asp.net高分求助:Object reference not set to an instance of an object
我查了论坛里很多相关的问题，大概是对象没有实例化的问题，但还是不知道解决方法，希望得到大家的帮助！

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 82:
Line 83:
Line 84: string teacherName = ds.Tables[ "teacherName "].NewRow().ToString();

贴出源码如下：

string strSql = "SELECT college.collegeName, teacher.teacherID, teacher.teacherName, college.collegeID FROM teacher INNER JOIN college ON teacher.collegeID = college.collegeID WHERE (teacher.teacherID = @teacherID) ";

string sconn=ConfigurationSettings.AppSettings[ "conn "];
SqlConnection conn= new SqlConnection(sconn);
conn.Open();
SqlParameter para = new SqlParameter( "@teacherID ",SqlDbType.NVarChar);

para.Value =Session[ "sUser "].ToString();
SqlDataAdapter da = new SqlDataAdapter(strSql,conn);
da.SelectCommand.Parameters.Add(para);

DataSet ds = new DataSet();
da.Fill(ds);
string teacherName = ds.Tables[ "teacherName "].NewRow().ToString();
conn.Close();





[解决办法]
string teacherName;
if(ds.Tables[0].Rows.Count> 0&&ds.Tables[0].Rows[0][ "teacherName "]!=DBNull.Value)
teacherName = ds.Tables[0].Rows[0][ "teacherName "].ToString();