在 SelectedIndexChanged 事件触发后调用 javascript window.onload

//我的问题是当我异步触发 ddlmodalitylist 的 SelectedIndexChanged 事件（ajax 调用）时，javascript 加载事件不会触发。也就是说，我必须从服务器端触发加载事件。

window.onload = body_Onload;
     function body_Onload() {
        //javascript code
       }
    protected void ddlModalityList_SelectedIndexChanged(object sender, EventArgs e)
        {
            ddlStudy.Items.Clear();
            ListItem selectedPair = ddlModalityList.SelectedItem;
            string str= selectedPair.Value;
           int ID= Convert.ToInt32(str);
           if (ID == -1)
           {
              // ddlStudy.Items.Clear();
               return;
           }

            DataTable dataTableStudy = null;
            dataTableStudy = objSqlDbComm.ExecuteDatasetQuery(strSQL).Tables[0];
            var dictioneryStudy = new Dictionary<int, string>();
            foreach (DataRow dr in dataTableStudy.Rows)
            {          
                dictioneryStudy.Add(Convert.ToInt32(dr["Study_ID"]), dr["Study_Desc"].ToString());
            }
            ddlStudy.DataTextField = "Value";
            ddlStudy.DataValueField = "Key";
            ddlStudy.DataSource = dictioneryStudy;
            ddlStudy.DataBind();
            ddlStudy.Items.Insert(0, new ListItem("[Select]", "-1"));
            ddlStudy.Items[0].Selected = true;
        }

Sys.Application.add_init(your_bodyload_function);