[cannibalherd]

if my file need .net 3.5 to run and machine has.net 4.5 + installed will my file open and run successfully?

[| x |]

Yes.
You can test this on virtual machine.

[Geofry Handsome]

if my file need .net 3.5 to run and machine has.net 4.5 + installed will my file open and run successfully?

no, Windows will ask you to install the missing components.
Usually 3.5 is included in Win 10.

[jmark7]

no, Windows will ask you to install the missing components.
Usually 3.5 is included in Win 10.

small correcting.
Usually 3.5 is not included in Win 10 and is required to be installed.

[Vuln]

It is always included as Windows Feature, but usually disabled by default (depending on how Win 10 was installed).
Enabling the feature will download it through Windows Update.
If it is disabled and you execute a .NET 2.x/3.x app, you will be asked to enable the feature. Your app won't just crash.

small correcting.
Usually 3.5 is not included in Win 10 and is required to be installed.

This is correct.

[benio2]

Use .net framework 4.0 for development right now if you want your software to be able to run on any Win10+ pc.