[英]I have a error Object cannot be cast from DBNull to other types?
我的代码是:我在做断点时正在检索数据氟利昂数据库,它在列表中显示数据,但它也给我一个错误
public static List<StudentScore> GetAllScore()
{
SqlConnection conn = MyDB.GetConnection();
string selectStm = "SELECT en.CourseID,en.Score,s.StudentID FROM EnrollmentTable en,Student s WHERE en.StudentID = s.StudentID";
SqlCommand command = new SqlCommand(selectStm, conn);
List<StudentScore> aStudentScore = new List<StudentScore>();
try
{
conn.Open();
SqlDataReader reader = command.ExecuteReader();
Console.WriteLine(reader.HasRows.ToString());
while (reader.Read())
{
StudentTable st = new StudentTable();
CourseTable cr = new CourseTable();
Enrollment enr = new Enrollment();
StudentScore score = new StudentScore();
enr.CourseData = cr;
enr.StudentData = st;
//score.EnrollmentData.StudentData.StudentID = reader["StudentID"].ToString();
//score.EnrollmentData.CourseData.CourseID = reader["CourseID"].ToString();
st.StudentID = reader["StudentID"].ToString();
cr.CourseID = reader["CourseID"].ToString();
score.Score = Convert.ToInt32(reader["Score"]);
score.EnrollmentData = enr;
aStudentScore.Add(score);
}
reader.Close();
return aStudentScore;
}
catch (SqlException ex)
{
throw ex;
}
finally
{
conn.Close();
}
}
}
}
它从数据库中获取数据,但向mw显示此错误.....无法将对象从DBNull强制转换为其他类型,所以这是什么意思,请告诉我如何解决?
这意味着您在数据库中有一个NULL
值。 您必须在代码中检查它,或将列模式更改为NOT NULL
。
st.StudentID = reader["StudentID"] == DBNull.Value ? null : reader["StudentID"].ToString();
cr.CourseID = reader["CourseID"] == DBNull.Value ? null : reader["CourseID"].ToString();
score.Score = reader["Score"] == DBNull.Value ? 0 : Convert.ToInt32(reader["Score"]);
您现在必须处理C#对象中的null
值。
您需要检查阅读器是否为DBNULL类型
在尝试对其进行转换之前,请在读取器上调用IsDBNull()以检查该列:
using (reader = server.ExecuteReader(CommandType.Text, TopIDQuery, paramet))
{
while (reader.Read())
{
var column = reader.GetOrdinal("TopID");
if (!reader.IsDBNull(column))
topID = Convert.ToInt32(reader[column]);
}
}
}
或者,与DBNull.Value进行比较:
var value = reader["TopID"];
if (value != DBNull.Value)
{
topID = Convert.ToInt32(value);
}
DBNull用于表示数据库中的空值。
您应该在转换ir之前检查该值是否不是DBNull。
object score = reader["Score"];
score.Score = score == DBNull.Value ? 0 : Convert.ToInt32(score);
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