You can call UserPrincipal.FindByIdentity
inside System.DirectoryServices.AccountManagement
:
using System.DirectoryServices.AccountManagement;
using (var pc = new PrincipalContext(ContextType.Domain, "MyDomainName"))
{
var user = UserPrincipal.FindByIdentity(pc, IdentityType.SamAccountName, "MyDomainName\\" + userName);
}