Sample Event Handler to set a field as a primary key (Enforce no duplicates)

how to prevent users from adding items with same titles as ones that already exist in the list.

using System;
using System.Collections.Generic;
using System.Text;
using Microsoft.SharePoint;

namespace SPSTIPS.SharePoint.EventHandlers
{
    public class TitlePrimaryEventHandler:SPItemEventReceiver
    {
        const string TITLE_QUERY = @"<Query><Where><Eq><FieldRef Name=""Title"" /><Value Type=""Text"">{0}</Value></Eq></Where></Query>";
        public override void ItemAdding(SPItemEventProperties properties)
        {
            if (properties.AfterProperties["Title"] != null)
            {
                //get the title of the new item
                string currentTitle = properties.AfterProperties["Title"].ToString();
                //get the web site object
                using (SPWeb web = properties.OpenWeb())
                {
                    //get the current list
                    SPList list = web.Lists[properties.ListId];
                    //query the list to check if there are items with the same title
                    SPQuery q = new SPQuery();
                    q.Query = string.Format(TITLE_QUERY, currentTitle);
                    SPListItemCollection itemsWithSameTitle = list.GetItems(q);
                    //if there are items, cancel the add, and show an error to the user.
                    if (itemsWithSameTitle.Count > 0)
                    {
                        properties.Cancel = true;
                        properties.ErrorMessage = "There is already an item with the title \"" + currentTitle + "\ in this list".";
                    }
                }
            }
        }
    }
}