Open MDI child in existing application

[John]

Hi,

I am trying to get a file with extension .xxx to open
as an MDI child inside an application (ABC.exe), which I
created. I made a file association so that it will open
with ABC.exe, but instead of opening it as an MDI child of
an existing ABC.exe, it opens it in a whole new ABC.exe.

An example of this would be similiar to when you click
on a header file, Visual Studio(VS) will open that file
inside an already existing VS instead of opening a whole
new VS. I want my application to have that same effect.

There was a similiar post by Jason Gleason which
mentioned using Running Object Table (ROT), but it seems
like ROT uses COM, but my app doesn't use COM. I want my
app to call a function so that it would open the file in a
certain way. Is there any way to do this? An example on
how this can be done would be really helpful and any help
would be appreciated. Thanks in advance.

John