Sdk: Remove requirement of setting RuntimeIdentifier in project file for .NET Framework Exe projects

[dsplaisted]

Right now we require that .NET Framework Exe projects set a RuntimeIdentifier property. This is because ASP.NET Core projects depend on the native library in the libuv package. In .NET Core we have a way of deploying both x86 and x64 versions and loading the correct one at runtime, but we don't have this for .NET Framework. That means we effectively don't support AnyCPU for projects that depend on Libuv (or other packages authored in the same way).

At a minimum, we should change it so that the RuntimeIdentifier is only required when there is a package in the dependency graph that has architecture-specific assets.

We should also try to get rid of the requirement of settingRuntimeIdentifier in web projects. This could include setting it to a default in the Web SDK targets when targeting .NET Framework, or updating the Libuv and Kestrel packages to deploy both architectures and load the right native DLL at runtime.

[davidfowl]

or updating the Libuv and Kestrel packages to deploy both architectures and load the right native DLL at runtime.

Kestrel does the right thing already and libuv already has the right bits inside of it. So changing the package would be a mistake. What you really want is to do what we do in .NET Core and have a runtime library that can look for things by RID. That library could be used by libraries that have native dependencies to properly load the right ones. This way we don't need to update each package with native dependencies to have custom msbuild tasks.

[dsplaisted]

@davidfowl

You're right, another option would be to make the same magic that works for .NET Core work for .NET Framework. My understanding was that this would require changes to the .NET Framework itself, so there would be a higher barrier to doing it and it wouldn't apply to versions of the Framework that already shipped.

If you wanted to update the packages to work on .NET Framework as AnyCPU, you don't need any custom MSBuild targets or tasks. You'd just need the packages to deploy the native libraries in different subfolders, which I think NuGet already does for .NET Core, and which in any case you can do by declaring them as content files. Then in the Libuv constructor before you use any of the DllImports from Libuv, you do a LoadLibrary on the right version of the native library, like this:

``` c#

if NET451

        string libuvPath;
        if (Marshal.SizeOf<IntPtr>() == 4)
        {
            libuvPath = "x86\\libuv.dll";
        }
        else
        {
            libuvPath = "x64\\libuv.dll";
        }
        NativeMethods.LoadLibrary(libuvPath);

endif

```

Then the DllImports for Libuv methods will resolve to the native library you already loaded.

[davidfowl]

We already have something like this for SQLLite https://github.com/aspnet/Microsoft.Data.Sqlite/blob/f04228cac9c29e2b8e0362749be47f9f6350df86/src/Microsoft.Data.Sqlite/Interop/NativeMethodsImpl.cs#L24. Maybe we should move this into a netstandard library more people should be using.

/cc @bricelam