Use the AllUsers or SiteUsers property of the SPWeb class, the GetAllAuthenticatedUsers method of the SPUtility class, or the GetUniqueUsers method of the SPAlertCollection class, to return the following:

• All the users of a site

• All the users in the site collection

• All authenticated users of a site

• A list of users for a collection of alerts.

Otherwise, use the Users property of the SPGroup, SPRole, or SPWeb class to return the users in a group, role definition, or site.

Use an indexer to return a single user from the collection. For example, if the collection is assigned to a variable named collUsers, use collUsers[index] in C#, or collUsers(index) in Visual Basic, where index is either the index number of the user in the collection or the user name of the user.

Every user has a unique member ID (ID property), has the permissions associated with that membership, and can be represented by an SPMember object. The following example assigns a user to an SPMember object, given a specified SharePoint Web site:

Dim oWebsite As SPWeb = SPContext.Current.Web
Dim oMember As SPMember = oWebsite.AllUsers("Domain\\User_Alias")

SPWeb oWebsite = SPContext.Current.Web;
SPMember oMember = oWebsite.AllUsers["Domain\\User_Alias"];

For general information about users and security, see Security, Users, and Groups.

The following code example modifies the e-mail address, display name, and notes for a specified user object.

Dim siteCollection As SPSite = SPControl.GetContextSite(Context)
Dim site As SPWeb = siteCollection.AllWebs("Site_Name")
Dim user As SPUser = site.AllUsers("User_Name")

user.Email = "E-mail_Address"
user.Name = "Display_Name"
user.Notes = "User_Notes"

user.Update()

SPSite oSiteCollection = SPContext.Current.Site;
using (SPWeb oWebsite = oSiteCollection.AllWebs["Website_Name"])
{
    SPUser oUser = oWebsite.AllUsers["User_Name"];

    oUser.Email = " E-mail_Address";
    oUser.Name = " Display_Name";
    oUser.Notes = " User_Notes";

    oUser.Update();
}

Note:
Certain objects implement the IDisposable interface, and you must avoid retaining these objects in memory after they are no longer needed. For information about good coding practices, see Best Practices: Using Disposable Windows SharePoint Services Objects.