What is the simplest way for VBA code (in Excel) to send some sort
of signal to a .NET process and tell it to do something? The VBA
code knows the task ID of the .NET process, because it started that
process (a Windows Forms app) using Shell(). It isn't necessary
to pass any information either way (although that wouldn't hurt);
all that is needed is a way to poke at the .NET process so that it
wakes up and looks for an instruction file that has just been
written by Excel.

Right now the .NET process is polling the disk, looking for new
instruction files. This actually works fine, but since there can
be long intervals between new instruction files, it would be better
if there were some way for Excel to notify the .NET process directly
when a new file is written. I'm sure there's a way to do this
using sockets and threads, but I'd like to add as little complexity
as possible, so if anyone knows a simpler way I hope you will let
me know about it.

John Brock wrote:
What is the simplest way for VBA code (in Excel) to send some sort
of signal to a .NET process and tell it to do something? The VBA
code knows the task ID of the .NET process, because it started that
process (a Windows Forms app) using Shell(). It isn't necessary
to pass any information either way (although that wouldn't hurt);
all that is needed is a way to poke at the .NET process so that it
wakes up and looks for an instruction file that has just been
written by Excel.

Right now the .NET process is polling the disk, looking for new
instruction files. This actually works fine, but since there can
be long intervals between new instruction files, it would be better
if there were some way for Excel to notify the .NET process directly
when a new file is written. I'm sure there's a way to do this
using sockets and threads, but I'd like to add as little complexity
as possible, so if anyone knows a simpler way I hope you will let
me know about it.

How is the .Net process polling, currently? If you are just testing for
a file existence, then I think you should try FileSystemWatcher on the
.net side.

I'm sure there is a way for Excel to signal the external app, but the
process sound more or less independent.

I can get around this of course by writing the file under a different
name and then renaming it. I'm still hoping though to find some
easy way to use the task ID to directly signal the .NET process
from Excel, or even pass it a string. But if I can't then
FileSystemWatcher it is.

If your .NET process has a window then you can use FindWindow to find
the window handle then SendMessage to send it a message. These are
Win32 API functions, but you should be able to call them from VBA with
suitable declarations.

http://msdn.microsoft.com/en-us/library/ms644950(VS.85).aspx

http://msdn.microsoft.com/en-us/library/ms633499(VS.85).aspx

http://msdn.microsoft.com/en-us/library/ms644931(VS.85).aspx

I can get around this of course by writing the file under a different
name and then renaming it. I'm still hoping though to find some
easy way to use the task ID to directly signal the .NET process
from Excel, or even pass it a string. But if I can't then
FileSystemWatcher it is.

Thanks. Looks interesting, but possibly overcomplicated for what
I'm doing.

Do you know what the .NET process needs to do to receive messages?
The process does have a window, but I'm not finding much information
about using SendWindow with .NET.