel sheet, eventually inputting first column value
        //as key and second column value as value in dictionary
        foreach (ExcelWorksheet sheet in ef.Worksheets)
        {


            foreach (ExcelRow row in sheet.Rows)
            {
                object[] industry = new object[2];
                var count = 0;

                //populate industry array with all values of row
                foreach (ExcelCell cell in row.AllocatedCells)
                {
                    if (cell.Value != null)
                    {
                        industry[count] = cell.Value;
                    }
                    count++;
                }
                count = 0;

                //add values of first 2 cells to naics dictionary object
                naics.Add(industry[0], industry[1]);
                //Console.WriteLine(industry[0] + " | " + industry[1]);

                //reinitialize for every record
                industry[0] = 0;
                industry[1] = 0;

            }
        }


        //Set values from query to client model object, uses
        //to populate allRecords Array
        using (var command = new SqlCommand(sql, con))
        {
            con.Open();
            using (var reader = command.ExecuteReader())
            {
                var list = new List<Client>();
                while (reader.Read())
                    list.Add(new Client {
                        //Solution: write as v_company = reader[0] as string;
                        v_company = reader.GetString(0),
                        client_pk = reader.GetInt32(1),
                        v_naic_1 = reader.GetString(2),
                        v_naic_2 = reader.GetString(3),
                        v_naic_3 = reader.GetString(4),
                        v_naic_4 = reader.GetString(5),
                        v_naic_5 = reader.GetString(6)

                    });
                allRecords = list.ToArray();

            }
        }


        Console.WriteLine(allRecords[0].v_naic_1);
        Console.ReadLine();

我可以算出匹配结果，此时我只想让 "list.Add(new Client..." 工作以便 while 循环完成。

Answer 1

您需要检查所有值的 IsDBNull：

if(!reader.IsDBNull(0))
{
  employee.FirstName = sqlreader.GetString(0);
}

或者您可以将其用作

reader[0] as string

另外最好使用列名而不是索引值，您可以通过列名获取列索引，如下所示：

int colIndex = reader.GetOrdinal(fieldname);

通过阅读 this 问题的所有答案，您可以找到有关如何从 reader 读取数据的更多信息和更好的方法。

使用.net core mvc 将空值插入对象列表时出错？

error inserting nulls into List of objects using .net core mvc?

c#

asp.net-core

ado.net

sqldatareader