What does it really mean to target a framework, and how do I maximize compatibility?

Question

Greetings all,

This has confused me ever since I first started coding in C#. My goal is to create an assembly that will run on the most recent .NET framework the user has, whatever that may be. I don't want to require .NET 4 to be installed, but I want to use it if the user has it, and more importantly, I want it to still work if the user has only .NET 4 and nothing below. I'm beginning to suspect this is not even possible.

I don't really understand what I'm selecting when I change the "Target Framework" in Visual Studio. Does that mean "will be compatible with this version and up"? Or "will be compatible with only this version"? It seems like it's the latter so far; my tests in virtual machines show .NET 4 targeted assemblies failing without .NET 4, and .NET 3.5 targeted assemblies failing without .NET 3.5. Is there no way to set this so I can achieve maximum compatibility?

UPDATE: To clarify, I have a program that targets .NET 2. The posters here seem to indicate that it should load with .NET 4. But in an environment with only .NET 4 and nothing before it, it fails to load.

UPDATE2: Okay, figured it out, but it's far more complex than the posters here seem to think. I'm opening a different question to discuss the intricacies, but the short version is, you need an app.config with <supportedRuntime version="v4.0" /> in it if you want to run non-.NET 4 assemblies on .NET 4. They won't load without it.

Answer 1

The frameworks are designed to be backwards-compatible; if you have a program written in .NET 2.0, you can run it in the 4.0 runtime, because none of the frameworks ever remove functionality that a prior version had (which is why we still have the non-generic collections like ArrayList, even though they're deprecated in favor of generic collections). However, the reverse is not necessarily true; a 4.0 app is not guaranteed to run in 2.0, because it MAY take advantage of new features of the new runtime that are not available in prior versions. In any case, if you want your app to attempt to run on runtime versions it does not specifically target, you must specify that in the app.config using SupportedRuntime elements.

To answer your specific question, you can do what you want with the following basic procedure:

• Develop your app to target the EARLIEST framework version that you want to support. This will disable access to newer features of newer runtimes (like Linq in 3.5, and dynamic typing in 4.0) ensuring your app will not require any feature that cannot be provided by any of the supported runtimes.

• Specify the frameworks that are acceptable to your application by using the SupportedRuntime element in your app.config file. This will tell the native code that initializes the runtime in which your app will run that if it can't find the targeted version, any of the others are acceptable. I believe the behavior is to look for the targeted framework first, and if not available it should use the newest supported runtime.

Answer 2

.NET is backward-compatible, this means if you select .NET Framework 2.0 as target framework, it will run on installed version 2.0, 3.0, 3.5 and 4.0. But if you select e.g version 4.0 as target framework, it will only run if you have version 4.0 installed.