How to run an exe application as part of new application?

No. You can't do it.

You would need to recode A.exe into a DLL and use it from there.

The simple answer is NO you can not do it.

But if you remove the requirement of using A.exe as child form, there are few things you can do. Say A.exe does some actions or performs some processing, you could leave A.exe as is and spawn it from B.exe. This does not mean it is running as child form, rather a full fledged self contained exe. The advantage is you don't have to change any ( or at least very little in A.exe)

Where as if you insist on using it child form, then follow what OriginalGriff suggested.

Hi

If A.exe also a .Net exe then you can add reference the A.exe to B.exe. The A.exe is in its own namespace.

Then set the B.exe's form's isMdiContainer property to true. Now you can open A.exe's form1 as child here

private void Form1_Load(object sender, EventArgs e)
{
    Form frm = new A.Form1();// A is the namespace of A.exe and it has a form1 form.
    frm.MdiParent = this;
    frm.Show();
}

There is actually a good article on CP about this:

Hosting EXE Applications in a WinForm project[^]