添加了using System.Data.SqlClient、using System.Data、using System.Configuration,还是提示SqlConnection找不到命空间……

报错在：SqlConnection conn = new SqlConnection(ConfigurationManager.AppSettings["DBConnectionString"]);
        SqlCommand cmd = new SqlCommand("select * from Users where userId='" + txtUserName.Text + "'", conn);
SqlDataReader sdr = cmd.ExecuteReader;
详细的代码如下，请各位看看了：
using System;
using System.Data;
using System.Data.SqlClient;
using System.Data.Sql;
using System.Data.ProviderBase;
using System.Data.SqlTypes;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;public partial class Register : System.Web.UI.Page
{
    protected void btnValidate_Click(object sender, EventArgs e)
    {
        int i = userNameValidate();
    }
    private int userNameValidate()
    {
        SqlConnection conn= new SqlConnection(ConfigurationManager.AppSettings["DBconnectionString"]);
        SqlCommand selectCmd=new SqlCommand("select * from Users where userId='"+txtName.Text.Trim()+"'",conn);
        int i=0;
        try
        {
            conn.Open();
            SqlDataReader sdr=selectCmd.ExecuteReader();
            if(sdr.Read())
            {
                i=1;
                lb1Message.Text="用户名已经存在！";
            }
            else
            {
                lb1Message.Text="用户名可用！";
            }
        }
        catch (System.Exception ee)
        {
            Response.Write("<scirpt language=javascript>alert('" + ee.Message.ToString() + "')</script>");
        }
        finally
        {
            conn.Close();
        }
        return i;
    }    protected void btnSubmit_Click(object sender, EventArgs e)
    {
        int i = userNameValidate();
        if (i == 0)
        {
            SqlConnection conn = new SqlConnection(ConfigurationManager.AppSettings["DBConnectionString"]);
            SqlCommand insertCmd = new SqlCommand("insert into Users values(@userId,@userPwd,@userName,@userSex,@userBirth,@userPhone,@userMobilePhone,@userCode,@userAddress,@userEmail,@userQQ)", conn);
            insertCmd.Parameters.Add("@userId", SqlDbType.VarChar, 15);
            insertCmd.Parameters.Add("@userPwd", SqlDbType.VarChar, 20);
            insertCmd.Parameters.Add("@userName", SqlDbType.VarChar, 20);
            insertCmd.Parameters.Add("@userSex", SqlDbType.VarChar, 2);
            insertCmd.Parameters.Add("@userBirth", SqlDbType.VarChar, 8);
            insertCmd.Parameters.Add("@userPhone", SqlDbType.VarChar, 15);
            insertCmd.Parameters.Add("@userMobilPhone", SqlDbType.VarChar, 11);
            insertCmd.Parameters.Add("@userCode", SqlDbType.VarChar, 6);
            insertCmd.Parameters.Add("@userAddress", SqlDbType.VarChar, 200);
            insertCmd.Parameters.Add("@userEmail", SqlDbType.VarChar, 100);
            insertCmd.Parameters.Add("userQQ", SqlDbType.VarChar, 20);
            insertCmd.Parameters["@userId"].Value = txtName.Text;
            insertCmd.Parameters["@userPwd"].Value = txtPwd.Text;
            insertCmd.Parameters["@userName"].Value = txtUname.Text;
            insertCmd.Parameters["@userSex"].Value = rltSex.SelectedItem.Text;
            insertCmd.Parameters["@userBirth"].Value = Convert.ToDateTime(ddlYear.SelectedValue + "-" + ddlMonth.SelectedValue + "-" + ddlDay.SelectedValue);
            insertCmd.Parameters["@userPhone"].Value = txtUle12.Text;
            insertCmd.Parameters["@userMobilePhone"].Value = txtUle13.Text;
            insertCmd.Parameters["@userCode"].Value = txtUzip.Text;
            insertCmd.Parameters["@userAddress"].Value = txtUaddr.Text;
            insertCmd.Parameters["@userEmail"].Value = txtUemail.Text;
            insertCmd.Parameters["@userQQ"].Value = txtUQQ.Text;
            try
            {
                conn.Open();
                int flag = insertCmd.ExcuteNonQuery();
                if (flag > 0)
                {
                    Response.Redirect("Login.aspx");
                }
                else
                {
                    Response.Write("<script language=javascript>alert('注册失败！')</script>");
                }
            }
            catch (System.Exception ee)
            {
                Response.Write("<script language=javascript>alert('" + ee.Message.ToString() + "')</script>");
            }
            finally
            {
                conn.Close();
            }
        }
    }
}