Getting an error(cannot insert null values,use dbnull instead)combobox taking null values.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;
using System.Configuration;
using Microsoft.VisualBasic;
//using System.Data.SqlClient;
//using System.Configuration;


namespace ESchool
{
    public partial class frmteacherallocation : frmDef_BaseRoot1
    {
        public ColFieldDescription mCFields = new ColFieldDescription();
       

        public frmteacherallocation()
        {
            InitializeComponent();
        }
        SqlConnection CON = new SqlConnection(ConfigurationManager.AppSettings["CONSTR"]);
        SqlCommand CMD;// = new SqlCommand();
        DataSet DSLoadData = new DataSet();
        SqlDataAdapter DAA;
        GblDefinitions GV = new GblDefinitions();
        public override bool mfnSaveForm()
        {
            bool FunctionReturnValue=false;
           
            DSLoadData.Tables[0].Constraints.Clear();
            DataRow drow = DSLoadData.Tables[0].NewRow();
            try
            {
                if (string.IsNullOrEmpty(cmbclassname.Text))
                {
                    MessageBox.Show("Select Class Name");
                    cmbclassname.Focus();
                    FunctionReturnValue = false;
                }
                else
                {
                    drow["Class_Name"] = cmbclassname.SelectedValue;
                }
                if (Information.IsDBNull(cmbSection.Text)) 
                {
                    MessageBox.Show("Select a section");
                    cmbSection.Focus();
                    FunctionReturnValue = false;
                }
                else
                {
                    drow["Section"] = cmbSection.SelectedValue;
                }
                if (string.IsNullOrEmpty(cmbstaffID.Text))
                {
                    MessageBox.Show("Select a staff id");
                    cmbstaffID.Focus();
                    FunctionReturnValue = false;
                }
                else
                {
                    drow["StaffID"] = cmbstaffID.SelectedValue;
                }
                if (string.IsNullOrEmpty(txtteachername.Text))
                {
                    MessageBox.Show("Field cant be empty");
                    txtteachername.Focus();
                    FunctionReturnValue = false;
                }
                else
                {
                    drow["StaffName"] = txtteachername.Text;
                }
                if (string.IsNullOrEmpty(cmbsubject.Text))
                {
                    MessageBox.Show("Select a subject");
                    cmbsubject.Focus();
                    FunctionReturnValue = false;
                }
                else
                {
                    drow["Subject"] = cmbsubject.SelectedValue;
                }
                DSLoadData.Tables[0].Columns["TeacherID"].AllowDBNull = true;
                DSLoadData.Tables[0].Rows.Add(drow);

                SqlCommand cmdinsert = new SqlCommand();
                cmdinsert.Connection = CON;
                DAA.InsertCommand = cmdinsert;
                cmdinsert.CommandText = "insert into Teacher_Allocation(StaffID,Class_Name,Section,Subject,StaffName)VALUES(@StaffID,@Class_Name,@Section,@Subject,@StaffName)";
                cmdinsert.Parameters.Add("@StaffID", SqlDbType.Int, 50, "StaffID");
                cmdinsert.Parameters.Add("@Class_Name", SqlDbType.Int, 50, "Class_Name");
                cmdinsert.Parameters.Add("@Section", SqlDbType.Int, 50, "Section");
                cmdinsert.Parameters.Add("@Subject", SqlDbType.Int, 50, "Subject");
                cmdinsert.Parameters.Add("@StaffName", SqlDbType.VarChar, 100, "StaffName");

                DAA.Update(DSLoadData.Tables[0]);
                MessageBox.Show("Records Saved Succesfully");

                SqlCommand CCCMM = new SqlCommand("select * from Teacher_Allocation ", CON);
                DSLoadData.Clear();
                DAA.Fill(DSLoadData, "Teacher_Allocation");
                ultraGrid1.DataSource = null;
                ultraGrid1.DataSource = DSLoadData.Tables[0].DefaultView;

                FunctionReturnValue = true;
              
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
            return FunctionReturnValue;
        }
        private void frmteacherallocation_Load(object sender, EventArgs e)
        {
            MProcLoad("all");
            CMD = new SqlCommand("select * from Teacher_Allocation ", CON);
            DAA = new SqlDataAdapter(CMD);
            DAA.Fill(DSLoadData, "Teacher_Allocation");
            ultraGrid1.DataSource = DSLoadData.Tables[0].DefaultView;
          //LODADATA();
           
        }
        //SqlCommand cmdLoad;
        //DataSet DSload;
        //SqlDataAdapter DALoad;
        private void LODADATA()
        {
            clsPopulateList Populate = new clsPopulateList();          
            DataSet dt = new DataSet();
            dt=GV.LoadDataset("select ClassID,ClassName from Class_Master");
            cmbclassname.DataSource = dt.Tables[0];          
            cmbclassname.DisplayMember = "ClassName";
            cmbclassname.ValueMember = "ClassID";
            dt.Clear();
            dt = GV.LoadDataset("select SubjectID,SubjectName from Subject_Master");
            cmbsubject.DataSource = dt.Tables[0];
            cmbsubject.DisplayMember = "SubjectName";
            cmbsubject.ValueMember = "SubjectID";
            dt.Clear();
            dt = GV.LoadDataset("select SectionID,SectionName from Section_Master");
            cmbSection.DataSource = dt.Tables[0];
            cmbSection.DisplayMember = "SectionName";
            cmbSection.ValueMember = "SectionID";
            dt.Clear();
            dt = GV.LoadDataset("select StaffID,Staffname from Staff_Details");
            cmbstaffID.DataSource = dt.Tables[0];
            cmbstaffID.DisplayMember = "StaffID";
            cmbstaffID.ValueMember = "Staffname";

        }
        public bool MProcLoad(string names)
        {
            clsPopulateList CPopulate = new clsPopulateList();
            bool result = false;
            switch (names)
            {
                case "all":
                    CPopulate.Load_Combo(cmbclassname, "Class_Master", "ClassName", "ClassID", "");
                    cmbclassname.DisplayMember = "ClassName";
                    cmbclassname.ValueMember = "ClassID";
                    CPopulate.Load_Combo(cmbsubject, "Subject_Master", "SubjectName", "SubjectID", "");
                    CPopulate.Load_Combo(cmbSection, "Section_Master", "SectionName", "SectionID", "");
                    cmbSection.DisplayMember = "SectionName";
                    cmbSection.ValueMember = "SectionID";
                    CPopulate.Load_Combo(cmbstaffID, "Staff_Details", "StaffID", "Staffname", "");
                    break;
            }
            return result;
        }

        private void cmbstaffID_SelectedIndexChanged(object sender, EventArgs e)
        {
            GblDefinitions GV = new GblDefinitions();
            txtteachername.Text =GV.gfnGetName("select staffname from  Staff_Details where StaffID='"+ cmbstaffID.Text+"'");
           
        }

      
    }
}

====================================================================
using System;
using System.Collections.Generic;
using System.Text;
using Microsoft.VisualBasic;
using System.Collections;
using System.Data;
using System.Diagnostics;
using System.Data.SqlClient;
using System.Windows.Forms;

namespace ESchool
{
    public class clsPopulateList
    {
        GblDefinitions GV = new GblDefinitions();
        GblDeclarations GD = new GblDeclarations();

        public IList gfnLoadCombo(string tblName, string DisplayTextCol, string DisplayKeyCol, string strQuery )
        {
            strQuery = null;
            List CmboList = new List();
            string strSQL = null;
            SqlCommand cmdCombo = new SqlCommand();
            SqlDataReader RdrCombo = default(SqlDataReader);
            try {
                if (strQuery == null) {
                    strSQL = " SELECT " + DisplayTextCol + "," + DisplayKeyCol + " FROM " + tblName + " ORDER BY " + DisplayTextCol;
                } else {
                    strSQL = strQuery;
                }
                cmdCombo.CommandText = strSQL;
                cmdCombo.Connection = GV.GetConnection();
                if (GD.gblTransBegin)
                    cmdCombo.Transaction = GD.gblSQLTrans;
                RdrCombo = cmdCombo.ExecuteReader();
                if (RdrCombo.HasRows)
                {
                    while (RdrCombo.Read())
                    {
                        CmboList.Add(new clsList(RdrCombo[0].ToString(),Convert.ToInt32( RdrCombo[1].ToString())));
                    }
                }
                RdrCombo.Close();
            } catch (Exception ex) {
                GV.gprocErrorHandler(ex.Source, ex.Message, "gfnLoadCombo", "clsPopulateList");
            }
            return (CmboList);
        }
        public void Load_Combo(ComboBox objCombo, string TableName , string DisplayTextColumn , string CellDataColumn , string sqlStmt )
        {

            //CellDataColumn = "";
            //sqlStmt = "";
            //TableName = "";
            // DisplayTextColumn = "";
            string strSQL = null;
            ArrayList ListItem = new ArrayList();
            SqlCommand cmdCombo = new SqlCommand();
            SqlDataReader RdrCombo ;//= default(SqlDataReader);
            try
            {
                if (string.IsNullOrEmpty(Strings.Trim(DisplayTextColumn)))
                {
                    DisplayTextColumn = CellDataColumn;
                }
                else if (string.IsNullOrEmpty(Strings.Trim(CellDataColumn)))
                {
                    CellDataColumn = DisplayTextColumn;
                }

                if (string.IsNullOrEmpty(Strings.Trim(sqlStmt)))
                {
                    strSQL = " SELECT " + DisplayTextColumn + "," + CellDataColumn + " FROM " + TableName + " ORDER BY " + DisplayTextColumn;
                }
                else
                {
                    strSQL = sqlStmt;
                }
                cmdCombo.CommandText = strSQL;
                cmdCombo.Connection = GV.GetConnection();
                if (GD.gblTransBegin)
                    cmdCombo.Transaction = GD.gblSQLTrans;
                RdrCombo = cmdCombo.ExecuteReader();
                if (RdrCombo.HasRows)
                {
                    while (RdrCombo.Read())
                    {
                        //ListItem.Add(new clsList(RdrCombo[0].ToString(),Convert.ToInt32(RdrCombo[1].ToString())));
                        objCombo.Items.Add(RdrCombo[0].ToString());
                    }
                }
                RdrCombo.Close();

                //if (ListItem.Count > 0)
                //{
                //    objCombo.DataSource = ListItem;
                //    objCombo.DisplayMember = "GetName";
                //    objCombo.ValueMember = "GetID";
                //}
                objCombo.SelectedIndex = -1;
            }
            catch (Exception ex)           
            {
                GV.gprocErrorHandler(ex.Source, ex.Message, "Load_Combo", "clsPopulateList");
            }
        }
        public void Load_GridCombo(ref DataGridViewComboBoxColumn objCombo, string TableName , string DisplayTextColumn , string CellDataColumn , string sqlStmt )
        {
            string strSQL = null;
            TableName = "";
            DisplayTextColumn = "";
            CellDataColumn = "";
            sqlStmt = "";
            ArrayList ListItem = new ArrayList();
            SqlCommand cmdCombo = new SqlCommand();
            SqlDataReader RdrCombo = default(SqlDataReader);
            try {
                if (string.IsNullOrEmpty(Strings.Trim(DisplayTextColumn)))
                {
                    DisplayTextColumn = CellDataColumn;
                } else if (string.IsNullOrEmpty(Strings.Trim(CellDataColumn)))
                {
                    CellDataColumn = DisplayTextColumn;
                }

                if (string.IsNullOrEmpty(Strings.Trim(sqlStmt)))
                {
                    strSQL = " SELECT " + DisplayTextColumn + "," + CellDataColumn + " FROM " + TableName + " ORDER BY " + DisplayTextColumn;
                }
                else
                {
                    strSQL = sqlStmt;
                }
                cmdCombo.CommandText = strSQL;
                cmdCombo.Connection = GV.GetConnection();
                if (GD.gblTransBegin)
                    cmdCombo.Transaction = GD.gblSQLTrans;
                RdrCombo = cmdCombo.ExecuteReader();
                if (RdrCombo.HasRows)
                {
                    while (RdrCombo.Read())
                    {
                        objCombo.Items.Add(RdrCombo[0].ToString());
                        //ListItem.Add(New clsList(RdrCombo(0).ToString, RdrCombo(1).ToString))
                    }
                }
                RdrCombo.Close();

            //If ListItem.Count > 0 Then
            //    objCombo.DataSource = ListItem
            //    objCombo.DisplayMember = "GetName"
            //    objCombo.ValueMember = "GetID"
            //End If
            } catch (Exception ex)
            {
                GV.gprocErrorHandler(ex.Source, ex.Message, "Load_GridCombo", "clsPopulateList");
            }
        }
        public void Load_GridItemCombo(ref DataGridViewComboBoxColumn objCombo, string TableName, string DisplayTextColumn1, string DisplayTextColumn2, string CellDataColumn, string sqlStmt)
        {
             sqlStmt = "";
            CellDataColumn = "";
            DisplayTextColumn2 = "";
            DisplayTextColumn1 = "";
            TableName = "";
            string strSQL = null;
            ArrayList ListItem = new ArrayList();
            SqlCommand cmdCombo = new SqlCommand();
            SqlDataReader RdrCombo ;//= default(SqlDataReader);
            try {
                if (string.IsNullOrEmpty(Strings.Trim(DisplayTextColumn1)))
                {
                    DisplayTextColumn1 = CellDataColumn;
                }
                else if (string.IsNullOrEmpty(Strings.Trim(CellDataColumn)))
                {
                    CellDataColumn = DisplayTextColumn1;
                }

                if (string.IsNullOrEmpty(Strings.Trim(sqlStmt)))
                {
                    if (string.IsNullOrEmpty(DisplayTextColumn2))
                    {
                        strSQL = " SELECT " + DisplayTextColumn1 + "," + CellDataColumn + " FROM " + TableName + " ORDER BY " + DisplayTextColumn1;
                    }
                    else
                    {
                        strSQL = " SELECT " + DisplayTextColumn1 + "," + DisplayTextColumn2 + "," + CellDataColumn + " FROM " + TableName + " ORDER BY " + DisplayTextColumn1;
                    }
                }
                else
                {
                    strSQL = sqlStmt;
                }
                cmdCombo.CommandText = strSQL;
                cmdCombo.Connection = GV.GetConnection();
                if (GD.gblTransBegin)
                    cmdCombo.Transaction = GD.gblSQLTrans;
                RdrCombo = cmdCombo.ExecuteReader();
                if (RdrCombo.HasRows)
                {
                    while (RdrCombo.Read())
                    {
                        if (string.IsNullOrEmpty(DisplayTextColumn2))
                        {
                            ListItem.Add(new clsList(RdrCombo[0].ToString(), Convert.ToInt32(RdrCombo[1].ToString())));
                        }
                        else
                        {
                            ListItem.Add(new clsList(RdrCombo[0].ToString()+ Constants.vbCr +Convert.ToInt32(RdrCombo[1].ToString()),Convert.ToInt32( RdrCombo[2].ToString())));
                        }
                    }
                }
                RdrCombo.Close();

                if (ListItem.Count > 0)
                {
                    objCombo.DataSource = ListItem;
                    objCombo.DisplayMember = "GetName";
                    objCombo.ValueMember = "GetID";
                }
            } catch (Exception ex)
            {
                GV.gprocErrorHandler(ex.Source, ex.Message, "Load_GridCombo", "clsPopulateList");
            }
        }

        //Not used anywhere
        public void Load_GridComboWithDS(ref DataGridViewComboBoxColumn objCombo, string TableName , string DisplayTextColumn , string CellDataColumn, string sqlStmt)
        {
            sqlStmt = "";
            CellDataColumn = "";
            DisplayTextColumn = "";
            TableName = "";
            string strSQL = null;
            SqlDataAdapter da = new SqlDataAdapter();
            DataSet ds = new DataSet();
            try {
                if (string.IsNullOrEmpty(Strings.Trim(DisplayTextColumn))) {
                    DisplayTextColumn = CellDataColumn;
                } else if (string.IsNullOrEmpty(Strings.Trim(CellDataColumn))) {
                    CellDataColumn = DisplayTextColumn;
                }

                if (string.IsNullOrEmpty(Strings.Trim(sqlStmt))) {
                    strSQL = " SELECT " + DisplayTextColumn + "," + CellDataColumn + " FROM " + TableName + " ORDER BY " + DisplayTextColumn;
                } else {
                    strSQL = sqlStmt;
                }
                SqlCommand cmdCombo = new SqlCommand(strSQL, GV.GetConnection());
                if (GD.gblTransBegin)
                    cmdCombo.Transaction = GD.gblSQLTrans;
                da.SelectCommand = cmdCombo;
                da.Fill(ds);

                objCombo.DataSource = ds.Tables[0];
                objCombo.DisplayMember = ds.Tables[0].Columns[0].ToString();
                objCombo.ValueMember = ds.Tables[0].Columns[1].ToString();
            } catch (Exception ex)
            {
                GV.gprocErrorHandler(ex.Source, ex.Message, "Load_GridComboWithDS", "clsPopulateList");
            }
        }

        //Not used anywhere, can be used to load Item Location
        public void Load_GridComboCell(ref DataGridViewComboBoxCell objCombo, string TableName , string DisplayTextColumn, string CellDataColumn, string sqlStmt)
        {
            CellDataColumn = "";
            sqlStmt = "";
            DisplayTextColumn = "";
            TableName = "";
            string strSQL = null;
            ArrayList ListItem = new ArrayList();
            SqlCommand cmdCombo = new SqlCommand();
            SqlDataReader RdrCombo ;//= default(SqlDataReader);
            try {
                if (string.IsNullOrEmpty(Strings.Trim(DisplayTextColumn)))
                {
                    DisplayTextColumn = CellDataColumn;
                } else if (string.IsNullOrEmpty(Strings.Trim(CellDataColumn)))
                {
                    CellDataColumn = DisplayTextColumn;
                }

                if (string.IsNullOrEmpty(Strings.Trim(sqlStmt)))
                {
                    strSQL = " SELECT " + DisplayTextColumn + "," + CellDataColumn + " FROM " + TableName + " ORDER BY " + DisplayTextColumn;
                }
                else
                {
                    strSQL = sqlStmt;
                }
                cmdCombo.CommandText = strSQL;
                cmdCombo.Connection = GV.GetConnection();
                if (GD.gblTransBegin)
                    cmdCombo.Transaction = GD.gblSQLTrans;
                RdrCombo = cmdCombo.ExecuteReader();
                if (RdrCombo.HasRows)
                {
                    while (RdrCombo.Read())
                    {
                        ListItem.Add(new clsList(RdrCombo[0].ToString(),Convert.ToInt32( RdrCombo[1].ToString())));
                    }
                }
                RdrCombo.Close();

                if (ListItem.Count > 0)
                {
                    objCombo.DataSource = ListItem;
                    objCombo.DisplayMember = "GetName";
                    objCombo.ValueMember = "GetID";
                }
            } catch (Exception ex)
            {
                GV.gprocErrorHandler(ex.Source, ex.Message, "Load_GridComboCell", "clsPopulateList");
            }
        }

    }
}
================================================================================
Teacher Allocation Table
USE [SchoolManagement]
GO
/****** Object:  Table [dbo].[Teacher_Allocation]    Script Date: 06/01/2011 17:35:21 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[Teacher_Allocation](
    [StaffID] [int] IDENTITY(1,1) NOT NULL,
    [Class_Name] [varchar](50) NOT NULL,
    [Section] [int] NOT NULL,
    [Subject] [int] NOT NULL,
    [Teacher_Name] [varchar](100) NOT NULL
) ON [PRIMARY]

GO
SET ANSI_PADDING OFF