Custom Login ASP.NET C#
using System;
using System.Data;
using System.Configuration;
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;
using System.Data.SqlClient;


public partial class Login : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }

    // Custom login control
    protected void Login1_Authenticate(object sender, AuthenticateEventArgs e)
    {
        try
        {
            string uname = Login1.UserName.Trim();
            string password = Login1.Password.Trim();

            bool flag = AuthenticateUser(uname, password);
            if (flag == true)
            {
                e.Authenticated = true;
                Login1.DestinationPageUrl = "Default.aspx";
            }
            else
                e.Authenticated = false;
        }
        catch (Exception)
        {
            e.Authenticated = false;
        }
    }

    private bool AuthenticateUser(string uname, string password)
    {
        bool bflag = false;
        string connString = "Server=DEVSERVER;User ID=sa;Password=whatpassword;Database=CommonUser";
string connstring2 = "Server=DEVSERVER;User ID=sa;Password=whatpassword;Database=Admins";
        string strSQL = "Select * from dbo.Users where Username ='" + uname + "' and Password ='" + password + "'";
        DataSet userDS = new DataSet();
        SqlConnection m_conn;
        SqlDataAdapter m_dataAdapter;
        SqlCommand m_Command;
        try
        {
            m_conn = new SqlConnection(connString);
            m_conn.Open();
            m_dataAdapter = new SqlDataAdapter(strSQL, m_conn);
            m_dataAdapter.Fill(userDS);
            m_conn.Close();
        }
        catch (Exception)
        {
            userDS = null;
        }

        if (userDS != null)
        {
            if (userDS.Tables[0].Rows.Count > 0)
                bflag = true;
        }
        return bflag;

    }
}