Callsite rendermodes

[SteveSandersonMS]

Add callsite rendermodes for Blazor full-stack web apps

Allows Blazor components to have a rendermode specified at the usage site. Among other use cases, this allows us to simplify the default project template by removing an otherwise pointless wrapper component.

We want to do this in preview 6 (to get customer feedback) but didn't land it in time for the normal code-complete cutoff.

Description

We already support specifying a fixed rendermode on a component type, e.g.:

@page "/my-component"
@attribute [RenderModeServer]

<h1>Here's some interactive stuff</h1>
...

However, it's commonly also desirable to specify a rendermode at the callsite for a component, e.g.:

<h1>Here's some interactive stuff</h1>
<Counter @rendermode="@RenderMode.Server" />

This allows the consumer to vary the rendermode according to their own preferences without having to do so on the parent component. In the default project template, it means that developers can have an interactive <HeadOutlet> just by writing:

<HeadOutlet @rendermode="@RenderMode.Server" />

... whereas without this change, they have to define some new component e.g., called AppHeadOutlet containing just:

@attribute [RenderModeServer]
<HeadOutlet />

... and then place <AppHeadOutlet /> at the intended call site.

Fixes #48793

Customer Impact

Allows the default migration experience from Blazor Server/WebAssembly to the new full-stack architecture to be more streamlined and not involve a weird, pointless-seeming wrapper component.

Regression?

• Yes
• No

[If yes, specify the version the behavior has regressed from]

Risk

• High
• Medium
• Low

Low because this only impacts completely new code using the new functionality. It shouldn't be possible for this to break existing apps.

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

[ghost]

Hi @SteveSandersonMS. Please make sure you've updated the PR description to use the Shiproom Template. Also, make sure this PR is not marked as a draft and is ready-to-merge.

To learn more about how to prepare a servicing PR click here.

[mkArtakMSFT]

@SteveSandersonMS do you want to retarget this to preview6? Or will there be a port PR?

[javiercn]

@SteveSandersonMS overall looks good.

Are we checking at some point that users don't intermingle server/webasembly components?

[SteveSandersonMS]

Are we checking at some point that users don't intermingle server/webasembly components?

Yes, both the server and webassembly renderers reject instructions to render the other type of component.

[ghost]

Hi @SteveSandersonMS. This PR was just approved to be included in the upcoming servicing release. Somebody from the @dotnet/aspnet-build team will get it merged when the branches are open. Until then, please make sure all the CI checks pass and the PR is reviewed.