SendMessage works, PostMessage doesn't work.

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Guest

When I use the SendMessage API I can sucessfully send and receive a user
defined message.
When I use the PostMessage API I can NOT sucessfully send and receive the
same user defined message.

I've got a C# class library project with two classes:

Class 1 is derives from : System.Windows.Forms.Form and overrides the base
WndProc method for the purpose of receiving and handeling user defined
messages:

protected override void WndProc(ref System.Windows.Forms.Message m)
{
switch (m.Msg)
{
case WM_MESSAGE1:
// do something here.
break;
case WM_MESSAGE2:
// do something here.
break;
default:
// do something here.
break;
}

Class 2 is created by Class 1 and runs as a background thread. At the
appropriate time Class 2 will either use:
SendMessage - when Class2 needs to wait for Class 1 to process the message
and possible return a response.
PostMessage - when Class2 does not need to wait for Class1 to processes
the message.

Class 2 uses this code to Send / Post the user defined messages:

[System.Runtime.InteropServices.DllImport("user32.dll")]
private static extern Int32 SendMessage(IntPtr hWnd, Int32 msg, Int32
wParam, IntPtr lParam);

[System.Runtime.InteropServices.DllImport("user32.dll")]
private static extern Int32 PostMessage(IntPtr hWnd, Int32 msg, Int32
wParam, IntPtr lParam);

private const Int32 WM_USER= (0x400);
private const Int32 WM_MESSAGE1 = (WM_USER + 101);
private const Int32 WM_MESSAGE2 = (WM_USER + 102);

// THIS CODE SUCCEEDS.
IntPtr lParam = System.IntPtr.Zero;
int res = SendMessage(Class1.Handle, WM_MESSAGE1, 0, lParam);

// THIS CODE FAILS.
IntPtr lParam = System.IntPtr.Zero;
PostMessage(Class1.Handle, WM_MESSAGE1, 0, lParam);

I'm logging the messages (m.Msg) that are arriving in Class1 - WndProc
function to a file for review. The log file shows that both the SendMessage
and PostMessage API calls cause the Class1 - WndProc method to fire, but in
the case of PostMessage the message that I'm looking for WM_MESSAGE1 never
arrives.

The log file also shows both the SendMessage and PostMessage cause exactly
the same additional messages to appear in the Class1 - WndProc method that I
did not explicitly issue. There are about 23 or so additional messages that
appear in the log prior to WM_MESSAGE1. But in the case of PostMessage
WM_MESSAGE1 never arrives. Just the other non-WM_MESSAGE1 messages appear in
the log.

Any idea why this might be?

Thanks in advance.
 
Class 2 is created by Class 1 and runs as a background thread. At the
appropriate time Class 2 will either use:
SendMessage - when Class2 needs to wait for Class 1 to process the message
and possible return a response.
PostMessage - when Class2 does not need to wait for Class1 to processes
the message.
Click to expand...


Any reason you're not using the Invoke and BeginInvoke methods of the
form instead?


Mattias
 
Thanks for the suggestion.

I'm really in need of a solution to the issue of the PostMessage command not
working as it should.

Does anyone have any idea why SendMessage would work, but PostMessage doesn't?

Thanks again.
 
I'm really in need of a solution to the issue of the PostMessage command not
working as it should.

Does anyone have any idea why SendMessage would work, but PostMessage doesn't?
Click to expand...

Can you post a complete sample that will let us reproduce the behavior
you're seeing?


Mattias
 
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