Compilation: every .cs file in .csproj

You should add a .cs file to the Sample.csproje, to the appropriate <ItemGroup> manually:

<Compile Include="MyButton.cs" />

otherwise you may get this:

I/MonoDroid( 2017): UNHANDLED EXCEPTION:
I/MonoDroid( 2017): Xamarin.Forms.Xaml.XamlParseException: Position 8:3. Type local:MyButton not found in xmlns clr-namespace:Sample

P.S. seems that MonoDevelop may add it automatically.