Using Multi-valued UserInfo fields (UserMulti)

Call me a dumb arse (maybe I was just tired at the time?!), but I had some issues the other day reading and writing users from a multi-valued UserInfo field.

I had a UserMulti type field declared in my solution, and needed to read and write to it via an application page.

The field declaration:

<Field ID="{084A686D-60CA-4E95-AB4D-55D8B1DF1602}" DisplayName="APTN" Name="ictRiskAPTN" StaticName="ictRiskAPTN" Group="Ince Risk"         List="UserInfo" ShowField="ImnName" Required="FALSE" Type="UserMulti" Description="By default... (omitted)" UserSelectionMode="PeopleOnly" UserSelectionScope="0" AllowMultiVote="TRUE" />

To read from the field:

[Note: Fields.AdditionalPeopleToNotify.Id is a property of a helper class that contains all the Ids, Displaynames and static names of my declared fields)]

var additionalRecipients = i[Fields.AdditionalPeopleToNotify.Id] as SPFieldUserValueCollection;
if (additionalRecipients != null)
{
if (additionalRecipients.Count > 0)
{
foreach (SPFieldUserValue uservalue in additionalRecipients)
{
//Do something with your user
}
}
}

To set the value of the field:

var i = listItems[0];
SPFieldUserValueCollection users = new SPFieldUserValueCollection();
SPPrincipal pr = web.EnsureUser("domain\\username") as SPPrincipal;
users.Add(new SPFieldUserValue(i.Web, pr.ID, pr.Name));
i[Fields.AdditionalPeopleToNotify.Id] = users;
i.SystemUpdate();

Or, for a more realistic example, set the value of the field based on a list of users or groups selected from a PeoplePicker control:

var i = listItems[0];
SPFieldUserValueCollection users = new SPFieldUserValueCollection(); 
if (peoplePicker.Entities.Count > 0)
{
foreach (PickerEntity entity in peoplePicker.Entities)
{
SPPrincipal pr = null;
if (entity.EntityData.ContainsKey("PrincipalType"))
{
var pT = entity.EntityData["PrincipalType"] as String;
if (pT != null)
{
switch (pT)
{
case "SharePointGroup":
pr = web.SiteGroups[entity.Key] as SPPrincipal;
break;
case "User":
pr = web.EnsureUser(entity.Key) as SPPrincipal;
break;
case "SecurityGroup":
pr = web.EnsureUser(entity.Key) as SPPrincipal;
break;
default:
break;
}
}
} 
if (pr != null)
{
users.Add(new SPFieldUserValue(item.Web, pr.ID, pr.Name));
}
}
}
i[Fields.AdditionalPeopleToNotify.Id] = users;
i.SystemUpdate();