Pass (or inject) some HTML/JS or pass some model to shared Net Core Razor Class Library - asp.net-core

Question

I would like to have a shared UI project with common interface for other apps, so I don't have to copy/paste the same _Layout.cshtml between all apps which need same UI.
After using this question as reference and this article I have created shared Razor Class Library project. And when I reference this project via NuGet, the _Layout.cshtml in this shared project gets called and renders header with navigation.
But my problem comes then if I want to add some extra HTML to header. Let say I have drop down with some link to main landing page, to user setting, some administration page and etc. But how could I add some extra navigation links specific to concrete app next to all common links? Is there a way to inject HTML? Or maybe to pass some DTO model to this _Layout.cshtml?
Dump image below to help you to see what I try to achieve.

Answer 1

There's two possible approaches. First, you can simply call a partial in your layout:
<partial name="_HeaderExtra" />
Then, just add an empty partial view to the RCL, so it's satisfied, i.e. _HeaderExtra.cshtml. Finally, in your actual application, create the same partial view in the same location as the one in the RCL, and add whatever extra HTML you want there. When the partial is pulled into your layout, it will looking your project first, before falling back to the RCL.
The second option is to use sections. Sections are a little bit more strict, and you must remember to always reimplement them if you do any view inheritance. Essentially, in your layout, at something like:
#await RenderSectionAsync("HeaderExtra", required: false)
Then, in any sublayouts or views:
#section HeaderExtra
{
<!-- add something here -->
}
If you've got a sublayout and you want to be able to further customize this section in views, then you need:
#section HeaderExtra
{
#await RenderSectionAsync("HeaderExtra", required: false)
<!-- add something here -->
}