C#

private void frmInward_Load(object sender, EventArgs e)
       {
           txtInwardNo.Text=ReturnInwardNo();
           LoadGridcolumns();
           LoadSuppliers();

       }

       private void LoadGridcolumns()
       {
           string SQL;
           OleDbConnection obcon = new OleDbConnection();

           SQL = "";
           SQL = "select '' as ItemGroup,'' as Item,Qty,Amount from InwardDetails";
           obcon.ConnectionString = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Application.StartupPath + "/MsBakery.mdb";
           obcon.Open();
           OleDbDataAdapter obda = new OleDbDataAdapter(SQL, obcon);
           DataSet ds = new DataSet();
           obda.Fill(ds,"InwardDetails");
           dgvInwardDet.DataSource = ds;
           dgvInwardDet.DataMember = "InwardDetails";

           dgvInwardDet.AutoResizeColumns();

       }

       private string ReturnInwardNo()
       {
           string InwardNo;
           InwardNo = "";

           OleDbConnection obcon = new OleDbConnection();
           OleDbCommand obcmd = new OleDbCommand();
           string SQL;
           bool status;

           status = false;

           try
           {
               obcon.ConnectionString = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Application.StartupPath + "/MsBakery.mdb";
               obcon.Open();
               SQL = "SELECT TOP 1 InwardNo FROM Inward ORDER BY InwardNo DESC";
               obcmd.CommandText = SQL;
               obcmd.Connection = obcon;

               OleDbDataReader obdr = obcmd.ExecuteReader();
               if (obdr.Read())
               {
                   InwardNo = Convert.ToString(Convert.ToInt32(obdr["InwardNo"].ToString()) + 1);
               }
               else
               {
                  return "1";
               }

           }

           catch (OleDbException ex)
           {
               MessageBox.Show(ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
               status = true;
           }

           finally
           {
               obcon.Close();
               if (!status)
               {
                   obcon = null;
                   obcmd = null;
               }

           }
           return InwardNo;
       }

       private void LoadSuppliers()
       {
           DataGridViewComboBoxColumn dgvcomcol = new DataGridViewComboBoxColumn ();
           //ComboBox cb = new ComboBox();

           dgvcomcol.MaxDropDownItems = 5;
           dgvcomcol.AutoComplete= true;
           //dgvcomcol.AutoSizeMode = DataGridViewAutoSizeColumnMode.Fill;
          // dgvcomcol.FlatStyle = FlatStyle.Flat;
           //dgvcomcol.DisplayMember = "Supplier";
           dgvcomcol.DisplayStyle = DataGridViewComboBoxDisplayStyle.Nothing;
           dgvcomcol.DisplayIndex = 0;
          // dgvcomcol.AutoSizeMode = DataGridViewAutoSizeColumnsMode.Fill ;
           dgvcomcol.HeaderText = "Supplier";
           PrepareSupplierCombo(dgvcomcol);
           //dgvcomcol.AutoSizeMode = DataGridViewAutoSizeColumnsMode.Fill;
           //dgvInwardDet.Columns[0].AutoSizeMode = DataGridViewAutoSizeColumnMode.Fill;
           //dgvcomcol.AutoSizeMode = DataGridViewAutoSizeColumnsMode.Fill;
           dgvInwardDet.Columns.Add(dgvcomcol);
           //dgvcomcol.DropDownWidth
           dgvcomcol.Width = 100;


           //dgvcomcol.AutoSizeMode=;


       }

       private void PrepareSupplierCombo(DataGridViewComboBoxColumn col)
       {

           OleDbConnection obcon = new OleDbConnection();
           string SQL;

           obcon.ConnectionString = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Application.StartupPath + "/MsBakery.mdb";
           obcon.Open();
           SQL = "SELECT SupplierName as Supplier,SupplierID  FROM Suppliers";
           OleDbDataAdapter obda = new OleDbDataAdapter(SQL, obcon);
           DataSet ds = new DataSet();
           obda.Fill(ds, "Suppliers");
           col.Items.Clear();
           col.DataSource = ds.Tables[0];
           col.DisplayMember = "Supplier";
           col.ValueMember = "SupplierID";
           //col.AutoSizeMode = DataGridViewAutoSizeColumnsMode.Fill;
           //col.AutoSizeMode = AutoSizeMode.GrowOnly;
           //col.Sorted = true;

       }

       private void dgvInwardDet_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
       {
           //dgvInwardDet_KeyDown += new DataGridViewEditingControlShowingEventHandler (dgvInwardDet_KeyDown);
           ComboBox com = e.Control as ComboBox;
           //ListBox lb = new ListBox();

           com.AutoCompleteMode = AutoCompleteMode.Suggest;
           com.AutoCompleteSource = AutoCompleteSource.ListItems;
           com.FlatStyle = FlatStyle.Flat;


          // com = Convert.ChangeType(e.Control, ComboBox);


           //dgvInwardDet.KeyDown += new KeyEventHandler(dgvInwardDet_KeyDown);
         //  dgvInwardDet.CellLeave -= new DataGridViewCellEventHandler(dgvInwardDet_CellLeave);
          // com.KeyDown += new KeyEventHandler(ComboBox_KeyDown);

       }