Comment question at http://blachniet.com/2013/11/10/email-addresses-as-user-names-in-asp-net-identity/#comment-1175501695

CodeInQuestion.cs

public Task CreateAsync(ApplicationUser user)
{
  if (user == null)
  {
    throw new ArgumentNullException("user cannot be null.");
  }

  if (string.IsNullOrEmpty(user.UserName))
  {
    throw new ArgumentNullException("username cannot be null or empty.");
  }

  if (string.IsNullOrEmpty(user.Email))
  {
    throw new ArgumentNullException("email cannot be null or empty.");
  }

  //compile errors like username or email already taken...
  var errors = GetErrors(user);

  if (errors.Any())
  {
    return Task.FromResult<identityresult>(IdentityResult.Failed(errors.ToArray()));
  }

  userBusiness.AddUser(user.ToUser());

  return Task.FromResult<identityresult>(null);
}