Xamarin Forms PCL消耗WCF并不总是返回值

Question

I have a simple WCF service that is validating a login and return an object representing a user.

It is being run with a Xamarin Forms PCL project. When I click the login button, the returned object returns null the first time, but when pressing a second time it returns the user object.

The object is being created in the async functions on completed method.

I am not quite sure what I am missing.

Here are pieces of code to help clarify.

Thanks for any and all assistance!

The function called by clicking the button...

private void buildLoginUser(string strUserName, string strPassword)
    {
        if (strUserName == null || strPassword == null) {
            strResult = "Please Enter User Name and Password.";
            DisplayAlert("Alert",strResult,"OK"); 
        } else {

            //IF USERNAME AND PASSWORD RECEIVED VALIDATE AGAINST DB
            //cl.ValidateUserAsync (strUserName, strPassword);
            //strResult = strUserRole;

            BuildClientService bcs = new BuildClientService ();
            //AuthenticateUser au = new AuthenticateUser ();

            TDMSClient cl = new TDMSClient ();
            cl = bcs.InitializeServiceClient();
            cl.ValidateUserAsync(strUserName, strPassword);
            //au.AuthenticateUserLogin (strUserName, strPassword, cl);
            //DisplayAlert("New User", lu.FullName, "OK");
            if (lu.FullName == null || lu.FullName == "") {
                DisplayAlert ("Alert", "NULL", "OK");
            } else {
                DisplayAlert ("Alert", lu.FullName, "OK");
            }

        }

The code that builds the user object...

public class BuildClientService
{
    public static readonly EndpointAddress EndPoint = new EndpointAddress("http://wcf.thompsoncs.net/TDMS.svc");

    public string strUserRole = "";

    BasicHttpBinding binding = CreateBasicHttp();

    private static BasicHttpBinding CreateBasicHttp()
    {
        BasicHttpBinding binding = new BasicHttpBinding
        {
            Name = "basicHttpBinding",
            MaxBufferSize = 2147483647,
            MaxReceivedMessageSize = 2147483647
        };
        TimeSpan timeout = new TimeSpan(0, 0, 30);
        binding.SendTimeout = timeout;
        binding.OpenTimeout = timeout;
        binding.ReceiveTimeout = timeout;
        return binding;
    }

    public TDMSClient InitializeServiceClient(){
        TDMSClient cl = new TDMSClient();

        cl = new TDMSClient (binding, EndPoint);
        cl.ValidateUserCompleted += clOnValidateUserCompleted;

        return cl;
    }

    private void clOnValidateUserCompleted(object sender, ValidateUserCompletedEventArgs validateUserCompletedEventArgs)
    {
        TDMS.Login.lu = validateUserCompletedEventArgs.Result;

    }
}

Thanks! I changed the code and I am now getting a UI thread error... uikit consistency error you are calling a uikit method that can only be invoked from the ui thread.

private async void buildLoginUser(string strUserName, string strPassword)
    {
        if (strUserName == null || strPassword == null) {
            strResult = "Please Enter User Name and Password.";
            DisplayAlert("Alert",strResult,"OK"); 
        } else {

            //IF USERNAME AND PASSWORD RECEIVED VALIDATE AGAINST DB

            BuildClientService bcs = new BuildClientService ();

            TDMSClient cl = new TDMSClient ();
            cl = bcs.InitializeServiceClient();

            var task = Task.Factory.StartNew(() => cl.ValidateUserAsync(strUserName, strPassword));

            await task.ContinueWith (e => {
                if (lu.FullName == null || lu.FullName == "") {
                    DisplayAlert ("Alert", "NULL", "OK");
                } else {
                    DisplayAlert ("Alert", lu.FullName, "OK");
                }
            });

        }

    }

Answer 1

You need to make your method into an Async method.

At the moment your ValidateUserAsync method returns straight away meaning that lu.FullName is null or empty.

To turn your method into an async method add the word aync to the method and await to the async line you would like to await.

private async void buildLoginUser(string strUserName, string strPassword)
{
    if (strUserName == null || strPassword == null) {
        strResult = "Please Enter User Name and Password.";
        DisplayAlert("Alert",strResult,"OK"); 
    } else {

        //IF USERNAME AND PASSWORD RECEIVED VALIDATE AGAINST DB
        //cl.ValidateUserAsync (strUserName, strPassword);
        //strResult = strUserRole;

        BuildClientService bcs = new BuildClientService ();
        //AuthenticateUser au = new AuthenticateUser ();

        TDMSClient cl = new TDMSClient ();
        cl = bcs.InitializeServiceClient();
        await cl.ValidateUserAsync(strUserName, strPassword);
        //au.AuthenticateUserLogin (strUserName, strPassword, cl);
        //DisplayAlert("New User", lu.FullName, "OK");
        if (string.IsNullOrEmpty(lu.FullName)) {
            DisplayAlert ("Alert", "NULL", "OK");
        } else {
            DisplayAlert ("Alert", lu.FullName, "OK");
        }

    }
}

Here's some more information and examples on Async/Await