C# 错误："；找不到列pollQuestion“；标题Gridview排序

下面我有一些排序方法的代码，希望这足够清楚。我试图在我的Gridview上按字母顺序对标题进行排序，根据用户的点击，它会变为ASC或DESC

protected void gvEditQuestions_Sorting(object sender, GridViewSortEventArgs e)
{
    DataTable dtSortTable = new DataTable();

    if (dtSortTable != null)
    {
        DataSet dsSortTable = gvEditQuestions.DataSource as DataSet;
        DataView dvSortedView = new DataView(dtSortTable);
        dvSortedView.Sort = e.SortExpression + " " + getSortDirectionString();
        ViewState["sortExpression"] = e.SortExpression;
        gvEditQuestions.DataSource = dvSortedView;
        gvEditQuestions.DataBind();
    }
}

private string getSortDirectionString()
{
    if (ViewState["sortDirection"] == null)
    {
        ViewState["sortDirection"] = "ASC";
    }
    else
    {
        if (ViewState["sortDirection"].ToString() == "ASC")
        {
            ViewState["sortDirection"] = "DESC";
            return ViewState["sortDirection"].ToString();
        }  
        if (ViewState["sortDirection"].ToString() == "DESC")
        {
            ViewState["sortDirection"] = "ASC";
            return ViewState["sortDirection"].ToString();
        }
    }
    return ViewState["sortDirection"].ToString();
}

protected void gvEditQuestions_PageIndexChanging(object sender, GridViewPageEventArgs e)
{
    gvEditQuestions.PageIndex = e.NewPageIndex;
    gvEditQuestions.DataSource = ViewState["sortDirection"];
    DataBind();
}