C# – Add rows dynamically to gridview

Be sure to access the page at the correct time. If you access it in the OnInit event, the ViewState will not have been mapped back on your grid.

PageLoad would be appropriate probably.

When you databind in AddNewRow, the original rows are lost. You must store the dropdown selected index, together with your textbox phone value in dtCurrentData. You can then set the index of the dropdowns in your RowDataBound event, using the values your saved in dtCurrentData.

private void AddNewRow()
{
  //Create a datatable 
  dtCurrentData = new DataTable();
  dtCurrentData.Columns.Add("phone");
  //new column for dropdown index
  dtCurrentData.Columns.Add("ddlIndex");

//Store values of each row to a new datarow. Add all rows to datatable
  foreach (GridViewRow gvRow in gvCommissions.Rows)
    {
      DataRow drcurrentrow = dtCurrentData.NewRow();                      
      drcurrentrow["phone"] = ((TextBox)gvRow.FindControl("txtphone")).Text;
      //get dropdown index
      drcurrentrow["ddlIndex"] = ((DropDownList)gvRow.FindControl("ddlLogin")).SelectedIndex;
      dtCurrentData.Rows.Add(drcurrentrow);
    }

  //create an empty datarow and also add it to the new datatable.
    DataRow dr = dtCurrentData.NewRow();
    dr["phone"] = "";
    //initial drop down index
    dr["ddlIndex"] = 0;
    dtCurrentData.Rows.Add(dr);

    //save to the viewstate like your AgentsTable
    ViewState["CurrentData"] = dtCurrentData;

 //Bind the new datatable to the grid
   gvCommissions.DataSource = dtCurrentData;
   gvCommissions.DataBind();
}

protected void gvCommissions_RowDataBound(object sender, GridViewRowEventArgs e)
{
 if (e.Row.RowType == DataControlRowType.DataRow)
  {
    DropDownList ddl = (DropDownList)e.Row.FindControl("ddlLogin");
    dtAgents = (DataTable)ViewState["AgentsTable"];
    ddl.DataSource = dtAgents;
    ddl.DataBind();
    ddl.Items.Insert(0, "");

    //get the dropdown index from CurrentData
    //use the current gridview's row index, since it matches the datatable
    if (ViewState["CurrentData"] != null)
    {
     DataTable dtCurrentData = (DataTable)ViewState["CurrentData"];
     ddl.SelectedIndex = Convert.ToInt32(dtCurrentData.Rows[e.Row.RowIndex]["ddlIndex"]);
    }

  }            
}