Question

我编写了以下代码来从SQL服务器中的表中提取所有记录。代码中没有错误，但是当我在fiddler中运行它时，我得到了空对象。

    [HttpGet]
    public List<Student> Get()
    {
        SqlDataReader reader = null;

        SqlConnection myConnection = new SqlConnection();

        myConnection.ConnectionString = @"Data Source=PALLAVI-PC\SQLEXPRESS;Initial Catalog=StudentDB;Integrated Security=True;MultipleActiveResultSets=True;";

        SqlCommand sqlCmd = new SqlCommand();
        sqlCmd.CommandType = CommandType.Text;
        sqlCmd.CommandText = "Select * from Tbl_Students;";
        sqlCmd.Connection = myConnection;

        myConnection.Open();

        reader = sqlCmd.ExecuteReader();

        int rowCount = 0;

        while (reader.Read())
        {
            rowCount++;
        }

        Student[] student = new Student[rowCount];
        for(int i=0;i<rowCount;i++)
        {
            student[i] = new Student();
        }

        int j = 0;

        while (reader.Read())
        {
          //  student[j] = new Student();
            student[j].Roll_Number = Convert.ToInt32(reader.GetValue(0));
            student[j].FirstName = reader.GetValue(1).ToString();
            student[j].LastName = reader.GetValue(2).ToString();
            student[j].Class = Convert.ToInt32(reader.GetValue(3));
            student[j].Gender = reader.GetValue(4).ToString();

            j++;

        }

        return student.ToList();
        myConnection.Close();
    }

我在表格中有5条记录。我能够获得5个json对象，但没有内容。

附加来自Fiddler的图像：

Student.cs

namespace WebAPIDemo.Models
{
   public class Student
   {
      public int Roll_Number { get; set; }
      public string FirstName { get; set; }
      public string LastName { get; set; }
      public int Class { get; set; }
      public string Gender { get; set; }
   }
 }

Answer 1

试一试，不计算记录数。我没有使用数组，我为每条记录创建了新的学生实例。

 public static List<Student> GetData()
    {
        List<Student> lstStudent = new List<Student>();
        using (SqlConnection con = new SqlConnection(@"Data Source=PALLAVI-PC\SQLEXPRESS;Initial Catalog=StudentDB;Integrated Security=True;MultipleActiveResultSets=True;"))
        {
            using (SqlCommand cmd = new SqlCommand("Select * from Tbl_Students;", con))
            {
                cmd.CommandType = CommandType.Text;
                if (con.State == ConnectionState.Closed)
                    con.Open();
                SqlDataReader reader = cmd.ExecuteReader();                   

                while(reader.Read())
                {
                    Student student =new Student();

                    student.Roll_Number = Convert.ToInt32(reader.GetValue(0));
                    student.FirstName = reader.GetValue(1).ToString();
                    student.LastName = reader.GetValue(2).ToString();
                    student.Class = Convert.ToInt32(reader.GetValue(3));
                    student.Gender = reader.GetValue(4).ToString();

                    lstStudent.Add(student);
                }                   

            }
        }
        return lstStudent;
    }

Answer 2

您的第一个while(reader.Read())是包含数据的实际读者对象。 DataReader允许仅转发对数据行的只读访问。下一个while循环甚至不会执行，因为Read()已经完成。所以你得到一个5的数组{ {1}}使用默认值初始化的对象。

而不是初始化Student个填充实例的数组，并添加到Student循环内的List<Student>。

无法在fiddler中显示表中提取的数据

2 个答案: