getting data on gridview

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ASP.NET

trying to getting value from dropdownlist and press search button and show data on gridview.giving values on dropdownlist is hardcode

//string searchTXt = GridView2.SelectedValue.ToString();
string query = "SELECT * FROM ['ISB VAS Nodes$'] where id='"+DropDownList1.SelectedValue+"'";
DataSet ds = new DataSet();
SqlDataAdapter da = new SqlDataAdapter(query, connstring);
da.Fill(ds);
if (ds.Tables[0].Rows.Count > 0)
{
    GridView2.DataSource = ds;
    GridView2.DataBind();
}