ASP.NET Core Identity invalid token on confirmation email

Question

This is a very similar question to this aspnet identity invalid token on confirmation email but the solutions are not valid because I am using the new ASP.NET Core 1.0 that

Answer 1

In my case, this problem was due to OnPostAsync method in RegisterModel encoding the callback url:

var callbackUrl = Url.Page(
                    "/Account/ConfirmEmail",
                    pageHandler: null,
                    values: new { userId = user.Id, code = code },
                    protocol: Request.Scheme);

await _emailSender.SendEmailAsync(Input.Email, "Confirm your email",
                    $"Please confirm your account by <a href='{HtmlEncoder.Default.Encode(callbackUrl)}'>clicking here</a>.");

This encoding (the application of HtmlEncoder.Default.Encode() to callbackUrl), made the url '&' become '&', thus invalidating the whole link.