Navigating to Master Detail Page from c# code

Question

I have a problem with navigating to MasterDetailPage. I know how to do it from xaml with button:

<Button IsVisible="False" Grid.Row="6" x:Name="Start" Text="START" 
VerticalOptions="Center" Margin="0,0,0,15" HorizontalOptions="Center" 
Command="{Binding Path=NavigateCommand}" 
CommandParameter="/MasterDetail/NavigationPage/MainPage" />

But how to achieve the same goal using code behind?

I was trying to do this like this:

MasterDetailPage mdet;
Page detpage;
Page mastpage;

mdet = new MasterDetailPage();
detpage = new MainPage();
mastpage = new MasterDetail();
mdet.Master = mastpage;
mdet.Detail = detpage;
App.Current.MainPage = mdet;

But as a result I get:

System.InvalidOperationException: Master and Detail must be set before adding MasterDetailPage to a container

EDIT. I'm sorry, I wrote the question a bit unclearly. To clarify what i need to achive: I have a page which is displayed when the application starts. In OnAppearing method I'm checking some conditions. If they're not met the initial page is showed with Button with Command="{Binding Path=NavigateCommand}" and CommandParameter="/MasterDetail/NavigationPage/MainPage". But if the conditions are met I want to go directly to MainPage (achieve the same what is done with mentioned button clicked but without clicking)

EDIT2. I have two pages, MasterDetail which is type of MasterDetailPage and has a master part definied in xaml and the MainPage which is ContentPage and is added to MasterDetail as detail part. When I'm adding the code Navigation.PushModalAsync(new MasterDetail()); nothing happens. To have any respons I need to do either Navigation.PushModalAsync(new MainPage()); or

var MaDe = new MasterDetail();
MaDe.Detail = new MainPage();
Navigation.PushModalAsync(MaDe);

But in both options after showing the detail page some buttons doesn't work. I have 4 buttons (well, in fact 4 grids with TapGestureRecognizers) which have similar code:

private async void TapImage_Tapped1(object sender, System.EventArgs e)
    {
        if (App.Current.Properties.ContainsKey("Boxisconf1"))
        {
            var masterDetail = App.Current.MainPage as MasterDetailPage;

            if (masterDetail == null || masterDetail.Detail == null)
                return;

            var navigationPage = masterDetail.Detail as NavigationPage;
            if (navigationPage == null)
            {
                masterDetail.Detail = new NavigationPage(new Box1());
                masterDetail.IsPresented = false;
                return;
            }

            var NavPage = new Box1();
            NavPage.Title = "Part 1";

            switch (isInsideButOutside1)
            {
                case 0:
                    NavPage.Children.Add(new Letter1 {});
                    NavPage.Children.Add(new Parcel1 {});
                    NavPage.Children.Add(new Exit1 { });
                    break;
            (...)
            }

            await navigationPage.Navigation.PushAsync(NavPage);

            navigationPage.Navigation.RemovePage(navigationPage.Navigation.NavigationStack[navigationPage.Navigation.NavigationStack.Count - 2]);
            masterDetail.IsPresented = false;
        }
        else
        {
            await Navigation.PushAsync(new Configure());
        }
    }

And in case of App.Current.Properties.ContainsKey("Boxisconf1") is true the function returns on the next if (masterDetail is null) and if the first condition is false I'm getting an exception: System.InvalidOperationException: PushAsync is not supported globally on Android, please use a NavigationPage.

Answer 1

Do you want to achieve result the code with C# like this GIF?

Here is code.

  var btn=new Button();
        btn.CommandParameter = new MasterDetailPage1();
        btn.Command = new Command<MasterDetailPage1>((key) =>
         {
             Navigation.PushAsync(key);
         });

If you do not want to new Navigation in the app.xaml.

You can just use following code.

   var btn=new Button();
        btn.CommandParameter = new MasterDetailPage1();
        btn.Command = new Command<MasterDetailPage1>((key) =>
         {
            // Navigation.PushAsync(key);
             Navigation.PushModalAsync(key);
         });

Here is running GIF.

Edit Just judge the flag, if meet the condition just navigate to other page. if not meet the condition,show a button.

public partial class MainPage : ContentPage
{
    public MainPage()
    {
        InitializeComponent();
        bool IsMeet = true ;
        if (IsMeet)
        {
            Navigation.PushModalAsync(new MasterDetailPage1());
        }
        else
        {
            var btn = new Button();
            btn.CommandParameter = new MasterDetailPage1();
            btn.Command = new Command<MasterDetailPage1>((key) =>
            {
                // Navigation.PushAsync(key);
                Navigation.PushModalAsync(key);
            });

            //  btn.Clicked += Btn_Clicked;
            Content = btn;

        }

    }