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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?

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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 (e.g. 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.

Steve Mitcham
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  • You mean if CLR 4.0 and CLR 4.5 were installed on the same machine, this will cause some incompatibilities? –  Feb 19 '15 at 02:39
  • 4.5 installed on a machine changes the CLR. See http://stackoverflow.com/questions/8810391/will-net-4-5-introduce-a-new-version-of-the-clr – Steve Mitcham Feb 19 '15 at 04:09
  • Is this a bug? I mean if someone create an application using .net 4.0, he should except the application to run without any problem if .net 4.0 is installed, regardless of any other stuff installed! –  Feb 19 '15 at 09:12
  • I wouldn't say so. .NET 4.0 is almost 5 years old and is approaching it's end of life, the issues that are incompatible are very edge case and most developers won't encounter them. Microsoft has one of the best backwards compatible systems in the industry. Most things written 5 years ago for other systems won't run without modification either, but most .NET applications will still run without problem – Steve Mitcham Feb 19 '15 at 12:49
  • Sounds like you don't answer the "side by side" part. – Lex Li Feb 20 '15 at 05:51
  • Updated to include details on side by side CLR functions. – Steve Mitcham Feb 20 '15 at 06:27
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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...

  • Just having the 4.5 runtime on the machine will cause the problems. The 4.5 runtime changes the CLR on the machine and you no longer have the ability to run 4.0 on its original framework. I actually just completed a project where we did actually run into these incompatibilities and it was a new 4.5 based component (forcing the update) that brought out the problems. – Steve Mitcham Feb 19 '15 at 04:11
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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.

Stephen Kennedy
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Lex Li
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