如何使用 Asp.Net Identity 检查 Azure 表中是否存在记录并返回自定义响应

我的 Xamarin.Forms 移动应用程序使用 Web Api 和 Asp.Net 标识来注册用户并将信息存储在名为 dbo.AspNetUsers 的默认 Azure 表中。我想在注册用户之前检查记录是否存在，如果用户已经存在，则返回相关消息（在这种情况下，电子邮件应该是唯一的）。

在我的 Web Api 中：

    [Route("Register")]
    public async Task<IHttpActionResult> Register(RegisterBindingModel model)
    {
        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }

        var user = new ApplicationUser() { UserName = model.Email, Email = model.Email, FirstName = model.FirstName, LastName = model.LastName, Region = model.Region };

        IdentityResult result = await UserManager.CreateAsync(user, model.Password);

        if (!result.Succeeded)
        {
            return GetErrorResult(result);
        }

        return Ok();
    }

在我的 Xamarin.Forms PCL 中：

public class RegisterBindingModel
{
    public string Email { get; set; }

    public string Region { get; set; }

    public string FirstName { get; set; }

    public string LastName { get; set; }

    public string Password { get; set; }

    public string ConfirmPassword { get; set; }

}

public async Task<bool> RegisterAsync (string email, string password, string confirmPassword, string firstName, string lastName, string region)
    {
        var client = new HttpClient();

        // create object to send to web api for registering
        var model = new RegisterBindingModel
        {
            Email = email,
            Password = password,
            ConfirmPassword = confirmPassword,
            FirstName = firstName,
            LastName = lastName,
            Region = region
        };

        var json = JsonConvert.SerializeObject(model);

        HttpContent content = new StringContent(json);

        content.Headers.ContentType = new MediaTypeHeaderValue("application/json");

        // add for API Key
        content.Headers.Add(AppConstants.API_KEY_NAME, AppConstants.API_KEY_VALUE);

        var response = await client.PostAsync(AppConstants.AZURE_WEB_API_REGISTER_URL, content);

        return response.IsSuccessStatusCode;
    }