checkedListBox1 填充过程出错，请帮助看一下！

每张单对应多个值,想将每张单子号从表中读出来，然后将 checkedListBox1(包含每张单取值范围) 其中数据比较，若相同
 则将其选中
string sql= " select pr from sd_tbl  ";
             SqlDataReader sqlread = boperate.getread(sql);
             if (sqlread.HasRows)
             {
                 while (sqlread.Read())
                 {                           string pr= sqlread["pr"].ToString().Trim();                     for (int i = 0; i < checkedListBox1.Items.Count; i++)
                     {
                         if(checkedListBox1.Items[i].ToString()==pr )                         { checkedListBox1.SetItemChecked(i,true );  }
                     }
                    
                                     }
             }