由于您的最后建议，看来我已经找到了错误！我正在尝试设置外键的值，如果ID ...

It seems I've found the error thanks to your last suggestion! I was trying to set the value of the foreign key instead if the ID...

对我有用的代码:

[HttpPost]
public async Task<ActionResult> Register(RegisterViewModel model)
{
  // verify the models' valid
  if (ModelState.IsValid)
  {
      var user = new ApplicationUser {
        /* ... */,
        CompanyID = model.CompanyID // entity we retrieved
      };

      //
      // SignInManager yatta yata
      //
  }
  return View(model); // Uh-oh fallback.
}