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I ran into situation when some MessageBoxes with error messages were not displayed at all. After closer investigation, I was able to narrow down the problem to cases when a very long string is passed as messageBoxText.

In such cases, call MessageBox.Show does not display anything, returns MessageBoxResult.No and, in most cases, there's a message in the Output window saying The thread 0xHEXNUMBER has exited with code 0 (0x0). For me, this method silently fails.

This is very strange - WPF is a very mature technology and I expect this code to work according to spec or throw some exception (e.g. OutOfMemory, StackOverflowException). I've debugged the program and no manged or unmanged exceptions are thrown and caught.

What is the root cause of MessageBox not being displayed? Is there some easy way to debug such things (since no exceptions are thrown or logged). What is the messageBoxText length limit and what does it depend on (I could check that empirically on my computer, but would be conclusive for one OS/platform/.NET Framework version at best)?

Repro:

Here's a code which demonstrates the problem (it can be used with standard WPF Application template in Visual Studio).

MainWindow.xaml:

<Window x:Class="LongMessageInMessageBox.MainWindow"

    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

    Title="MainWindow" Height="350" Width="525">

<Grid>

  <Grid.RowDefinitions>

      <RowDefinition Height="Auto" />

      <RowDefinition Height="Auto" />

  </Grid.RowDefinitions>

  <Grid.ColumnDefinitions>

      <ColumnDefinition Width="Auto" />

  </Grid.ColumnDefinitions>



  <Button Grid.Row="0" Grid.Column="0" Click="ShortMessage_Clicked">Short Message</Button>

  <Button Grid.Row="1" Grid.Column="0" Click="LongMessage_Clicked">Long Message</Button>



</Grid>

MainWindow.xaml.cs:

 using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using System.Windows;

using System.Windows.Controls;

using System.Windows.Data;

using System.Windows.Documents;

using System.Windows.Input;

using System.Windows.Media;

using System.Windows.Media.Imaging;

using System.Windows.Navigation;

using System.Windows.Shapes;



namespace LongMessageInMessageBox

{

    public partial class MainWindow : Window

    {

        public MainWindow()

        {

            InitializeComponent();

        }



        /// Method which shows that displaying short message works correctly

        private void ShortMessage_Clicked(object sender, RoutedEventArgs e)

        {

            Console.WriteLine("ShortMessage_Clicked");

            var result = MessageBox.Show("Short Message");

            Console.WriteLine(result);

        }



        /// Method which shows that displaying a long message does not work

        private void LongMessage_Clicked(object sender, RoutedEventArgs e)

        {

            Console.WriteLine("LongMessage_Clicked");

            var longTextBuilder = new System.Text.StringBuilder(10000);

            longTextBuilder.Append("Long Message n");

            for (int i = 1; i <= 100000; i++)

            {

                longTextBuilder.Append(" ").Append(i);

            }

            var result = MessageBox.Show(longTextBuilder.ToString());

            Console.WriteLine(result);

        } 

    }

}

Sample from Output window:

ShortMessage_Clicked

OK

ShortMessage_Clicked

OK

LongMessage_Clicked

No

The thread 0x55bc has exited with code 0 (0x0).

MessageBox class in WPF (and WinForms) wraps the Win32 MessageBox function, and doesn't throw an exception. In the old Win32 world error is reported by setting the last-error code, and if you don’t call GetLastError to check, you can just keep going without knowing something is broken.

From Reference Source

//so it just translates the return code to a MessageBoxResult



MessageBoxResult result = Win32ToMessageBoxResult (UnsafeNativeMethods.MessageBox (new HandleRef (null, owner), messageBoxText, caption, style));

I have not tested the capacity of the lpText string in the Win32 MessageBox function. But it is not surprising if the capacity limit is something like 1024, 4096 or 65536, after all, MessageBox function is designed to display short message.