c# - Error in ui kit Consistency error

Question

I am having a issue accesing a text box in a view controller .cs file

 async partial void loginUser(UIButton sender)

{

    // Show the progressBar as the MainActivity is being loade


    Console.WriteLine("Entered email : " + txtEmail.Text);


        // Create user object from entered email
        mCurrentUser = mJsonHandler.DeserialiseUser(txtEmail.Text);
        try
        {

            Console.WriteLine("Starting network check");
            // Calls email check to see if a registered email address has been entered
            if (EmailCheck(txtEmail.Text) == true)
            {
                await  CheckPassword();
            }
            else
            {
                UIAlertView alert = new UIAlertView()
                {
                    Title = "Login Alert",
                    Message = "Incorrect email or password entered"
                };
                alert.AddButton("OK");
                alert.Show();

            }


        }
        catch (Exception ex)
        {
            System.Diagnostics.Debug.WriteLine("An error has occured: '{0}'", ex);
        }

It is within this funciton that it complains it cannot access a text box which is on a aynsc method

    public Task CheckPassword()
    {

   return Task.Run(() =>
    {
            // Creates instance of password hash to compare plain text and encrypted passwords.
            PasswordHash hash = new PasswordHash();
            // Checks password with registered user password to confirm access to account.
            if (hash.ValidatePassword(txtPassword.Text ,mCurrentUser.password)==true)
            {
                Console.WriteLine("Password correct");


                UIAlertView alert = new UIAlertView()
                {
                    Title = "Login Alert",
                    Message = "Password Correct Loggin In"
                };
                alert.AddButton("OK");
                alert.Show();
                //insert intent call to successful login here.

            }
            else
            {
                UIAlertView alert = new UIAlertView()
                {
                    Title = "Login Alert",
                    Message = "Incorrect email or password entered"
                };
                alert.AddButton("OK");
                alert.Show();

            }
            Console.WriteLine("Finished check password");
        });
    }

Its this line the error occurs:

txtPassword.Text 

The error is as follows:

UIKit.UIKitThreadAccessException: UIKit Consistency error: you are calling a UIKit method that can only be invoked from the UI thread.

Also my Password Correct does not show even though if it is a correct password. Do i have to run the UI Alerts on a seperate thread?

Answer 1

Any UIKit methods must be called from the UI thread (or Main thread, Main queue, etc.). This ensures consistency in the UI. Xamarin adds a check to all UIKit methods in debug mode and throws that exception if you try to use a background thread to change the UI.

The solution is simple: only modify the UI from the UI thread. That essentially means if you're using a class with "UI" in front it, you should probably do it from the UI thread. (That's a rule of thumb and there are other times to be on the UI thread).

How do I get my code on this mythical UI thread? I'm glad you asked. In iOS, you have a few options:

• When in a subclass of NSObject, InvokeOnMainThread will do the trick.
• From anywhere, CoreFoundation.DispatchQueue.MainQueue.DispatchAsync will always work.

Both of those methods just accept an Action, which can be a lambda or a method.

So in your code, if we add an InvokeOnMainThread (because I think this is in your UIViewController subclass)...

 public Task CheckPassword()
 {

     return Task.Run(() =>
     {
        // Creates instance of password hash to compare plain text and encrypted passwords.
        PasswordHash hash = new PasswordHash();
        // Checks password with registered user password to confirm access to account.
        InvokeOnMainThread(() => {
            if (hash.ValidatePassword(txtPassword.Text ,mCurrentUser.password)==true)
            {
                Console.WriteLine("Password correct");


                UIAlertView alert = new UIAlertView()
                {
                    Title = "Login Alert",
                    Message = "Password Correct Loggin In"
                };
                alert.AddButton("OK");
                alert.Show();
                //insert intent call to successful login here.

            }
            else
            {
                UIAlertView alert = new UIAlertView()
                {
                    Title = "Login Alert",
                    Message = "Incorrect email or password entered"
                };
                alert.AddButton("OK");
                alert.Show();

            }
        });
        Console.WriteLine("Finished check password");
    });
}