UnauthorizedAccessException was unhandled by user code in SharePoint event receiver

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In this article we will be seeing how to resolve "UnauthorizedAccessException was unhandled by user code" in SharePoint event receiver.

I had one custom list "MyList" which contains two columns "Title" and "Service Account".

I had one group with Full control "Service Team". In the "Service Account" column I need to add all the users from the "Service Team" group.
I have written ItemAdded event receiver for adding the users from the "Service Team" group to the "Service Account" column.

public override void ItemAdded(SPItemEventProperties properties)
       {
           base.ItemAdded(properties);
           SPSecurity.RunWithElevatedPrivileges(delegate()
           {
               SPWeb web = properties.Web;
               SPListItem item = properties.ListItem;
               SPGroup group = web.Groups["Service Team"];
               web.AllowUnsafeUpdates = true;
               SPFieldUserValueCollection UserCollection = new SPFieldUserValueCollection();
               foreach (SPUser user in group.Users)
               {
                   SPFieldUserValue UserName = new SPFieldUserValue(web, user.ID, user.LoginName);
                   UserCollection.Add(UserName);
               }
               item["Service Account"] = UserCollection;
               item.Update();
           });
       }

When the user with contribute access tried to add the column, users from the "Service Team" were not added to the column and I was getting the following error when I was debugging.

Even though I was using SPSecurity.RunWithElevatedPrivileges I was getting the error. That is because "when using SPSecurity.RunWithElevatedPrivileges, we need to open an elevated site context through a new SPSite object in order to really be elevated". Then I tried with the following code which added users to the columns successfully.

public override void ItemAdded(SPItemEventProperties properties)
       {
           base.ItemAdded(properties);
           SPSecurity.RunWithElevatedPrivileges(delegate()
           {
               using(SPSite site=new SPSite (properties.SiteId))
                {
                    using (SPWeb web = site.RootWeb)
                    {
                        SPListItem item = properties.ListItem;
                        SPGroup group = web.Groups["Service Team"];
                        web.AllowUnsafeUpdates = true;
                        SPFieldUserValueCollection UserCollection = new SPFieldUserValueCollection();
                        foreach (SPUser user in group.Users)
                        {
                            SPFieldUserValue UserName = new SPFieldUserValue(web, user.ID, user.LoginName);
                            UserCollection.Add(UserName);
                        }
                        item["Service Account"] = UserCollection;
                        item.Update();
                    }
                }
           });
       }