protected void Page_Load(object sender, EventArgs e)
    {
        if (Session["UserName"] != null)
        {

            if (Session["roles"].ToString() != "Admin")
            {
                ScriptManager.RegisterStartupScript(this, this.GetType(), "alert", "alert('Only Admin Can Access this Page');window.location ='HomePageAdmin.aspx';", true);
            }
        }

        else
        {
            ScriptManager.RegisterStartupScript(this, this.GetType(), "alert", "alert('Please Login To Proceed');window.location ='HomePageAdmin.aspx';", true);
        }
    }

    protected void btnRegister_Click(object sender, EventArgs e)
    {
        SqlConnection SqlConn;
        SqlCommand cmdInsert, cmdSelect, cmdSelect1, cmdSelect2;
        SqlDataReader dtrRead;
        SqlDataReader dtra;
        SqlDataReader dtrb;


        string connStr = ConfigurationManager.ConnectionStrings["ConnectionNWind"].ConnectionString;

        SqlConn = new SqlConnection(connStr);

        SqlConn.Open();


        String strSearch = "Select SUserName From Staff_Login Where SUserName=@userName";
        cmdSelect = new SqlCommand(strSearch, SqlConn);
        cmdSelect.Parameters.AddWithValue("@userName", txtSUN.Text);



        String stra = "Select SIC From Staff_Login where SIC =@IC";
        cmdSelect1 = new SqlCommand(stra, SqlConn);
        cmdSelect1.Parameters.AddWithValue("@IC", txtSIC.Text);


        String strb = "Select SEmailAdd From Staff_Login where SEmailAdd =@Add";
        cmdSelect2 = new SqlCommand(strb, SqlConn);
        cmdSelect2.Parameters.AddWithValue("@Add", txtSEA.Text);





        dtrRead = cmdSelect.ExecuteReader();
       dtra = cmdSelect1.ExecuteReader();
        dtrb = cmdSelect2.ExecuteReader();


        if (dtrRead.Read())
        {

            Response.Write("<script>alert(' - User already exist')</script>");

        }

         else if (dtra.Read())
        {

            Response.Write("<script>alert(' - IC already exist')</script>");

        }
        else if(dtrb.Read()){

            Response.Write("<script>alert(' - Email already exist')</script>");

        }
        else
        {
            dtrRead.Close();
            dtra.Close();
            dtrb.Close();
            String strInsert = "Insert Into Staff_Login (SUserName,SPassword,SfirstName,SlastName,SIC,gender,ShomeAdd,SemailAdd,SContactNum,StaffStatus)Values (@userName, @passW,@fName, @lName, @icNo, @gender, @hAdd, @eAdd, @SCN, @StaffStatus )";


            cmdInsert = new SqlCommand(strInsert, SqlConn);
            cmdInsert.Parameters.AddWithValue("@userName", txtSUN.Text);
            cmdInsert.Parameters.AddWithValue("@fName", txtSFN.Text);
            cmdInsert.Parameters.AddWithValue("@lName", txtSLN.Text);
            cmdInsert.Parameters.AddWithValue("@icNo", txtSIC.Text);
            cmdInsert.Parameters.AddWithValue("@Gender", rblSG.SelectedItem.ToString());
            cmdInsert.Parameters.AddWithValue("@hAdd", txtSHA.Text);
            cmdInsert.Parameters.AddWithValue("@eAdd", txtSEA.Text);
            cmdInsert.Parameters.AddWithValue("@SCN", txtSCN.Text);
            cmdInsert.Parameters.AddWithValue("@passW", txtSP.Text);
            cmdInsert.Parameters.AddWithValue("@StaffStatus", ddlStatus.SelectedItem.ToString());
            cmdInsert.ExecuteNonQuery();


            lblMessage.Text = "You have been successfully register as member!";

            txtSUN.Text = "";
            txtSFN.Text = "";
            txtSLN.Text = "";
            txtSIC.Text = "";
            txtSHA.Text = "";
            txtSEA.Text = "";
            txtSCN.Text = "";
            txtSP.Text = "";



        }

        SqlConn.Close();
        dtrRead.Close();
        //dtra.Close();
        //dtrb.Close();
    }


}

I  wan use the diffrent datareader to display out the diffrent message bu i get an error  protected void Page_Load(object sender, EventArgs e)
    {
        if (Session["UserName"] != null)
        {

            if (Session["roles"].ToString() != "Admin")
            {
                ScriptManager.RegisterStartupScript(this, this.GetType(), "alert", "alert('Only Admin Can Access this Page');window.location ='HomePageAdmin.aspx';", true);
            }
        }

        else
        {
            ScriptManager.RegisterStartupScript(this, this.GetType(), "alert", "alert('Please Login To Proceed');window.location ='HomePageAdmin.aspx';", true);
        }
    }

    protected void btnRegister_Click(object sender, EventArgs e)
    {
        SqlConnection SqlConn;
        SqlCommand cmdInsert, cmdSelect, cmdSelect1, cmdSelect2;
        SqlDataReader dtrRead;
        SqlDataReader dtra;
        SqlDataReader dtrb;


        string connStr = ConfigurationManager.ConnectionStrings["ConnectionNWind"].ConnectionString;

        SqlConn = new SqlConnection(connStr);

        SqlConn.Open();


        String strSearch = "Select SUserName From Staff_Login Where SUserName=@userName";
        cmdSelect = new SqlCommand(strSearch, SqlConn);
        cmdSelect.Parameters.AddWithValue("@userName", txtSUN.Text);



        String stra = "Select SIC From Staff_Login where SIC =@IC";
        cmdSelect1 = new SqlCommand(stra, SqlConn);
        cmdSelect1.Parameters.AddWithValue("@IC", txtSIC.Text);


        String strb = "Select SEmailAdd From Staff_Login where SEmailAdd =@Add";
        cmdSelect2 = new SqlCommand(strb, SqlConn);
        cmdSelect2.Parameters.AddWithValue("@Add", txtSEA.Text);





        dtrRead = cmdSelect.ExecuteReader();
       dtra = cmdSelect1.ExecuteReader();
        dtrb = cmdSelect2.ExecuteReader();


        if (dtrRead.Read())
        {

            Response.Write("<script>alert(' - User already exist')</script>");

        }

         else if (dtra.Read())
        {

            Response.Write("<script>alert(' - IC already exist')</script>");

        }
        else if(dtrb.Read()){

            Response.Write("<script>alert(' - Email already exist')</script>");

        }
        else
        {
            dtrRead.Close();
            dtra.Close();
            dtrb.Close();
            String strInsert = "Insert Into Staff_Login (SUserName,SPassword,SfirstName,SlastName,SIC,gender,ShomeAdd,SemailAdd,SContactNum,StaffStatus)Values (@userName, @passW,@fName, @lName, @icNo, @gender, @hAdd, @eAdd, @SCN, @StaffStatus )";


            cmdInsert = new SqlCommand(strInsert, SqlConn);
            cmdInsert.Parameters.AddWithValue("@userName", txtSUN.Text);
            cmdInsert.Parameters.AddWithValue("@fName", txtSFN.Text);
            cmdInsert.Parameters.AddWithValue("@lName", txtSLN.Text);
            cmdInsert.Parameters.AddWithValue("@icNo", txtSIC.Text);
            cmdInsert.Parameters.AddWithValue("@Gender", rblSG.SelectedItem.ToString());
            cmdInsert.Parameters.AddWithValue("@hAdd", txtSHA.Text);
            cmdInsert.Parameters.AddWithValue("@eAdd", txtSEA.Text);
            cmdInsert.Parameters.AddWithValue("@SCN", txtSCN.Text);
            cmdInsert.Parameters.AddWithValue("@passW", txtSP.Text);
            cmdInsert.Parameters.AddWithValue("@StaffStatus", ddlStatus.SelectedItem.ToString());
            cmdInsert.ExecuteNonQuery();


            lblMessage.Text = "You have been successfully register as member!";

            txtSUN.Text = "";
            txtSFN.Text = "";
            txtSLN.Text = "";
            txtSIC.Text = "";
            txtSHA.Text = "";
            txtSEA.Text = "";
            txtSCN.Text = "";
            txtSP.Text = "";



        }

        SqlConn.Close();
        dtrRead.Close();
        //dtra.Close();
        //dtrb.Close();
    }


}

I wan use  the diffrent data reader to display the diffrent erro message but i get an error There is already an open DataReader associated with this Command which must be closed first. 
How i wan to solve it?
if (dtrRead.Read())
        {
            Response.Write("<script>alert(' - User already exist')</script>");
        }
         else if (dtra.Read())
        {
            Response.Write("<script>alert(' - IC already exist')</script>");
        }
        else if(dtrb.Read()){
            Response.Write("<script>alert(' - Email already exist')</script>");
        }

yours Above Script is displaying same Messages. i think you are tired of doing CODE :)

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