OnInit() called twice

[dkurok]

Describe the bug

The OnInit()-event is called twice / handler runs twice

To Reproduce

Steps to reproduce the behavior:

1. Visual Studio 2019; create new project using Razor components template

2. In MainLayout.razor add the following to the end of the file:
   `@functions{ protected override void OnInit()
   {
   base.OnInit();
   Console.WriteLine("OnInit called");

   }
   }`

3. Place a breakpoint on the Console.WriteLine-call in OnInit()

4. Debug and see in Output -> ASP.NET Core Web Server:
   The breakpoint will be hit twice! From what I've read it should only be hit once at the initialization of the component

Expected behavior

OnInit() should only be called once

Additional context

Visual Studio Enterprise 2019 Version 16.0.0 on Windows 10, 1809

[dkurok]

Addon: It is .NET Core 3.0 (3.0.100-preview3-010431) (otherwise there wouldn't be no Razor components template)

[Andrzej-W]

In razor components template prerendering is enabled. In YourApp\YourApp\Pages\Index.cshtml file there is line

<app>@(await Html.RenderComponentAsync<App>())</app>

Change it to

<app></app>

Of course now prerendering doesn't work.

[dkurok]

Great, this works!
Is there somewhere a deeper explanation for that behaviour and intention of the prerendering? I want to use a dynamic nav-menu (rendered from DB-content depending on user's rights); I fill a List at the very beginning (in OnInit()) but saw this gets called twice. Can I "ask" for a status like "isInPrerendering" or alike? found just a very small introduction of prerendering in the announcements of ASP.NET Core 3.0 preview

[Andrzej-W]

@dkurok Temporary solution for "isPrerendering": #8404 (comment)
Discussion about problems during prerendering: #8786
It is now closed and should be releases in preview 4.
Here you can see prerendering in action (great demo by Steven Sanderson)
https://www.youtube.com/watch?v=0RfUPr0KrSM&feature=youtu.be&t=2818

[danroth27]

Thanks for answering, @Andrzej-W