ASP.NET WEB API外部登录

我使用Visual Studio 2013项目向导在ASP.NET中创建了WEB API项目。它为社交登录创建了这个函数:

// GET api/Account/ExternalLogin
[OverrideAuthentication]
[HostAuthentication(DefaultAuthenticationTypes.ExternalCookie)]
[AllowAnonymous]
[Route("ExternalLogin", Name = "ExternalLogin")]
public async Task<IHttpActionResult> GetExternalLogin(string provider, string error = null)
{
    if (error != null)
    {
        return Redirect(Url.Content("~/") + "#error=" + Uri.EscapeDataString(error));
    }
    if (!User.Identity.IsAuthenticated)
    {
        return new ChallengeResult(provider, this);
    }
    ExternalLoginData externalLogin = ExternalLoginData.FromIdentity(User.Identity as ClaimsIdentity);
    if (externalLogin == null)
    {
        return InternalServerError();
    }
    if (externalLogin.LoginProvider != provider)
    {
        Authentication.SignOut(DefaultAuthenticationTypes.ExternalCookie);
        return new ChallengeResult(provider, this);
    }
    ApplicationUser user = await UserManager.FindAsync(new UserLoginInfo(externalLogin.LoginProvider,
        externalLogin.ProviderKey));
    bool hasRegistered = user != null;
    if (hasRegistered)
    {
        Authentication.SignOut(DefaultAuthenticationTypes.ExternalCookie);
         ClaimsIdentity oAuthIdentity = await user.GenerateUserIdentityAsync(UserManager,
            OAuthDefaults.AuthenticationType);
        ClaimsIdentity cookieIdentity = await user.GenerateUserIdentityAsync(UserManager,
            CookieAuthenticationDefaults.AuthenticationType);
        AuthenticationProperties properties = ApplicationOAuthProvider.CreateProperties(user.UserName);
        Authentication.SignIn(properties, oAuthIdentity, cookieIdentity);
    }
    else
    {
        IEnumerable<Claim> claims = externalLogin.GetClaims();
        ClaimsIdentity identity = new ClaimsIdentity(claims, OAuthDefaults.AuthenticationType);
        Authentication.SignIn(identity);
    }
    return Ok();
}

然后我用c#写了一个客户端代码来调用这个函数:

public async Task LogInAsync(string url, string provider)
{
    using (HttpClient client = new HttpClient())
    {
      string request = url + "/api/Account/ExternalLogin";
      var query = HttpUtility.ParseQueryString(string.Empty);
      query["provider"] = provider;
      query["error"] = "";
      request += "?" + query.ToString();
      HttpResponseMessage responseMessage = await client.GetAsync(request);
      if (responseMessage.IsSuccessStatusCode)
      {
        string responseContent = await responseMessage.Content.ReadAsStringAsync();
      }
    }
}

奇怪的是，我从服务器收到这个错误响应:

StatusCode: 400, ReasonPhrase: 'Bad Request'， Version: 1.1, Content:System.Net.Http。StreamContent, Headers: {Pragma: no-cache
X-SourceFiles:utf - 8 = ? B RTpcUHJvamVjdFxEYXRpbmdcRGF0aW5nLlNlcnZlclxhcGlcQWNjb3VudFxFeHRlcm5hbExvZ2lu ?=Cache-Control: no-cache Date: Tue, 08 Nov 2016 15:12:33 GMT
服务器:Microsoft-IIS/8.0 X-Powered-By: ASP。NET内容-长度:24内容类型:文本/平原;charset=UTF-8 Expires: -1}

当我尝试在web浏览器中导航各自的链接时，出现同样的错误。在服务器调试时，不会命中此函数的相应入口点。我做错了什么?它是GET动词，所以我应该能够以任何一种方式成功地访问它。

最让我困惑的是，这个函数默认包含在每个WEB API项目中，但是，我找不到任何参考或提及人们在实践中如何使用它。

AuthorizeEndpointPath = new PathString("/api/Account/ExternalLogin")