IdentityExtensions.cs

using System;
using System.Security.Principal;

internal static class IdentityExtensions
{
    internal static string GetNormalizedLogin(this IIdentity identity)
    {
        if (identity == null)
        {
            throw new ArgumentNullException(nameof(identity));
        }

        string s = identity.Name;
        if (s == null)
        {
            return String.Empty;
        }

        int stop = s.IndexOf("\\", StringComparison.OrdinalIgnoreCase);
        return (stop > -1) ? s.Substring(stop + 1, s.Length - stop - 1) : s;
    }
}