Tukukkk

I am learning Xamarin Forms and am attempting to add in validation. What I want to do is if a valid email address is not input, then I want to outline the field in red and display a hidden label.

I get this error:

System.NullReferenceException has been thrown

In my my XF_Login.iOS

This is my syntax:

    <ContentPage.Content>  

    <StackLayout Orientation="Vertical" Padding="30" Spacing="40">  

        <BoxView HeightRequest="10"/>  

        <Frame BackgroundColor="#BF043055" HasShadow="False">  

            <StackLayout Orientation="Vertical" Spacing="10"> 

                <Entry x:Name="Email" Text="{Binding Email}" Placeholder="Email"   

                       PlaceholderColor="Red" HeightRequest="50" 

                       TextColor="Black"/>

                <Label x:Name="emailerror"  Text="Error this is invalid format.  Please udpate" TextColor="Red" /> 

            </StackLayout>  

        </Frame>  

        <Button Command="{Binding SubmitCommand}" Text="Register" TextColor="White"  

                FontAttributes="Bold" FontSize="Large" HorizontalOptions="FillAndExpand"  

                BackgroundColor="#088da5" Clicked="OnRegisterTap" />  

    </StackLayout>  

</ContentPage.Content> 



//Constructor

public RegisterPage()

{

        //Setting the label to invisible until needed

        emailerror.IsVisible = false;



        //Wiring up the event for the email check

        Email.Completed += new EventHandler(Email_Completed);



        InitializeComponent();

}



//Custom event

void Email_Completed(object sender, EventArgs e)

{

        bool isEmail = Regex.IsMatch(Email.Text, @"A(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?)Z", RegexOptions.IgnoreCase);



        if (isEmail == false)

        {

            emailerror.IsVisible = true;

            Email.BackgroundColor = Color.FromHex("#2c3e50");

            Email.TextColor = Color.Red;

        }

 }

    void OnRegisterTap(object sender, EventArgs e)

    {

       App.Current.MainPage = new NavigationPage(new AlreadyRegistered());

}

public partial class AlreadyRegistered : ContentPage

{

    public AlreadyRegistered()

    {

        InitializeComponent();

    }

}

 //Error Method

 namespace XF_Login.iOS

{

public class Application

{

    // This is the main entry point of the application.

    static void Main(string args)

    {

        // if you want to use a different Application Delegate class from "AppDelegate"

        // you can specify it here.

        UIApplication.Main(args, null, "AppDelegate");

    }

}

}   

First, in a XAML code behind you must call InitializeComponent in the constructor before you reference any XAML elements. InitializeComponent handles loading the XAML elements, if you do not call it the refs will be null.

Second, Completed only fires when Return is pressed on an Entry field. To detect when a user moves to another field, try using Unfocused instead.