将DropDownList SelectedValue传递给SQL查询抛出输入字符串的格式不正确

我正在开发ASP。NET webforms页,用于查询数据库以获取研究表中的条目列表。这些条目将被传递到dropdownlist中。从dropdownlist中选择研究时,studyID将传递给GetResponses方法,以从存储过程中检索相关数据。

我收到 Input String was not in a Correct Format 使用以下代码段:

 private DataTable LoadStudies(int iStudyID )
{
    ddlStudies.Items.Clear();
    ddlStudies.SelectedValue = "0";

    DataTable dt = new DataTable();
    using (PROTOTYPING db = new PROTOTYPING(ConfigurationManager.ConnectionStrings["SQL"].ConnectionString))
    {
        var query = (from d in db.Studies
            where d.StudyStatus == 0 //Closed...
            orderby d.StudyName
            select new
            {
                d.StudyName,
                d.StudyID,
            });
        if (query.Count() > 0)
        {
            foreach (var a in query)
            {
                ddlStudies.Items.Add(new ListItem(a.StudyID.ToString()));
            }
        }

        dt.Dispose();
        DataView dv = new DataView(dt);
        return dt;
    }
}

在当前写入的Page\u Load上引发错误,如下所示:

protected void Page_Load(object sender, EventArgs e)
{
    int iUserID = 0;

    if (Session["UserID"] == null)
    {
        Response.Redirect("Default.aspx");
    }
    iUserID = Convert.ToInt32(Session["UserID"]);
    int iRole = 0;
    iRole = Convert.ToInt32(Session["RoleID"]);

    if (!Page.IsPostBack)
    {

        LoadStudies(Convert.ToInt32(ddlStudies.SelectedValue));
        GetResponses(Convert.ToInt32(ddlStudies.SelectedValue));

        ddlStudies.DataSource = LoadStudies(Convert.ToInt32(ddlStudies.SelectedValue));
        ddlStudies.DataTextField = "StudyName";
        ddlStudies.DataValueField = "StudyID";
        ddlStudies.DataBind();
    }
}

如何解决将dropdownlist的数据源分配给LoadStudies方法时引发的错误?