When settings DisableImplicitNamespaceImports=false, DisableImplicitNamespaceImports_DotNet is not set to any value:

So this condition '$(DisableImplicitNamespaceImports_DotNet)'!='true' in

is true and the implicit namespaces are still generated. So the only way to disable them is to set both properties to true explicitly.

cc: @dsplaisted @JunTaoLuo