如何设置表单列的默认值

using(SPSite site = new SPSite("site url"))
{
SPWeb web = site.RootWeb;
SPList list = web.Lists["My List with the field"];
SPField fld = list.Fields["My User Or Group Field"];

SPUser usr = web.EnsureUser("DOMAIN\\login");
SPFieldUserValue defValue = new SPFieldUserValue();
defValue.LookupId = usr.ID;

fld.DefaultValue = defValue.ToString();
fld.Update();

list.Update();
}