The library hostpolicy.dll was not found

This error message is unhelpful. The actual problem is a missing emitEntryPoint property:

  "buildOptions": {
    ...
    "emitEntryPoint": true
  },

Once this is added, the compiler will let you know about any other problems (like a missing static void Main() method). Successfully compiling the project will result in an output that dotnet run can execute.

Update for dotnet core 2.0 and beyond: the file appname.runtimeconfig.json (for both debug and release configuration) is needed in the same path as appname.dll.

It contains:

{
  "runtimeOptions": {
    "tfm": "netcoreapp2.0",
    "framework": {
      "name": "Microsoft.NETCore.App",
      "version": "2.0.0"
    }
  }
}

then dotnet.exe exec "path/to/appname.dll" [appargs] works.

If I'm not mistaken, one scenario when you can hit the issue is this: You have an integration project that references another application project (not library). In this case, dependentProject.runtimeconfig.json won't be copied to your integration project's output folder and you won't be able to run dependentProject.exe binary because it will throw The library hostpolicy.dll was not found..

There is a Github issue for this and a workaround.

Edit: Should be fixed in .NET SDK 5.0.200.