How to add an event into the Windows Message Pump queue?

Question

Using VB.NET/C# - .Net Framework 3.5. I am currently in a handler for a WinForms event (for example). In the handler I would like to put my own event for the windows message pump but at the end of its queue (assuming it pumps messages FIFO). I would like my handler and any additional handlers for the current event to execute and later on pick up my custom event that I inserted into the queue and call its handler. Any sample code on how I can insert an event into the windows event queue (don't care what language)?

Answer 1

If you want to add a message to the queue, then you just need to P/Invoke the PostMessage function. As the documentation says, this function:

Places (posts) a message in the message queue associated with the thread that created the specified window and returns without waiting for the thread to process the message.

Sample P/Invoke declaration:

[return: MarshalAs(UnmanagedType.Bool)]
[DllImport("user32", SetLastError = true)]
static extern bool PostMessage(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam);

• hWnd is a handle to your window, the one whose window procedure should receive the message. In WinForms, this can be retrieved using the Handle property.

• msg is the message you want to post. Of course, you'll need to figure out which one to use here. If one of the standard messages works for your purposes, then you can use that. Otherwise, just define a user-defined message. User-defined messages have a range of values >= WM_USER. So:

  const uint WM_USER = 0x0400;
  const uint UWM_MYGREATMESSAGE = WM_USER + 1;
  const uint UWM_MYOTHERMESSAGE = WM_USER + 2;
  

• Obviously, wParam and lParam contain message-specific information. If you're using one of the standard messages, their documentation will tell you what to specify here. If you're using a user-defined message, then you can pretty much pass whatever you want here, including IntPtr.Zero if you don't need any custom data.

Answer 2

I really enjoyed the engineering example @CodyGray has given. It's been a while since I was doing Win32 stuff in Delphi :).

Still I couldn't keep my fingers afk and had to share this idea I have about what Denis might need because I think it may be a little simpler.

Say you have a block of code. Let's call it A. Let's say that it is composed of 10 statements. Let's call them a0 thru a9. So our block of code looks something like this:

{
    a0;
    a1;
    ...
    a4;
    a5;
    ...
    a8;
    a9;
}

Now say you end up writing this block as the event handler for a Windows Forms control:

void button1_Click(object sender, EventArgs e) {
    a0;
    a1;
    ...
    a8;
    a9;
}

Suddenly you realise that you want to execute a0 thru a4 immediately and defer the execution of a5 thru a9 to a later time. Later meaning as soon as possible but after the message pump became idle.

All you need to do is:

void button1_Click(object sender, EventArgs e) {
    a0;
    a1;
    a2;
    a3;
    a4;

    this.BeginInvoke( () => {
        a5;
        a6;
        a7;
        a8;
        a9;
    } );
}