錯誤是「對象引用未設置爲對象的實例」

Question

我有一個問題，當我運行我的代碼時發生錯誤，「對象引用未設置爲對象的實例」。

PLZ建議我關於這一點。

代碼

protected void btn_Save_Click(object sender, EventArgs e) 
    { 
     string str = ("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:/Users/Geeta/Desktop/eTimeTrackLite1.mdb;Persist Security Info=False;"); 
     OleDbConnection conn = new OleDbConnection(str); 
     conn.Open(); 
     string query = "insert into Employees ([EmployeeName],EmployeeCode,DeviceCode,Company,Department,Designation,Grade,Team,Location,EmploymentType,Category,HolidayGroup,ShiftGroup,ShiftRoster,Dateofjoining,Dateofconfirmation,Status,DateofResigning,[Sex]) values ('" + txt_empname.Text + "','" + txt_code.Text + "', '" + txt_dcode.Text + "', '" + dp_company.SelectedItem.ToString() + "', '" + dp_department.SelectedItem.ToString() + "', '"+dp_designation.SelectedItem.ToString()+"', '"+dp_grade.SelectedItem.ToString()+"', '"+dp_team.SelectedItem.ToString()+"', '"+dp_location.SelectedItem.ToString()+"', '"+dp_emptype.SelectedItem.ToString()+"', '"+dp_category.SelectedItem.ToString()+"', '"+dp_holigroup.SelectedItem.ToString()+"', '"+dp_shiftgroup.SelectedItem.ToString()+"', '"+dp_shiftroster.SelectedItem.ToString()+"', '"+dp_day.SelectedItem.ToString()+"', '"+dp_month.SelectedItem.ToString()+"', '"+dp_year.SelectedItem.ToString()+"', '"+dp_cday.SelectedItem.ToString()+"', '"+dp_cmonth.SelectedItem.ToString()+"', '"+dp_cyear.SelectedItem.ToString()+"', '"+dp_status.SelectedItem.ToString()+"', '"+dp_rday.SelectedItem.ToString()+"', '"+dp_rmonth.SelectedItem.ToString()+"', '"+dp_ryear.SelectedItem.ToString()+"', '"+rdbtn_male.Checked.ToString()+"', '"+rdbtn_female.Checked.ToString()+"')"; 
     OleDbCommand cmd = new OleDbCommand(query, conn); 
     cmd.ExecuteNonQuery(); 
     conn.Close(); 
     BindGridData(); 
    } 

「謝謝」

Answer 1

爲了避免這種問題，不使用的ToString是有調用對象的風險爲空，使用轉換.ToString（）代替。 http://msdn.microsoft.com/en-us/library/system.convert.tostring.aspx