Generating unique incremental integer ID for database tables

using System;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;
public partial class _Default : System.Web.UI.Page
{
    string strConnString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
    string str;
    SqlCommand com;
    int count;
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            autogenerated();
          
        }
    }

    void autogenerated()
    {
        SqlConnection con = new SqlConnection(strConnString);
        str = "select count(*) from employee";
        com = new SqlCommand(str, con);
        con.Open();
        count = Convert.ToInt16(com.ExecuteScalar()) + 1;
        txt_empid.Text = count.ToString();
        txt_empid.Enabled = false;
        con.Close();
    }
    protected void btn_insert_Click(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(strConnString);
        con.Open();
        str = "insert into employee values('" + txt_empid.Text.Trim() + "','" + txt_empname.Text.Trim() + "'," + txt_sal.Text.Trim() + ")";
        com = new SqlCommand(str, con);
        com.ExecuteNonQuery();
        con.Close();
        Label4.Text = "Records successfully Inserted";
        txt_empname.Text = "";
        txt_sal.Text = "";
        autogenerated();
    }
}