Open multiple instances of MessageBox and automatically close after several seconds

There is an undocumented MessageBoxTimeout function in windows that you could use: MessageBoxTimeout in user32.dll (use via PInvoke).

Example:

public class MessageBoxWithTimeout
{
  [DllImport("user32.dll", SetLastError = true)]
  [return: MarshalAs(UnmanagedType.U4)]
  private static extern uint MessageBoxTimeout(IntPtr hwnd,
    [MarshalAs(UnmanagedType.LPTStr)]  String text,
    [MarshalAs(UnmanagedType.LPTStr)] String title,
    [MarshalAs(UnmanagedType.U4)] uint type, 
    Int16 wLanguageId, 
    Int32 milliseconds);

  public static uint Show(IntPtr hWnd, string message, string caption, uint messageBoxOptions,Int32 timeOutMilliSeconds)
  {
     return MessageBoxTimeout(hWnd, message, caption, messageBoxOptions, 0, timeOutMilliSeconds);
  }
}

In your code:

MessageBoxWithTimeout.Show( your parameters here );

However, you should think about your design. A message box by definition is blocking your dialog, so multiple messageboxes do not make sense. If you post your actual problem, maybe there is a better solution.

The following code can be used a as a starting point. It's based on the related answer I gave recently.

async void MainForm_Load(object sender, EventArgs e)
{
    Func<Func<Form>, Task<Form>> showAsync = (createForm) =>
    {
        var tcs = new TaskCompletionSource<Form>();
        var form = createForm();
        form.Tag = Task.Factory.StartNew(
            () => form.ShowDialog(), 
            CancellationToken.None, 
            TaskCreationOptions.None,
            TaskScheduler.FromCurrentSynchronizationContext());
        form.Load += (sIgnore, eIgnore) =>
            tcs.TrySetResult(form);
        return tcs.Task;
    };

    var forms = new Stack<Form>();
    for (var i = 0; i < 4; i++)
        forms.Push(await showAsync((() =>
            new Form { Text = "Hello #" + i })));

    var closeFormTasks = forms.Select((form) => (Task)form.Tag);

    var delay = Task.Delay(5000);
    var task = await Task.WhenAny(delay, Task.WhenAll(closeFormTasks));

    if (task == delay)
    {
        while (forms.Any())
        {
            var form = forms.Pop();
            form.Close();
            await (Task)form.Tag;
        }
    }

    MessageBox.Show("All closed.");
}