using System.Security.Principal;

String sGroupAdministrators = new SecurityIdentifier(WellKnownSidType.BuiltinAdministratorsSid, null).Translate(typeof(NTAccount)).Value;
sGroupAdministrators = sGroupAdministrators.Substring(sGroupAdministrators.IndexOf('\\') + 1);

Console.WriteLine("Local Administrator Group=" + sGroupAdministrators);

using System.Security.Principal;

String sLocalAdministratorUser = new SecurityIdentifier(WellKnownSidType.AccountAdministratorSid, WindowsIdentity.GetCurrent().User.AccountDomainSid).Translate(typeof(NTAccount)).Value;
sLocalAdministratorUser = sLocalAdministratorUser.Substring(sLocalAdministratorUser.IndexOf('\\') + 1);

Console.WriteLine("Local Administrator User=" + sLocalAdministratorUser);

String sLocalAdministratorUser = new SecurityIdentifier(WellKnownSidType.AccountAdministratorSid, null).Translate(typeof(NTAccount)).Value;
sLocalAdministratorUser = sLocalAdministratorUser.Substring(sLocalAdministratorUser.IndexOf('\\') + 1);
Console.WriteLine("Local Administrator User=" + sLocalAdministratorUser);

sLocalAdministratorUser = sLocalAdministratorUser.Substring(sLocalAdministratorUser.IndexOf('\\') + 1);

using System.DirectoryServices.AccountManagement;
using (PrincipalContext ctx = new PrincipalContext(ContextType.Machine))
using (GroupPrincipal grp = GroupPrincipal.FindByIdentity(ctx, IdentityType.Name, "Administrators"))
{
    if (grp is not null)
    {
        foreach (Principal p in grp.GetMembers(recursive: true))
        {
            Console.WriteLine(p.Name);
        }
    }
}

string sLocalAdministratorUser = "";

ManagementScope scope = new(@"\\.\root\CIMV2");
scope.Connect();

ObjectQuery query = new("SELECT Name from Win32_UserAccount WHERE SID LIKE 'S-1-5-21%-500'");
ManagementObjectSearcher searcher = new(scope, query);

ManagementObjectCollection results = searcher.Get();

foreach (var item in results)
{
	sLocalAdministratorUser = item["Name"].ToString();
	break;
}

Console.WriteLine("Local Administrator User=" + sLocalAdministratorUser);