Make apphost generation work correctly during incremental build

[sbomer]

The apphost generation currently will not overwrite an existing apphost. The result is that changing properties that influence the apphost (setting the target to WinExe/Exe, or adding resources to the managed dll that get copied over to the apphost via #2545) won't result in a new apphost being created for incremental builds, which seems incorrect.

See the discussion in #2467 and the related issue #2473.

[nguerrera]

I wonder if we can actually solve this on top of #2545 with just use Inputs="$(IntermediateAssembly)" Outputs="$(ApphostDestinationPath)" on _CreateAppHost + removing the File.Exists check. If the output type changes, the managed assembly will also change. Are there other properties that can impact the apphost?

cc @peterhuene

[rainersigwald]

If there are additional properties or items that matter beyond the assembly, you can use a pattern like we do for C#: collect all relevant properties into an item and write its hash to a file with WriteOnlyWhenDifferent="True".

See https://github.com/Microsoft/msbuild/blob/90506c8f6c5e4b82a09bafa6ae62a41e9e7367bb/src/Tasks/Microsoft.Common.CurrentVersion.targets#L3404-L3431 (right now that only does items but you could add properties too).

[wli3]

https://github.com/dotnet/sdk/pull/2413/files#diff-fc9ca72fce3be7d1f0119f6666a307bdR148

I think genshim's incremental build is correct. Normal apphost should be similar to that

[nguerrera]

It should be similar to that if necessary, but I think the standard apphost might be a pure function of the managed assembly.