添加数据库记录问题？

下面是代码：
SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["LocalSqlServer"].ConnectionString);
          string SqlStrInsert = "Insert into nianjian(chepai,bencisongjianriqi) values(' " + this.TBchepaihao.Text.ToString().Trim() + " ',' " +this.Ddlnianjianriqi.Value.ToString("d").Trim())";
          string SqlSelect = "Select * from nianjian where chepai='" + this.TBchepaihao.Text.ToString().Trim()+ "'";
        SqlCommand cmd = new SqlCommand(SqlSelect, conn);
        SqlCommand cmdInsert = new SqlCommand(SqlStrInsert, conn);

       try
        {
            conn.Open();
            SqlDataReader sdr = cmd.ExecuteReader();
            if (sdr.Read())
            {
                lblMessage.Text = "您输入的车牌号已经存在，请输入其它车牌号";
                sdr.Close();
            }         
  else
            {
                sdr.Close();
                try
                {
                    if (conn.State.ToString() == "Closed")
                        conn.Open();
                    cmdInsert.ExecuteNonQuery();
                }
                catch (Exception ex)
                {
                    Response.Write("数据库错误，错误原因：" + ex.Message);
                    Response.End();
                }
                lblMessage.Text = "信息添加成功";
            }
        }
        catch (System.Exception ee)
        {
            Response.Write("<script language=javascript>alert(" + ee.Message.ToString() + ")</script>");
        }
        finally
        {
            cmd.Dispose();
            cmdInsert.Dispose();
            if (conn.State.ToString() == "Open")
                conn.Close();
        }
    }
求助：为什么每次输入相同的车牌号就是不会提示我要显示的信息：你的车牌号已经存在。。。。(红色的部分不执行）
就是会显示数据库的错误原因：。。。
不知道错在哪里？请高手看看!