log in process code in asp.net

OR
User authenticate in asp.net

-->User authenticate required when Unauthorized user not access
some pages in your website

--> Table name  tbl_Student
--> field  (Id,Name,Address,.....User_Id,Password)
-->Add following namespace in your code behind file
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    using System.Data;
    using System.Data.SqlClient;

--> And then create following  object  above page load event
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    SqlConnection cn;
    SqlCommand cmd;
    SqlDataAdapter da;   
    DataSet ds;

--> Follwing code paste in your log_in button on click  event
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    cmd = new SqlCommand("select * from tbl_Student where
    User_Id=' "+txtUser.Text+" ' and Password=' "+txtPwd.Text+" ' ", cn);
    da = new SqlDataAdapter(cmd);
    ds = new DataSet();
    da.Fill(ds);
    if(ds.Tables[0].Rows.Count>0)
    {
        //when user id and password match then
        string Id=ds.Tables[0].Rows[0][0].ToString();
        Session["UserSession"]=Id;
         //Create session and use this session in entire application
        Response.Redirect("User/Home.aspx");
        //Successfully log in
    }
    else
    {
        //when user id and password does not match then
        Label1.Text="User name and Password does not match";
    }