Are .NET versions backwards compatible?

Solution 1:

Backwards compatible is a bad term to use. .NET 1.0, 1.1, and 2.0 are their own frameworks that have no compatibility between each other. .NET 3.0 and 3.5 are super sets of the 2.0 framework, using the .NET 2.0 base framework, with additional dll's to provide additional features (3.0 included things like WCF and WWF, 3.5 had things like LINQ). In terms of application requirements:

• 1.0 apps requires .NET 1.0 to be installed.
• 1.1 apps requires .NET 1.1 to be installed.
• 2.0 apps requires .NET 2.0, 3.0, or 3.5 to be installed.
• 3.0 apps requires .NET 3.0 or 3.5 to be installed.
• 3.5 apps requires .NET 3.5 to be installed.

I'm fairly certain that .NET 4.0 follows the same model as 2.0 - 3.5 (2.0 base with additional feature dlls). So installing .NET 3.5 will cover you for .NET 2.0 - 3.5 applications. You'll need to install .NET 1.1 if you run any 1.1 apps (same for 1.0 apps).

Solution 2:

.NET is backwards compatible to an extent, but you have to compare between versions of the framework. What they do state is that it is side-by-side compatible, which is the problem you're faced with. There is definitely a reason to have other versions installed. An app can be written to target a specific version and if this version is not present on a machine then the app will fail.