Question

我尝试从中提取数据的SQL Server中的表包含以下列：

名称|性别|工资|系

在下面的代码中，我要做的就是从表中提取员工信息，从该信息中创建员工对象，然后将该对象添加到员工列表中。这是代码：

namespace LINQQueriesPart2
{
    class Program
    {
        List<Employee> whatever = EmployeeDB.getEmployees();

        IEnumerable<Employee> query = from y in whatever
                                      where y.name[0] == 'M'
                                      select y;

        foreach (Employee x in query)
        {
            Console.WriteLine(x.name);
        }
    }
}

namespace ConnectingToSql
{
    public static class EmployeeDB
    {
        public static List<Employee> getEmployees()
        {
            List<Employee> returnList = new List<Employee>();

            String connectionString = "server=DESKTOP-T5KDKOP;"
                                    + "database=MCP_Exam_70-483_Prep;"
                                    + "Trusted_Connection=yes";

            if (SQLConnectingTools.CheckConnection(connectionString))
            {
                SqlConnection connection = new SqlConnection(connectionString);
                connection.Open();
                SqlCommand command = new SqlCommand("SELECT * FROM employees", connection);

                SqlDataReader reader = command.ExecuteReader();
                while (reader.Read())
                {
                    returnList.Add(
                       new Employee
                       {
                           name = reader.GetString(0),
                           gender = reader.GetChar(1),
                           salary = reader.GetInt16(2),
                           department = reader.GetString(3)
                       }
                   );
               }
               reader.Close();

               connection.Close();
           }
           else
           {
               Console.WriteLine("SQL connection is not successful.");
           }
           return returnList;
       }
   }

   public class Employee
   {
       public String name { get; set; }
       public char gender { get; set; }
       public int salary { get; set; }
       public String department { get; set; }

       public Employee() { }
       }
   }

当我在调试模式下运行上面的代码时（在Visual Studio中），代码中断并且以下代码以黄色突出显示：

returnList.Add(
    new Employee
    {
        name = reader.GetString(0),
        gender = reader.GetChar(1),
        salary = reader.GetInt16(2),
        department = reader.GetString(3)
    }
);'

Answer 1

首先，您应该检查结果中是否存在任何行。使用此代码，您可以检查：

if (reader.HasRows)

如果employee表和Employee对象的设计不同，并且您尝试进行无效的转换或结果中没有数据，则可能会收到System.NotSupportedException或System.InvalidCastException。您将收到System.NotSupportedException。

我对你的代码做了一些改进，这应该有效：

using (SqlConnection cn = new SqlConnection(connectionString))
{
    string commandText = "SELECT * FROM employees";
    using (SqlCommand cmd = new SqlCommand(commandText, cn))
    {
        cmd.CommandText = commandText;

        cn.Open();
        SqlDataReader reader = cmd.ExecuteReader();
        if (reader.HasRows)
        {
            while (reader.Read())
            {
                var employeeColumns = new object[4];
                var colCount = reader.GetSqlValues(employeeColumns);
                returnList.Add(
                    new Employee
                    {
                        name = employeeColumns[0].ToString(),
                        gender = employeeColumns[1].ToString()[0],
                        salary = Convert.ToInt16(employeeColumns[2].ToString()),
                        department = employeeColumns[3].ToString()
                    }
                );
            }
        }
    }
}

只需将上面的代码复制到if块（在此行之后）：

if (SQLConnectingTools.CheckConnection(connectionString))

尝试将对象添加到List时获取NotSupportedException

1 个答案: