Use LINQ to check if user is in a group. Create an extension method.

public static bool InGroup(this SPUser user, SPGroup group)
{
  return user.Groups.Cast<SPGroup>()
    .Any(g => g.ID == group.ID);
}
```EDIT 2011-01-22: There is a shortcoming of this method. You won't get a user which is in group through a AD group. You'll get only users and ad groups. [But there is another method to check if a user is inside an AD group](/2012/01/16/check-if-a-user-is-in-a-ou/ "See my post about how to retrieve users from AD groups with PrincipalSearcher"). How could we combine them?... I think we must start from group this time, not from user:

public static bool HasUser(this SPGroup user, SPUser user) { var users = group.Users.Cast(); var samAccount = Regex.Replace(user.LoginName, @".*\\(.*)", “$1”, RegexOptions.None); var exists = users.Any(u => u.LoginName.Equals(user.LoginName)); if (!exists) { var ctx = new PrincipalContext(ContextType.Domain); foreach (var u in users) { var login = u.LoginName; var groupName = Regex.Replace(login, @".*\\(.*)", “$1”, RegexOptions.None); var grp = GroupPrincipal.FindByIdentity(ctx, IdentityType.Name, groupName); if (grp == null) continue; var principals = grp.GetMembers(true); exists = principals.Any(p => p.SamAccountName.Equals(samAccount, StringComparison.InvariantCultureIgnoreCase)); if (exists) break; } } return exists; }