Oren says -

For some frameworks, like .NET 4.5, that’s all you need to do. However, targeting .NET Standard and .NET 4.x is far from “the world.” We can do better!

He goes on to explain how one could add a LanguageTargets property to your project file that will enable you to use platform-specific toolsets, like Windows Xaml or Android Resources, in our projects. At first I thought this was necessary if I wanted my shared project to be consumable by an Android project.

Since shared projects in our stack do not contain any platform-specific code, adding the LanguageTargets property was unnecessary. <TargetFrameworks>netstandard1.4,net461></TargetFrameworks> was all we needed in most shared project csprojs.

We did have one shared project which uses some Nuget packages that do not yet explicitly list their support for netstandard1.4. In this project, we had to use
<TargetFrameworks>net461;netstandard1.4</TargetFrameworks>
<PackageTargetFallback>xamarinios10;</PackageTargetFallback>
For some reason, we do not have to specify a PackageTargetFallback for Android here, even though this shared project is used by both Xamarin Android and Xamarin Forms iOS projects.

In the solution containing some shared projects and our Xamarin Android/Xamarin Forms clients, we do have one misleading error relating to project targeting.

The project 'Mobile.Core' cannot be referenced. The referenced project is targeted to a different framework family (.NETFramework) Source project: iOSClient

Mobile.Core, which iOSClient claims it cannot reference, has this in its csproj:

<TargetFrameworks>net461;netstandard1.4</TargetFrameworks>
<PackageTargetFallback>xamarinios10;</PackageTargetFallback>

So, it should be able to be referenced. When we reverse the order so that <TargetFrameworks> lists netstandard1.4 first, we got the opposite error: the Android project and other projects targeting net461 claim they cannot reference Mobile.Core because it is targeting a different framework family. The errors are misleading as those projects continue to run functionality defined in Mobile.Core without a problem.

While re-targeting our projects to support .NET Standard 1.4 in addition to .NET Framework 4.6.1, another topic that confused me was the exact syntax of the various TargetFrameworks and PackageTargetFallbacks I could use. Is it Xamarin.iOS10, like some blog posts used, or xamarinios10? Is net461 valid, or should I just use net46 like I've seen in blog posts? After some digging, I found the schema for Nuget Target Frameworks which answered these questions.

Another issue we ran into - the new, SDK style csproj format removes the need to define DefaultNamespace, AssemblyName in AssemblyInfo.cs. Those are generated along when the csproj is built. If you were using the defaults for both of these, you can simply remove those attributes. If you were not using the defaults, you'll want to move the attributes you previously had in AssemblyInfo.cs for these properties to your csproj so that they stay the same as they were before. You can now set these from the Properties pane of a project (right click on your project and click Properties, then go to the Package submenu).

No post that lists specific bugs and implementation details for tooling is complete without version info, so here's what I have installed:

Visual Studio 15.2 (26430.13)
.NET Framework 4.7.02053 installed
Nuget Package Manager 4.2.0
Xamarin 4.5.0.486
Xamarin.Android SDK 7.3.1.2
Xamarin.iOS and Xamarin Mac SDK 10.10.0.37