Implement Forgot User name and send user name in Email in ASP.Net using C# and VB.Net

Hi Phonghue,

   kindly refer below Sample.

HTML

Email Address:
<asp:TextBox ID="txtEmail" runat="server" Width="250" />
<br />
<asp:Label ID="lblMessage" runat="server" />
<br />
<asp:Button Text="Forgot Username" runat="server" OnClick="SendEmail" />

Namespaces

C#

using System.Net;
using System.Net.Mail;
using System.Drawing;
using System.Configuration;
using System.Data.SqlClient;

VB.Net

Imports System.Net
Imports System.Net.Mail
Imports System.Drawing
Imports System.Configuration
Imports System.Data.SqlClient

Code

C#

protected void SendEmail(object sender, EventArgs e)
{
    string username = string.Empty;
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT Username FROM Users WHERE Email = @Email"))
        {
            cmd.Parameters.AddWithValue("@Email", txtEmail.Text.Trim());
            cmd.Connection = con;
            con.Open();
            using (SqlDataReader sdr = cmd.ExecuteReader())
            {
                if (sdr.Read())
                {
                    username = sdr["Username"].ToString();
                }
            }
            con.Close();
        }
    }
    if (!string.IsNullOrEmpty(username))
    {
        MailMessage mm = new MailMessage("sender@gmail.com", txtEmail.Text.Trim());
        mm.Subject = "Username Recovery";
        mm.Body = string.Format("Your Username is {0}.<br /><br />Thank You.", username);
        mm.IsBodyHtml = true;
        SmtpClient smtp = new SmtpClient();
        smtp.Host = "smtp.gmail.com";
        smtp.EnableSsl = true;
        NetworkCredential NetworkCred = new NetworkCredential();
        NetworkCred.UserName = "sender@gmail.com";
        NetworkCred.Password = "<Password>";
        smtp.UseDefaultCredentials = true;
        smtp.Credentials = NetworkCred;
        smtp.Port = 587;
        smtp.Send(mm);
        lblMessage.ForeColor = Color.Green;
        lblMessage.Text = "Username has been sent to your email address.";
    }
    else
    {
        lblMessage.ForeColor = Color.Red;
        lblMessage.Text = "This email address does not match our records.";
    }
}

VB.Net

Protected Sub SendEmail(ByVal sender As Object, ByVal e As EventArgs)
    Dim username As String = String.Empty
    Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Using con As SqlConnection = New SqlConnection(constr)
        Using cmd As SqlCommand = New SqlCommand("SELECT Username FROM Users WHERE Email = @Email")
            cmd.Parameters.AddWithValue("@Email", txtEmail.Text.Trim())
            cmd.Connection = con
            con.Open()
            Using sdr As SqlDataReader = cmd.ExecuteReader()
                If sdr.Read() Then
                    username = sdr("Username").ToString()
                End If
            End Using
            con.Close()
        End Using
    End Using

    If Not String.IsNullOrEmpty(username) Then
        Dim mm As MailMessage = New MailMessage("sender@gmail.com", txtEmail.Text.Trim())
        mm.Subject = "Username Recovery"
        mm.Body = String.Format("Your Username is {0}.<br /><br />Thank You.", username)
        mm.IsBodyHtml = True
        Dim smtp As SmtpClient = New SmtpClient()
        smtp.Host = "smtp.gmail.com"
        smtp.EnableSsl = True
        Dim NetworkCred As NetworkCredential = New NetworkCredential()
        NetworkCred.UserName = "sender@gmail.com"
        NetworkCred.Password = "<Password>"
        smtp.UseDefaultCredentials = True
        smtp.Credentials = NetworkCred
        smtp.Port = 587
        smtp.Send(mm)
        lblMessage.ForeColor = Color.Green
        lblMessage.Text = "Username has been sent to your email address."
    Else
        lblMessage.ForeColor = Color.Red
        lblMessage.Text = "This email address does not match our records."
    End If
End Sub