how can maintain viewstate for gridview for different postback

protected void subjectdropdown_SelectedIndexChanged(object sender, EventArgs e)
       {
           studmarkgrid.Columns.RemoveAt(0);
           comp.ACADAMICYEAR = acyeardropdown.SelectedItem.Text;
           comp.MEDIUM = mediumdropdown.SelectedItem.Text;
           string name = classdropdown.SelectedItem.Text;
           string[] divide = name.Split('-');
           comp.CLASSNAME = divide[0].ToString();
           comp.SECTIONNAME = Convert.ToChar(divide[1].Trim().ToString());
           comp.EXAMNAMES = examnamedropdown.SelectedItem.Text;
           comp.SUBJECTNAME = subjectdropdown.SelectedItem.Text;
           string examname=examnamedropdown.SelectedItem.Text;
           if (examname == "Annual Exam" || examname == "Half yearly Exam" || examname == "Quartely Exam")
           {
               DataTable getmarkfdb = inter.getmarksfromdatabaseahq(comp);
               if (getmarkfdb != null)
               {
                   if (getmarkfdb.Rows.Count > 0)
                   {
                       TemplateField adtmep = new TemplateField();
                       adtmep.ShowHeader = true;
                       adtmep.HeaderText = "InternalMark";
                       adtmep.ItemTemplate = new gridviewtemplate(DataControlRowType.DataRow, "InternalMark", "InternalMark", "TextBox");

                       studmarkgrid.Columns.Add(adtmep);
                       TemplateField extmep = new TemplateField();
                       extmep.ShowHeader = true;
                       extmep.HeaderText = "ExternalMark";
                       extmep.ItemTemplate = new gridviewtemplate(DataControlRowType.DataRow, "ExternalMark", "ExternalMark", "TextBox");

                       studmarkgrid.Columns.Add(extmep);
                       ViewState["Temptable"] = getmarkfdb;
                       studmarkgrid.DataSource = getmarkfdb;
                       studmarkgrid.DataBind();



                   }
               }


                   else
                   {
                       DataTable result = inter.getstudentlistformark(comp);
                       TemplateField inmarktemp3 = new TemplateField();
                       inmarktemp3.ShowHeader = true;
                       inmarktemp3.HeaderText = "InternalMark";
                       inmarktemp3.ItemTemplate = new gridviewtemplatewithoutbind(DataControlRowType.DataRow, "InternalMark", "InternalMark", "TextBox");

                       studmarkgrid.Columns.Add(inmarktemp3);
                       TemplateField extertemp = new TemplateField();
                       extertemp.ShowHeader = true;
                       extertemp.HeaderText = "ExternalMark";
                       extertemp.ItemTemplate = new gridviewtemplatewithoutbind(DataControlRowType.DataRow, "ExternalMark", "ExternalMark", "TextBox");
                       studmarkgrid.Columns.Add(extertemp);

                       ViewState["Temptable"] = result;
                       studmarkgrid.DataSource = result;
                       studmarkgrid.DataBind();
                       Button1.Visible = true;
                       Button2.Visible = true;


               }

           }

           else
           {
               DataTable getmarkfdb = inter.getmarksfromdatabaseothers(comp);
                      if (getmarkfdb.Rows.Count > 0)
                   {

                       TemplateField extmep = new TemplateField();
                       extmep.ShowHeader = true;
                       extmep.HeaderText = "ExternalMark";
                       extmep.ItemTemplate = new gridviewtemplate(DataControlRowType.DataRow, "ExternalMark", "ExternalMark", "TextBox");

                       studmarkgrid.Columns.Add(extmep);
                       ViewState["Temptable"] = getmarkfdb;
                       studmarkgrid.DataSource = getmarkfdb;
                       studmarkgrid.DataBind();

                   }

                   else
                   {
                       DataTable result = inter.getstudentlistformark(comp);

                       TemplateField extemp= new TemplateField();
                       extemp.ShowHeader = true;
                       extemp.HeaderText = "ExternalMark";
                       extemp.ItemTemplate = new gridviewtemplatewithoutbind(DataControlRowType.DataRow, "ExternalMark", "ExternalMark", "TextBox");
                       studmarkgrid.Columns.Add(extemp);
                       ViewState["Temptable"] = result;
                       studmarkgrid.DataSource = result;
                       studmarkgrid.DataBind();
                      // ViewState["stdmarkgrid"] = studmarkgrid;
                   }
               }
           Button1.Visible = true;
           Button2.Visible = true;

protected void Button1_Click(object sender, EventArgs e)
       {
           comp.ACADAMICYEAR = acyeardropdown.SelectedItem.Text;
           comp.MEDIUM = mediumdropdown.SelectedItem.Text;
           string clas = classdropdown.SelectedItem.Text;
           string[] cs = clas.Split('-');
           comp.CLASSNAME = cs[0].ToString();
           comp.SECTIONNAME =Convert.ToChar(cs[1].Trim().ToString());
           comp.EXAMNAMES = examnamedropdown.SelectedItem.Text;
           comp.SUBJECTID = subjectdropdown.SelectedValue.ToString();
          // System.Web.UI.WebControls.GridView grid = ViewState["stdmarkgrid"] as System.Web.UI.WebControls.GridView;

           bool studm = inter.inserstumark(comp,studmarkgrid);
           if (studm)
           {
               string msg = "Record is successfully Inserted!";
               Response.Write("<script language=JavaScript> alert('" + msg + "'); <" +
   "/script>");

           }
           else
           {
               string msg = "Record is not Inserted!";
               Response.Write("<script language=JavaScript> alert('" + msg + "'); <" +
   "/script>");
           }


       }