I've created a new ASP.NET 5 project and I faced with something that I didn't understand well about how the new ASP.NET works.
In my references there are DNX 4.5.1 and DNX Core 5.0. When I install a new package from Nu-Get, some packages install successfully in both and some tell me they aren't supported by DNX Core 5.0.
Can I use many versions of DNX for supporting my packages or I must bound the things with DNX Core 5.0 to enjoy the advantages of ASP.NET 5, like cross-platform and others?
ASP.NET 5 projects are multi-targeted, which means you can compile for multiple platforms simultaneously. In your case .NET 4.5.1 targets ASP.NET 5 on the full .NET runtime (4.5.1) and .NET Core (5.0). When you compile your project, ASP.NET 5 actually creates output for both of these targets so - assuming you get the code to compile for both platforms - the application can run on either of them. It's possible to run additional targets to your projects.
When you actually run your application though, you have to pick a specific version of the .NET runtime you want to run under. In Visual Studio there's a drop down in the project settings where you can select that runtime that actually executes. When you use command line publish you also specify which platform to publish to with a parameter (if omitted it uses the active DNX runtime I believe).
As to the differences - .NET Core is a trimmed down version of the full .NET runtime, that is cross-platform and can run on Windows and other platforms like Linux and OSX. Because it has to run on other platforms a lot of the platform specific Windows features are not supported in this runtime, and as the platform is still evolving some APIs are simply not implemented yet in .NET Core. This is the reason you are likely to see many compile errors when you target .NET Core with existing code/libraries. In order to run on .NET core existing .NET code typically needs at least some minor adjustments to account for the smaller API footprint and potentially serious redesign to work around missing functionality.
If you need to use existing code and depend on assemblies/NuGet packages that are based on previous versions of .NET you have to stick with the full .NET version. You can still take advantage of the new ASP.NET 5 features and eco-system, as well as getting the benefit of the full .NET runtime and full API surface you are used to from previous .NET versions. The downside is that the runtime is Windows only and has a bigger resource footprint.
.NET Core is a new runtime and you should treat it as such. Migrating to this platform is likely not a trivial task. The benefits of this platform is a much leaner footprint and that it can run across multiple OS platforms.
It's likely that it'll take some time for the full potential of .NET Core to come to fruition as Microsoft expands the feature set and cross-platform compatibility but we won't really know what this will look like since it is still in Beta (well RC but it's really a beta with major changes still coming in RC2).
