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Why does my modal page have a back button?

Project and problem explanation
In my case I have a page which instructs the user how to move a certain machine. This page is supposed to be a modal page since the page should not be navigated away from without performing or completely canceling the action.
I have a mainpage with a listview which opens the details page of any of the items contained in the listview on click through the onSelect method:
Navigation.PushAsync(new FooPage(string Name)); /* gets called in the
onSelect method if selection is not null*/
The model is retrieved within the details page's viewmodel after passing the name to it and then using a model manager injected into the viewmodel with Unity to retrieve it.
-- we now both have a mainpage and detailspage on the stack which of only the detailspage has a backbutton --
On the detailspage we have a button called "Teach" with a x,y,z field next to it after clicking it this method gets called:
private async Task Button_Clicked(object sender, EventArgs e)
{
await Navigation.PushModalAsync(new ManualTeachPage());
}
which then, as it should, creates the page but then for some reason decides to add a back button to it:
Debugging and Research
This behavior is not visible on Android which does not have a visible back button or navigation bar on this page but does have a backbutton on the Details-page as well.
I have used modal pages before but I have never seen this kind of behavior, I have tried using the Navigation property of the Application.Mainpage itself which resulted in the exact same result except for one case.
I thought it might have something to do with me switching out the Application.Mainpage at one point(there is a stack of tutorial pages the user has to go through), seems like calling the pushModalAsync one line AFTER setting the new mainpage then it DOES push the page as a modal page and performs like one (without a back button) but does not do so after this point.
No bug reports on Bugzilla about this either as far as I have seen, neither have I found anything on the internet about this particular problem.
Note that when clicking the back button on the teachPage it returns to the detailspage. When the teachPage gets pushed it does actually get pushed onto the ModalStack.
update 1
Checked again if the modal page which I was talking about was the only modal page on the modal Stack, it was. NavigationPage.SetHasBackButton(this, false); does not seem to work either as suggested by Diego Rafael Souza.
update 2
I thought I would temporarily disable the backbutton by using the onBackButtonPressed within the teachpage until I had found a solution. Turns out this doesn't work for UWP anymore, this method does not get called anymore on this page or any page for that matter. It does work for the Android Hardware button though.
update 3
I tried using:
SystemNavigationManager.GetForCurrentView().AppViewBackButtonVisibility =
AppViewBackButtonVisibility.Collapsed;
This does hide the backbutton on UWP but for some reason #if WINDOWS_UWP doesn't work at all. If I do this without the #if the program won't build for Android. No solution yet, not only this but when using this fix the other pages still had the onBackButtonPressed method disabled.
update 4
after updating to the newest version of xamarin forms the OnBackButtonPressed started working again. The backbutton still appears on the page but I now have disabled it.
Recreating the problem
I recreated the problem in this small test project:
https://www.dropbox.com/sh/6btsud3uvw503ee/AAAiaGb3TwhMrZMJb2j-rd36a?dl=0

In your function Button_Clicked() in your example, in page DetailPage, where you call your Modal, right after calling PushModalAsync(new TeachPage());, use:
NavigationPage.SetHasNavigationBar(this, false);
NavigationPage.SetHasBackButton(this, false);
My guess is when you tried using the above SetHasBackButton, you were doing it on the Modal itself, but the back button you were actually seeing came from your DetailPage.
Once you add this, the modal pops up, and the back button disappears. If you set a "Back" button on your modal in order to close it, you can easily get your NavBar and BackButton back in your DetailPage by adding in an OnAppearing() Function in your DetailPage code-behind like so:
protected override void OnAppearing()
{
base.OnAppearing(); // do the usual stuff OnAppearing does
NavigationPage.SetHasNavigationBar(this, true); //get your navbar back
NavigationPage.SetHasBackButton(this, true); //get your back button back
}

Back button controlling for my Windows Phone 8.1 Silverlight App

I am developing a Windows Phone 8.1 Silverlight app. In my app, I need to override the back button code.
So I tried this,
protected override void OnBackKeyPress(CancelEventArgs e)
{
//Navigating to this page when back button is pressed
DisplayPage mynewPage = new DisplayPage();
this.Content = mynewPage;
e.Cancel = true;
}
But this code is not working. What am I doing wrong? Please help me out.
EDIT:
When I place this code on MainPage, it works! But I don't want to place it there. I want to place it on other pages.

Remove "e.Cancel = true" (it cancels the navigation). Simply just navigate to the new page.
EDIT:
For navigating to another page, I would rather suggest using NavigationService. Check out this page for examples.
EDIT:
Code snippet:
protected override void OnBackKeyPress(CancelEventArgs e)
{
//Navigating to this page when back button is pressed
NavigationService.Navigate(new Uri("/DisplayPage.xaml", UriKind.Relative));
}

How to go to previous page in windows phone 8 when back button is pressed

I have a home page or landing page in my windows phone c# based app where user enters login details and upon successful login user is redirected to page2 . Here the user will see a list box with few items . Upon selecting an item from this list box a new page called "Threadx" opens.(where x is the each page that opens upon clicking the x item in the list box)
While user is on this Thread page "Threadx" he may receive the toast notifications and the thread gets updated with new replies or answers on that thread.
But When user clicks on back button the "ThreadX" page doesn't get closed and instead it goes to its previous state where it has less number of messages , and so on until the app gets closed.
protected override void OnNavigatedTo(NavigationEventArgs e)
{
base.OnNavigatedTo(e);
if (e.NavigationMode == NavigationMode.Back)
{
return;
}
}
I would like to know if this "Threadx" page can be closed upon clicking back button without affecting other "Threadx+1","Threadx+2"..."Threadx+n" pages.
Any help would be really appreciated.

Normally windows keeps the pages on it's stack when you leave a page and navigate to another page. If you want to navigate to the previous page on pressing the Back Button you can do following things:
Add following line to OnNavigatedTo method:
Windows.Phone.UI.Input.HardwareButtons.BackPressed += HardwareButtons_BackPressed;
Add definition for HardwareButtons_BackPressed method:
private void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e)
{
e.Handled = true;
if (Frame.CanGoBack)
Frame.GoBack();
}
Don't forget to add Windows.Phone.UI.Input to the namespace list.

The other way I got it worked was using the below code in the onnavigateto method in my "thread" page and it worked for me. Let me know if there is an elegant way of doing it or better way of doing it .
if (e.NavigationMode == NavigationMode.Back)
{
NavigationService.Navigate(new Uri("/View/Page2.xaml", UriKind.Relative));
}

Windows Phone: RemoveBackEntry after navigate failing

I've got an intermediate "Loading" page for my game: I send them there and it has "Loading..." text that displays while the rather hefty game page loads up:
private void OnLoaded(object sender, RoutedEventArgs e)
{
Dispatcher.BeginInvoke(() =>
{
try
{
NavigationService.Navigate(new Uri("/GamePage.xaml", UriKind.Relative));
NavigationService.RemoveBackEntry();
}
catch (InvalidOperationException)
{
}
});
}
Then when you hit the back button you go to the main page rather than back to the loading screen. There's no other logic on the page.
However I just got a store submission declined: apparently on the Samsung Focus and Odyssey the navigation entry for the loading page didn't get removed, and the user is sent back to the loading page when they hit the back button, rather than back to the main menu. I'm guessing RemoveBackEntry failed.
This looks like it should work, I can't reproduce the error and I don't have a Focus or Odyssey to work with. Does anybody know what might be going wrong?

I think you should replace:
NavigationService.RemoveBackEntry();
with:
while (NavigationService.CanGoBack)
{
NavigationService.RemoveBackEntry();
}

You have to remember that NavigationService.Navigate will perform the navigation asynchronously. So when you call NavigationService.RemoveBackEntry(), the current page might not yet be on the BackStack.
To fix that, call RemoveBackEntry in OnNavigatedTo of GamePage.

Windows Phone NavigationService.RemoveBackEntry method missing?

I'm trying to tighten navigation in my WP8 app and I'm having an issue where I want to exit when the user is on the main xaml page (which could be navigated to from many pages within my app). The simplest way about it seemed to let the back button handle exiting, but in order to do this it seems you need to clear the back button's history so that it can exit.
MSDN says to use the NavigationService.RemoveBackEntry method to clear out back entries (and thus allowing the app to close naturally once the user clicks Back again). But I'm not seeing this method! What gives?
http://msdn.microsoft.com/en-us/library/windowsphone/develop/system.windows.navigation.navigationservice.removebackentry(v=vs.105).aspx

yes your point is correct. you can clear the back entry and exit by the following piece of code.
protected override void OnNavigatedTo(NavigationEventArgs e)
{
while (NavigationService.CanGoBack)
{
NavigationService.RemoveBackEntry();
}
}

you can use this to terminate the app in Windows Phone 8 (Just remember to save all your program data before calling this ;-))
App.Current.Terminate();