是否可以保证同时安装两个（或更多）版本的.NET Framework？

Question

Based on many articles I have read, I came to the conclusion that it is not guaranteed for a .NET Framework to be 100% backward compatible with the previous versions. So my question is: is it guaranteed for two (or more) versions of the .NET Framework to be installed side by side without any problems (is this documented somewhere by Microsoft)?

For example, can .NET 1.1 be installed side by side with .NET 4.5, or will I have to re-compile my project to work on .NET 4.5?

Answer 1

There's a difference between the Framework version and the runtime version. You can have different runtimes operate side-by-side without interfering if your application is properly marked with the expected runtime in its configuration file.

Programs written for a particular runtime (eg 1.1, 2.0, 4.0) can run independently against their given framework with no side effects from the other frameworks. That being said, you must modify your config file to ensure that a .NET 2.0 application runs with the .NET 2.0 CLR when the 4.0 CLR is installed on the machine

The CLR's are affected by the version of the Framework installed on the machine. For example, 4.0 and 4.5 both run with the 4.0 CLR and having 4.5 installed introduces some incompatibilities. That being said, these issues are usually very specific and most applications will not be affected by them.

Here's Microsoft's page on compatibility between Framework versions.

Answer 2

Your question seems to be in two parts:

1. Are .NET Frameworks backward compatible?
2. Can you install more than one .NET Framework at the same time on the same computer?

Yes you can install multiple versions of the ".NET Framework" at the same time without problems. Whether or not an app runs as expected is a different story as explained in the MSDN doco.

MSDN:

You can load multiple versions of the .NET Framework on a single computer at the same time . This means that you can install the .NET Framework without having uninstall previous versions Tell me more

...and

In general, you should not uninstall any versions of the .NET Framework that are installed on your computer . There are two reasons for this: If an application that you use depends on a specific version of the .NET Framework, that application may break if that version is removed. Some versions of the .NET Framework are in-place updates to earlier versions. For example, the .NET Framework 3.5 is an in-place update to version 2.0, and the .NET Framework 4.5.2 is an in-place update to versions 4, 4.5, and 4.5.1. Golly, tell me more

Backward Compatibility

Though newer versions of the framework are generally backward compatible (with respect to contracts) there may be unforseen behavioural changes visible at runtime.

As MSDN states:

The .NET Framework 4.5 and its point releases are backward-compatible with apps that were built with earlier versions of the .NET Framework. In other words, apps and components built with previous versions will work without modification on the .NET Framework 4.5

...however:

In practice, this compatibility can be broken by seemingly inconsequential changes in the .NET Framework and changes in programming techniques. For example, performance improvements in the .NET Framework 4 can expose a race condition that did not occur on earlier versions. More

So an app without appropriate config may be upscaled to use later versions of libraries leading to undesirable results. Sadly nothing can be done for a 4.0 app if .NET 4.5 is installed because the actual CLR was changed. More...

As Rick Strahl writes:

Note that this in-place replacement is very different from the side by side installs of .NET 2.0 and 3.0/3.5 which all ran on the 2.0 version of the CLR. The two 3.x versions were basically library enhancements on top of the core .NET 2.0 runtime. Both versions ran under the .NET 2.0 runtime which wasn't changed (other than for security patches and bug fixes) for the whole 3.x cycle. The 4.5 update instead completely replaces the .NET 4.0 runtime and leaves the actual version number set at v4.0.30319. More...

Answer 3

So far, we only see the following frameworks can be run side by side,

• .NET 1.x (already obsolete, so don't want to comment more)
• .NET 2.0/3.0/3.5 as a whole
• .NET 4.0/4.5/4.5.1/4.6 as a whole
• .NET Core

And you can see there is no guarantee from Microsoft whether a new version can be installed side by side or be an in-place upgrade of the previous version, and it really depends on what feature sets the new version brings.

We might observe some general rules (such as the frameworks with the same CLR version should be in-place upgrade), but that's still not something guaranteed by Microsoft, and can be broken by a future framework release.