How to override existing behaviour in list,site....

You can follow the below URL to create and deploy a custom event handler

1. Create basic event handler
http://msdn.microsoft.com/en-us/library/ms437502.aspx

2. custom Event Handler Deployment using Feature..
You need two files.
-Feature.xml
-Elements.xml

http://msdn.microsoft.com/en-us/library/ms475328.aspx

adding an entry to user field

Those have spent hour to figure why the list does not store login name directly will know extra effort is required to get / set username from / to user field. In fact, it is stored in the following format:

ID ;# account name

In this case, you need the following code snippet to insert very into user field with the help of SPFieldUserValue


using (SPSite site = new SPSite("http://portal"))
{
using (SPWeb web = site.OpenWeb())
{
SPList list = web.Lists["Example For Users"];
SPListItem item = list.Items[0];

SPFieldUserValue userValue = new SPFieldUserValue(web, web.CurrentUser.ID, web.CurrentUser.LoginName);
item["User Name"] = userValue;
item.Update();
}
}

Get a user from a list item

Very often we need to obtain username or group name from the list which has column type set to "person and group". The below code snippet might be handy



using (SPSite site = new SPSite(http://mysite))
{
using (SPWeb web = site.OpenWeb())
{
SPList list = web.Lists["myList"];
SPListItem item = list.Items[0];

SPFieldUserValue userValue = new SPFieldUserValue(web, item["User Name"].ToString());
if (userValue.User.LoginName == web.CurrentUser.LoginName)
{
//current user found in the list
}
}
}