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Will my .NET 1.x run on .NET 4.0?

Short answer:
Yes.

Should you?
No.

Why?
Read the long answer.

Long answer:

The .NET Framework 4 is backward-compatible with applications that were built with the .NET Framework versions 1.1, 2.0, 3.0, and 3.5. In other words, applications and components built with previous versions of the .NET Framework will work on the .NET Framework 4.

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. Similarly, using a hard-coded path to .NET Framework assemblies, performing an equality comparison with a particular version of the .NET Framework, and getting the value of a private field by using reflection are not backward-compatible practices. In addition, each version of the .NET Framework includes bug fixes and security-related changes that can affect the compatibility of some applications and components.- Source: MSDN
The .NET Framework has two main components:
  • The common language runtime (CLR)
  • The class library (BCL, etc).
The common language runtime is the foundation of the .NET Framework. 3.5 uses CLR version 2.0; and so does .NET Framework 2.0 or 3.0, while .NET framework 1.1 uses v1.1.

The compatibility goal for the .NET framework is that applications and components from previous versions should work smoothly on the latest framework; except for a set of known changes.

So, to make your .NET 1.1 application run properly and stable on the target machine, you would be better to install .NET 1.1 on it to support your application.

Alternatively, you also can try to upgrade your .NET v1.1 application to .NET 3.x.

Here's how.
  • In your IDE, select the Properties-->Application-->Target Framework
  • Change the target framework to a recent one, and rebuild.

Source: MSDN

The compatibility goal for the .NET framework is that applications and components from previous versions should work smoothly on the latest framework; except for a set of known changes.

So, to make your .NET 1.1 application run properly and stable on the target machine, you would be better to install .NET 1.1 on it to support your application.

This means, following may/may-not "properly" with each other:
  • .NET 4.0 will not run "properly" .NET 3.x or earlier applications
  • .NET 3.5 will run .NET 3.0 & .NET 2.0 applications, but not .NET 1.1
  • .NET 3.0 will run .NET 2.0 applications, but not .NET 1.1
  • .NET 2.0 will not run .NET 1.1


Alternatively, you also can try to upgrade your .NET v1.1 application to .NET 3.x.

Here's how:
  • In your IDE, select the Properties-->Application-->Target Framework
  • Change the target framework to a recent one, and rebuild.
Though, any app (web/desktop) can be migrated to the desired framework, here is a step by step indepth article. How to: Upgrade an ASP.NET Web Application to ASP.NET 4.

If you still plan to use the 1.x on 4.0, don't forget to check the .NET Framework 4 Migration Issues.

Cheers!