Universal Apps MessageBox: "The name 'MessageBox' does not exist in the current context"

I want to use MessageBox for showing download errors in my WP8.1 app.

I added:

using System.Windows;

but when I type:

MessageBox.Show("");

I get error:

"The name 'MessageBox' does not exist in the current context"

In Object Browser I found that such class should exist and in "Project->Add reference... ->Assemblies->Framework" is shown that all assemblies are referenced.

Do I miss something? Or is there another way how to show something like messagebox?

For Universal Apps, the new APIs require you to use await MessageDialog().ShowAsync() (in Windows.UI.Popups) to bring it into line with Win 8.1.

var dialog = new MessageDialog("Your message here");
await dialog.ShowAsync();

Just wanted to add to ZombieSheep's answer: also, customization is quite simple

        var dialog = new MessageDialog("Are you sure?");
        dialog.Title = "Really?";
        dialog.Commands.Add(new UICommand { Label = "Ok", Id = 0 });
        dialog.Commands.Add(new UICommand { Label = "Cancel", Id = 1 });
        var res = await dialog.ShowAsync();
        
        if ((int)res.Id == 0)
        { *** }

try this:

 using Windows.UI.Popups;

code:

private async void Button_Click(object sender, RoutedEventArgs e)
    {
                    
        MessageDialog msgbox = new MessageDialog("Would you like to greet the world with a \"Hello, world\"?", "My App");
        
        msgbox.Commands.Clear();
        msgbox.Commands.Add(new UICommand { Label = "Yes", Id = 0 });
        msgbox.Commands.Add(new UICommand { Label = "No", Id = 1});
        msgbox.Commands.Add(new UICommand { Label = "Cancel", Id = 2 });
        
        var res = await msgbox.ShowAsync(); 

        if ((int)res.Id == 0)
        {
            MessageDialog msgbox2 = new MessageDialog("Hello to you too! :)", "User Response");
            await msgbox2.ShowAsync();
        }

        if ((int)res.Id == 1)
        {
            MessageDialog msgbox2 = new MessageDialog("Oh well, too bad! :(", "User Response");
            await msgbox2.ShowAsync();
        }

        if ((int)res.Id == 2)
        {
            MessageDialog msgbox2 = new MessageDialog("Nevermind then... :|", "User Response");
            await msgbox2.ShowAsync();
        }
            

    }

To Trigger some Function When "Yes" or "No" is clicked, You can also use:

msgbox.Commands.Add(new UICommand("Yes", new UICommandInvokedHandler(this.TriggerThisFunctionForYes)));
msgbox.Commands.Add(new UICommand("No", new UICommandInvokedHandler(this.TriggerThisFunctionForNo)));

You could also make a class like the next one. Below the code a usage example:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Windows.UI.Popups;

namespace someApp.ViewModels
{
    public static class Msgbox
    {
        static public async void Show(string mytext)
        {
            var dialog = new MessageDialog(mytext, "Testmessage");
            await dialog.ShowAsync();
        }
    }

}

The example to use it in a class

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace someApp.ViewModels
{
    public class MyClass{

        public void SomeMethod(){
            Msgbox.Show("Test");
        }

    } 
}

public sealed partial class MainPage : Page {
    public MainPage() {
        this.InitializeComponent();
    }

    public static class Msgbox {
        static public async void Show(string m) {
            var dialog = new MessageDialog( m);            
            await dialog.ShowAsync();
        }
    }

    private void Button_Click(object sender, RoutedEventArgs e) { 
        Msgbox.Show("This is a test to see if the message box work");
        //Content.ToString();
    }
}

For new UWP apps (starting from Windows 10) Microsoft recommends using [ContentDialog][1] instead.

Example:

private async void MySomeMethod()
{
    ContentDialog dlg = new ContentDialog()
    {
        Title = "My Content Dialog:",
        Content = "Operation completed!",
        CloseButtonText = "Ok"
    };

    await dlg.ShowAsync();
}

Usage:

private void MyButton_Click(object sender, RoutedEventArgs e)
{
   MySomeMethod();
}

Remark: You can use different styles etc. for the ContentDialog. Please refer to the link above for various usages for ContentDialog. [1]: https://docs.microsoft.com/en-us/uwp/api/windows.ui.xaml.controls.contentdialog