Does installing .net 4.0 break any .net 1.1 apps?

I had a .net 1.1 app fail after installing .net 4.0, and I don't know if 4.0 is the cause. At the very end of 1.1's machine.config, the app was reporting an error unrecognized system.data section. (I ran it in vs2008 debugger to find this.) I commented out the section, it was empty anyway, and the app started to run again. The app has been running cor开发者_Go百科rectly for years, on xp, vista and win7-64.

This is the section in machine.config causing the problem:

<system.data>
  <DbProviderFactories></DbProviderFactories>
</system.data>

Installing any version of .NET other than 1.1 can certainly break .NET 1.1 applications. This is because Microsoft fixed several bugs in .NET 1.1 in a breaking manner. I believe that installing .NET 4.0 will also install one or more of those service packs for .NET 1.1, so you could see one of the breaks.

Just in case things may be GAC related, see:

.NET 4.0 has a new GAC, why?

Also, is it possible that the unrecognized system.data section in the machine.config was placed there manually? And didn't make trouble until the CLR 4.0 was installed?

Microsoft has stated that all versions of .Net can be installed in parallel without interfering with each other (at least thats what they said at MSDN Live). Each version is installed in a separate directory.

If you are talking about ASP.Net then you must consider what version your application is configured for in IIS.