Q:

Why is my List.GetUserEffectivePermissions() method not working?

I'm developing a process in C# with the SharePoint 2013 Client Side Object Model. I need to retrieve the SharePoint List Permissions of a given user, that will be different than the user that is executing the code.
using SP = Microsoft.SharePoint.Client;

SP.ClientContext SpContext = new SP.ClientContext("SITEURL");
SP.Web SiteWeb = SpContext.Web;
SP.List Lst = SpContext.Web.Lists.GetByTitle("LIST");
var ClientUserEffPerms = Lst.GetUserEffectivePermissions(@"<domain>\<username>");

SpContext.Load(SiteWeb, S => S.EffectiveBasePermissions);
SpContext.Load(Lst, L => L.EffectiveBasePermissions);
SpContext.ExecuteQuery();

After this code executes, the ClientUserEffPerms.Value (BasePermissions) object does not represent the permissions of the given user correctly.  The object isn't null, but it represents the user as having no permissions.  The user has at minimum view and edit permissions and I can confirm this by viewing/editing List Items using the web browser as this user.
The code executing user has permission to enumerate permissions at both the Web and List level.  I've confirmed this with the code below, both booleans resolve to true.
bool SvcUserHasSiteEnumPermsPerm = SiteWeb.EffectiveBasePermissions.Has(SP.PermissionKind.EnumeratePermissions);
bool SvcUserHasListEnumPermsPerm = Lst.EffectiveBasePermissions.Has(SP.PermissionKind.EnumeratePermissions);

Can anyone help me determine what is wrong with my GetUserEffectivePermissions() method?

A:

When you call GetUserEffectivePermissions you need to pass in the full claims token version of the login name, which looks something like this:
i:0#.w|domain\user
You can get this by loading the LoginName property on a user object:
clientContext.Load(clientContext.Web.CurrentUser, i => i.LoginName);
clientContext.ExecuteQuery();

Of course, that's for the current user, so you'll need to acquire the user you actually want first.

