public async Task<IActionResult> Login([FromBody]LoginViewModel model)
{
    if (ModelState.IsValid)
    {
        var result = await _signInManager.PasswordSignInAsync(model.Email, model.Password, true, lockoutOnFailure: false);
        if (result.Succeeded)
        {
            _logger.LogInformation(1, "User logged in.");
            return Json(User.Identity.Name);
        }
        else
        {
            return BadRequest("Invalid login attempt.");
        }
    }
    else if (HttpContext.User.Identity.IsAuthenticated)
    {
        return Json(User.Identity.Name);
    }
    return BadRequest("Cannot login.");
}

public class LoginViewModel
{
    [Required]
    [EmailAddress]
    public string Email { get; set; }

    [Required]
    [DataType(DataType.Password)]
    public string Password { get; set; }
}

services.AddMvc().AddJsonOptions(options =>
{
    options.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver();
    options.SerializerSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore;
    options.SerializerSettings.Converters.Add(new VersionConverter());
});