Sample Image - NotifyBalloon.png

Introduction

This article shows one approach to displaying a balloon tool tip for a notify icon created using the FCL's NotifyIcon class. This relatively new feature of notification icons is not supported by the NotifyIcon class and adding this feature to my own code was not immediately obvious without some creative coding. This is why I am presenting it here. The other reason is that I am hoping somebody can tell me that I'm doing it all wrong and that there is a much more 'correct' solution.

This article also serves as an example of the use of the platform invoke facility. It shows how to create a data structure required by a Win32 API function and how to pass that structure to the API function.

The way I have accomplished the task involves the following two steps:

Using the Win32 API get the handle to the hidden FCL created notify message window.
2. Use the Win32 API to send the 'balloon tip' message to the notify icon.

A Little Background

Notification icons communicate with their parent applications by sending windows messages to a window supplied when the notify icon is created. The FCL NotifyIcon class does not supply any information about this window (one of the disadvantages of working at a higher level of abstraction). The FCL creates a hidden window for a NotifyIcon and converts the windows messages sent to it into .NET consumable events.

In order to use the Win32 API to communicate with the notification icon you must obtain the Win32 window handle of the window receiving the messages, and also the numeric ID of the icon.

The Solution

Using Spy++ I determined the class name of the hidden window created by the FCL. I then used the Win32 API to find the handle of that window (looking only at the windows owned by my thread). The ID was determined by trial and error! I found that if you have one notify icon created by your app the ID will be 1 (not 0). Once I have the window handle I then call Shell_NotifyIcon() to send it the 'balloon' message. The only trick to this part is defining the data structure required to be sent to this API function.

The Caveats

This solution is a basically a hack for a few reasons. Here are the assumptions I made that may not always be true.

I assumed that calling the Win32 API function GetCurrentThreadId() would return me the correct thread ID.
I assume the name of the window class created by the FCL is fixed. Of course this may not be true in the future.
I assume that the ID of the icon is 1 for the first created icon.
Part of the reason I am posting this code is to maybe get some feedback on how I can reduce the number of assumptions that are made. So let me know if you come up with anything.

Here is the code for the NotifyIcon class that shows the balloon.

public class NotifyIcon
{
    [StructLayout(LayoutKind.Sequential)]
        public struct NotifyIconData
    {
        public System.UInt32 cbSize; // DWORD
        public System.IntPtr hWnd; // HWND
        public System.UInt32 uID; // UINT
        public NotifyFlags uFlags; // UINT
        public System.UInt32 uCallbackMessage; // UINT
        public System.IntPtr hIcon; // HICON
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst=128)]
        public System.String szTip; // char[128]
        public System.UInt32 dwState; // DWORD
        public System.UInt32 dwStateMask; // DWORD
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst=256)]
        public System.String szInfo; // char[256]
        public System.UInt32 uTimeoutOrVersion; // UINT
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst=64)]
        public System.String szInfoTitle; // char[64]
        public System.UInt32 dwInfoFlags; // DWORD
        //GUID guidItem; > IE 6
    }

    public enum NotifyCommand {Add = 0, Modify = 1, Delete = 2, SetFocus = 3, 
                               SetVersion = 4}
    public enum NotifyFlags {Message = 1, Icon = 2, Tip = 4, State = 8, Info = 16, 
                             Guid = 32}

    [DllImport("shell32.Dll")]
    public static extern System.Int32 Shell_NotifyIcon(NotifyCommand cmd, 
                                                       ref NotifyIconData data);

    [DllImport("Kernel32.Dll")]
    public static extern System.UInt32 GetCurrentThreadId();

    public delegate System.Int32 EnumThreadWndProc(System.IntPtr hWnd, 
                                                   System.UInt32 lParam);

    [DllImport("user32.Dll")]
    public static extern System.Int32 EnumThreadWindows(System.UInt32 threadId, 
                                        EnumThreadWndProc callback, 
                                        System.UInt32 param);

    [DllImport("user32.Dll")]
    public static extern System.Int32 GetClassName(System.IntPtr hWnd, 
                                                  System.Text.StringBuilder className,
                                                  System.Int32 maxCount);

    private System.IntPtr m_notifyWindow;
    private bool m_foundNotifyWindow;

    // Win32 Callback Function
    private System.Int32 FindNotifyWindowCallback(System.IntPtr hWnd, 
                                                  System.UInt32 lParam)
    {
        System.Text.StringBuilder buffer = new System.Text.StringBuilder(256);
        GetClassName(hWnd, buffer, buffer.Capacity);

		// but what if this changes?  - anybody got a better idea?
        if(buffer.ToString() == "WindowsForms10.Window.0.app1") 
        {
            m_notifyWindow = hWnd;
            m_foundNotifyWindow = true;
            return 0; // stop searching
        }
        return 1;
    }

    public void ShowBalloon(uint iconId, string title, string text, uint timeout)
    {
        // find notify window
        uint threadId = GetCurrentThreadId();
        EnumThreadWndProc cb = new EnumThreadWndProc(FindNotifyWindowCallback);
        m_foundNotifyWindow = false;
        EnumThreadWindows(threadId, cb, 0);
        if(m_foundNotifyWindow)
        {
            // show the balloon
            NotifyIconData data = new NotifyIconData();
            data.cbSize = (System.UInt32)
                          System.Runtime.InteropServices.Marshal.SizeOf(
                                                                typeof(NotifyIconData));
            data.hWnd = m_notifyWindow;
            data.uID = iconId;
            data.uFlags = NotifyFlags.Info;
            data.uTimeoutOrVersion = 15000;
            data.szInfo = text;
            data.szInfoTitle = title;
            Shell_NotifyIcon(NotifyCommand.Modify, ref data);
        }
    }
}

Here is the code that shows the usage of the class.

private void OnShowBalloon(object sender, System.EventArgs e)
{
    NotifyBalloonDemo.NotifyIcon notifyIcon = new NotifyBalloonDemo.NotifyIcon();
    notifyIcon.ShowBalloon(1, "My Title", "My Text", 15000);
}

So come on people, show me how to do this correctly.

Displaying a Notify Icon's Balloon Tool Tip