用户名和电子邮件验证有效，但当我按下“提交”按钮时，它仍会继续

Question

我将这些代码作为我的用户名和电子邮件验证（如果它们存在于数据库中）。到目前为止，我能够使它们工作（验证确实出现）但是，即使例如用户名存在，我点击提交，仍然推动？我在这里失踪了什么？

protected void txtUser_TextChanged(object sender, EventArgs e)
    {
        if (!string.IsNullOrEmpty(txtUser.Text))
        {
            SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["RegistrationConnectionString"].ConnectionString);
            conn.Open();
            SqlCommand cmd = new SqlCommand("Select * from UserData where Username=@Username", conn);
            cmd.Parameters.AddWithValue("@Username", txtUser.Text);
            SqlDataReader dr = cmd.ExecuteReader();
            if (dr.HasRows)
            {
                lblusercheck.Text = "Username exists";
                lblusercheck.ForeColor = System.Drawing.Color.Red;
                imgstatus.Visible = true;
                imgstatus.ImageUrl = "Images1/work/NotAvailable.png";
            }
            else
            {
                lblusercheck.Text = "Username available!";
                lblusercheck.ForeColor = System.Drawing.Color.LimeGreen;
                imgstatus.Visible = true;
                imgstatus.ImageUrl = "Images1/work/Available.png";
            }
            conn.Close();


        }
    }

    protected void txtEmail_TextChanged(object sender, EventArgs e)
    {
        if (!string.IsNullOrEmpty(txtEmail.Text))
        {
            SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["RegistrationConnectionString"].ConnectionString);
            conn.Open();
            SqlCommand cmd = new SqlCommand("Select * from UserData where Email=@Email", conn);
            cmd.Parameters.AddWithValue("@Email", txtEmail.Text);
            SqlDataReader dr = cmd.ExecuteReader();
            if (dr.HasRows)
            {
                lblemailcheck.Text = "Email already used";
                lblemailcheck.ForeColor = System.Drawing.Color.Red;
                ImageEmail.Visible = true;
                ImageEmail.ImageUrl = "Images1/work/NotAvailable.png";
            }
            else
            {
                lblemailcheck.Text = "Email Approved";
                lblemailcheck.ForeColor = System.Drawing.Color.LimeGreen;
                ImageEmail.Visible = true;
                ImageEmail.ImageUrl = "Images1/work/Available.png";
            }
            conn.Close();
        }

这是我的提交按钮代码：

protected void btn_Registration_Click(object sender, EventArgs e)
    {
        try
        {
            SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["RegistrationConnectionString"].ConnectionString);
            conn.Open();
            string insertQuery = "insert into UserData(Username,Firstname,Lastname,Email,Password,CustomerType,DeliveryAddress,Zip,ContactNumber)values(@Username,@Firstname,@Lastname,@Email,@Password,@CustomerType,@DeliveryAddress,@Zip,@ContactNumber)";
            SqlCommand scm = new SqlCommand(insertQuery, conn);
            scm.Parameters.AddWithValue("@Username", txtUser.Text);
            scm.Parameters.AddWithValue("@Firstname", txtFN.Text);
            scm.Parameters.AddWithValue("@Lastname", txtLN.Text);
            scm.Parameters.AddWithValue("@Email", txtEmail.Text);
            scm.Parameters.AddWithValue("@Password", BusinessLayer.ShoppingCart.CreateSHAHash(txtPW.Text));
            scm.Parameters.AddWithValue("@CustomerType", RadioButtonList1.SelectedItem.ToString());
            scm.Parameters.AddWithValue("@DeliveryAddress", txtAddress.Text);
            scm.Parameters.AddWithValue("@Zip", txtZip.Text);
            scm.Parameters.AddWithValue("@ContactNumber", txtContact.Text);

            scm.ExecuteNonQuery();
            Session["Contact"] = txtContact.Text;
            Session["Email"] = txtEmail.Text;
            Session["DeliveryAddress"] = txtAddress.Text;
            label_register_success.Text = ("Registration Successful!");
            //Response.Redirect("Home.aspx");
            conn.Close();
        }
        catch (Exception ex)
        {
            Response.Write("Error:" + ex.ToString());
        }
    }