show save record in datagridview in c# using windowsapplic

Question

  Note it is windows application.

 
Run mode screen as follows;
 
Course Code Combo box
 
Datagridview as follows;
 
Days sess1 sess 2 sess3 sess4
1 checkbox checkbox checkbox checkbox
 
In Database record as follows;
 
course Code Days sess1 sess2 sess3 sess4
REO 1 checkboxchecked checkboxchecked checkboxchecked checkboxchecked 
 

Then i have one button in the run mode called Search Button when click the REO Course in the combo box, in the datagridview that particular course details to be displayed in the datagridview.
 

For that search button code as follows;
 
int irow;
sql = "select [Days],[sess1],[sess2],[sess3],[sess14] from Tb_Session_Structure where Cmn_Minor_code = '" + cb_Course_Code.Text.ToString() + "'";
 
dr = GFun.ReadAcessSql(sql);
while (dr.Read())
{
for (irow = 0; irow < DGv_Session.RowCount; irow++)
{
DGv_Session.Rows[irow].Cells[0] = dr[0].ToString().Trim();
DGv_Session.Rows[irow].Cells[1] = dr[1].ToString().Trim();
DGv_Session.Rows[irow].Cells[2] = dr[2].ToString().Trim();
DGv_Session.Rows[irow].Cells[3] = dr[3].ToString().Trim();
dr.Close();
}
}
 

 
when i click the Search button error shows as follows;
 
Cannot implicitly convert type string to System.Windows.Forms.DataGridViewCell
 
what is the problem in my above code.
in the search button what is the mistake i made.
 
please help me.
 
Regards,
 Rao