I created one msi for my product it's working well but I need to install .net 4.0 as a prerequisite. I came across burn and bootstrapper but have a couple of questions:
Should I have a separate project for the main msi and the bootstrapper?
My customer needs an msi (not an exe), so can the bootstrapper be an msi?
MSI (Windows Installer) does not support "nested" installs, where one .msi package runs for a while, then pauses, runs a separate installer package, then picks up from where it paused. MSI uses some Windows resources that are essentially system-global, so the "inner" install tromps on the "outer" one.
The way to do multiple installs as a single user experience is to run each install in sequence, one at a time. That is the purpose of the bootstrapper, also known as a chainer: to run the chain of installs, link by link. The bootstrapper is not itself an install package; it is a tool for running one or more install packages.
That is why the bootstrapper cannot be an .msi: it is not itself an installer package. A bootstrapper can be as simple as a batch file or script, or you can use an industrial-strength tool such as a WiX burn bundle, Flexera AdminStudio, etc.
Be aware that there are some installer packages that ship as .exe -- for examples, the .NET Framework or SQL Server installers. These are essentially self-extracting archives that contain one or more .msi packages, dump a temporary copy of that payload then run the Windows Installer service on it. In some sense they are "bootstrappers" plus package, all rolled into one. A bootstrapper can run packages (and other bootstrappers), but not vice-versa.
You might think that you could have your outer package run a custom action that launches the inner package. We've all tried it... and found out the hard way that it doesn't work generally, even when you appear to get away with it on some specific target system.
You'll have to persuade your customer that Windows Installer does not, cannot, work the way he thinks it does. Sadly, sometimes the only way to do this is to replace the customer.
