Why does a MessageBox called via WinAPI give different errors on different computers?

Question

There is a nessesity to use WinAPI for my project. While I was studying WinAPI, I read about the method of getting errors Marshal.GetLastWin32Error().
I coped the example from the website learn.microsoft.com and added console project to .Net Core 5.0.

using System;
using System.Runtime.InteropServices;

internal class Win32
{
    // Use DllImportAttribute to inport the Win32 MessageBox
    // function.  Set the SetLastError flag to true to allow
    // the function to set the Win32 error.
    [DllImportAttribute("user32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
    public static extern int MessageBox(IntPtr hwnd, String text, String caption, uint type);
}

class Program
{

    static void Run()
    {
        // Call the MessageBox with normal parameters.

        Console.WriteLine("Calling Win32 MessageBox without error...");

        Win32.MessageBox(new IntPtr(0), "Press OK...", "Press OK Dialog", 0);

        // Get the last error and display it.
        int error = Marshal.GetLastWin32Error();

        Console.WriteLine("The last Win32 Error was: " + error);

        // Call the MessageBox with an invalid window handle to
        // produce a Win32 error.

        Console.WriteLine("Calling Win32 MessageBox with error...");

        Win32.MessageBox(new IntPtr(123132), "Press OK...", "Press OK Dialog", 0);

        // Get the last error and display it.

        error = Marshal.GetLastWin32Error();

        Console.WriteLine("The last Win32 Error was: " + error);
    }

    static void Main(string[] args)
    {
        Run();
    }
}
// This code example displays the following to the console:
//
// Calling Win32 MessageBox without error...
// The last Win32 Error was: 0
// Calling Win32 MessageBox with error...
// The last Win32 Error was: 1400

The first computer that I tried worked well.
After that I bought the new one and tried my project there.
The code began to work with the mistake: the error 1400 always came back after the calling messagebox, but the window showed and returned 1 (IDOK).

The first computer:

System manufacturer: MSI GF75 Thin 10UEK;
Windows: Windows10 Home x64;
Update: 20H2 (KB504237);
BIOS: E17FSIMS.102
Processor: Intel i7-10750H;

The second computer:

System manufacturer: HP ProBook 440 G8;
Windows: Windows10 Pro x64;
Update: 20H2 (KB504237)
BIOS: T70 Ver. 01.03.01
Processor: Intel i7-1165G7

If I call GetLastWin32Error before the calling of messagebox, I'll get a error 0.
Has somebody got an experience like that? The problem is not very serious because of checking what window returns. But I'd like to know what has happened.

What happens if you decorate your `Main()` method with `[STAThread]` ? — Matthew Watson, Jul 18 '21 at 09:37
Are you sure you were running the exact same code? Seems impossible to reproduce. Sounds very strange. — Simon Mourier, Jul 18 '21 at 10:21

score 2 · Accepted Answer · answered Jul 18 '21 at 10:52

2

Marshal.GetLastWin32Error calls GetLastError.

Calling GetLastError only makes sense when the function in question returned a failed code. For MessageBox, that is 0.

You call GetLastWin32Error regardless, so you cannot trust the value it returns.

To quote the documentation:

Most functions that set the thread's last-error code set it when they fail. However, some functions also set the last-error code when they succeed. If the function is not documented to set the last-error code, the value returned by this function is simply the most recent last-error code to have been set; some functions set the last-error code to 0 on success and others do not.

MessageBox is not documented to set last error to 0 on success.

answered Jul 18 '21 at 10:52

GSerg

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The Console.WriteLine calls before the MessageBox call does sets last error to 0. – Simon Mourier Jul 18 '21 at 11:25
@SimonMourier Then, in the process of calling `MessageBox`, the framework invokes some other API, e.g. `GetProcAddress`, which may or may not set the last error code. Then the `MessageBox` itself is invoked, which itself may call other API functions that may set the last error code - which could be because they genuinely failed and `MessageBox` handled the error, or because they pre-set the last error to an error code and then don't bother to clear it on success. And finally `MessageBox` returns success without bothering to clear the last error to zero. – GSerg Jul 18 '21 at 11:29
Do some testing and you'll see – Simon Mourier Jul 18 '21 at 14:21

Why does a MessageBox called via WinAPI give different errors on different computers?

1 Answers1