API的异步方法会调整变量，然后该变量处于其原始状态

Question

Alright so I use an eID-cardreader to get Name etc from an ID-card and then I use the following method to look if there's already someone in the database with the exact same info:

    private async void GetCustomer()
    {
        using (HttpClient client = new HttpClient())
        {
            HttpResponseMessage response = await client.GetAsync("http://localhost:32576/api/customer?name=" + EIDCustomer.CustomerName + "&address=" + EIDCustomer.Address);
            if(response.IsSuccessStatusCode)
            {
                string json = await response.Content.ReadAsStringAsync();

                ApplicationVM.customer = JsonConvert.DeserializeObject<Customer>(json);
            }
            else
            {
                Console.Write("api fail");
            }
        }
    }

Alright so ApplicationVM.customer is the following: " public static Customer customer = null; " GetCustomer() is being called in the following method:

private Customer GetCustomerFromIDReader()
{
    Customer c = new Customer();

    try
    {
        //BEID_ReaderSet.initSDK();
        BEID_ReaderSet readerSet = BEID_ReaderSet.instance();
        BEID_ReaderContext reader = readerSet.getReader();

        if(reader.isCardPresent())
        {
            BEID_EIDCard card = reader.getEIDCard();
            BEID_EId doc = card.getID();

            string firstName = doc.getFirstName();
            string lastName = doc.getSurname();
            int postCode = Int32.Parse(doc.getZipCode());
            string straatEnNr = doc.getStreet().Trim();
            string gemeente = doc.getMunicipality();
            c.CustomerName = firstName + "" + lastName;
            c.Address = straatEnNr + ";" + postCode + ";" + gemeente;
            c.Picture = card.getPicture().getData().GetBytes();
        }
        else
        {
            c = null;
        }

        BEID_ReaderSet.releaseSDK();
    }
    catch (BEID_Exception ex)
    {
        Console.Write("Er is iets misgelopen bij het inlezen van de eID");
        return null;
    }

    EIDCustomer = c;

    GetCustomer();

    return c;
}

EIDCustomer is a property of the type Customer. Alright so then the program is supposed to check if c is null, if it's not then it checks if ApplicationVM.customer is null. If so you get redirected to a window where you're asked to make a new account. If ApplicationVM.customer is not null, you get to go to the next window. This is the code:

public ICommand LoginCommand
{
    get
    {
        return new RelayCommand(Login);
    }
}

public void Login()
{
    ApplicationVM appvm = App.Current.MainWindow.DataContext as ApplicationVM;

    Customer c = new Customer();

    c = GetCustomerFromIDReader();

    if(c == null)
    {
        Console.Write("Er is iets fout gelopen bij het inlezen van de e-IDkaart");
    }
    else if (ApplicationVM.customer == null)
    {
        appvm.ChangePage(typeof(KlantNieuwVM));
    }
    else
    {
        appvm.ChangePage(typeof(KlantenInterfaceVM));
    }
}

( I work with UserControls to change pages, I'm always on MainWindow, but I don't think that's of any importance to the question. )

This is the method being called in the API:

public Customer Get(string name, string address)
{
    return CustomerDA.CheckCustomerLogin(name, address);
}

CustomerDA.CheckCustomerLogin is just an sql statement which checks if there's someone in the database with the exact same name and address.

Answer 1

The problem you are experiencing is that your GetCustomer() method is asynchronous and you are not awaiting it. So when you hit the HttpResponseMessage line:

HttpResponseMessage response = await client.GetAsync("");

control is returned to your command and execution continues prior to the request coming back.

You can fix this by making your command async.

When you use the async/await pattern, the entire call chain needs to be awaitable, or else you will have this problem.

GetCustomerFromIDReader

private async Task<Customer> GetCustomerFromIDReader()
{
    Customer c = new Customer();

    // .... redacted for clarity

    await GetCustomer();

    return c;
}

Command usage

public ICommand LoginCommand
{
    get
    {
        return new RelayCommand(async () => await Login());
    }
}

public async Task Login()
{
    ApplicationVM appvm = App.Current.MainWindow.DataContext as ApplicationVM;

    Customer c = new Customer();

    c = await GetCustomerFromIDReader();

    if(c == null)
    {
        Console.Write("Er is iets fout gelopen bij het inlezen van de e-IDkaart");
    }
    else if (ApplicationVM.customer == null)
    {
        appvm.ChangePage(typeof(KlantNieuwVM));
    }
    else
    {
        appvm.ChangePage(typeof(KlantenInterfaceVM));
    }
}

Lastly change your GetCustomer to return a Task and not be void.

GetCustomer

private async Task GetCustomer()
{
    using (HttpClient client = new HttpClient())
    {
        HttpResponseMessage response = await client.GetAsync("");
        if(response.IsSuccessStatusCode)
        {
            string json = await response.Content.ReadAsStringAsync();

            ApplicationVM.customer = JsonConvert.DeserializeObject<Customer>(json);
        }
        else
        {
            Console.Write("api fail");
        }
    }
}

Answer 2

As I explain in my MSDN article on async best practices , avoid async void .

So, your async void method should be async Task :

private async Task GetCustomerAsync()
{
  ...
}

And when it's called, it should be await ed, which in turn makes the calling method async :

private async Task<Customer> GetCustomerFromIDReaderAsync()
{
  ...

  EIDCustomer = c;

  await GetCustomerAsync();

  return c;
}

And the same with the method that calls that one:

public async Task LoginAsync()
{
  ApplicationVM appvm = App.Current.MainWindow.DataContext as ApplicationVM;

  Customer c = new Customer();

  c = await GetCustomerFromIDReaderAsync();

  if(c == null)
  {
    Console.Write("Er is iets fout gelopen bij het inlezen van de e-IDkaart");
  }
  else if (ApplicationVM.customer == null)
  {
    appvm.ChangePage(typeof(KlantNieuwVM));
  }
  else
  {
    appvm.ChangePage(typeof(KlantenInterfaceVM));
  }
}

Finally, you end up with the ICommand implementation, where you could use async void (since it is logically an event handler):

public ICommand LoginCommand
{
  get
  {
    return new RelayCommand(async () => { await LoginAsync(); });
  }
}

However, you'll want to consider several points when dealing with asynchronous commands. This simple example will not disable the ICommand while it is executing, nor will it handle exceptions cleanly. I have another MSDN article specifically on asynchronous MVVM commands .