[Blazor] `@key` is not respected for interactive WebAssembly components after publish

[MackinnonBuck]

Summary

Components with InteractiveWebAssembly render modes cannot receive parameter updates from SSR updates on published apps. If the component receives any changed parameters, it always gets destroyed and re-initialized regardless of whether a matching @key was present.

Steps to reproduce

1. dotnet new blazor -int WebAssembly
2. Add a CounterButton component in the .Client project:

   @inject NavigationManager NavigationManager
   
   <p role="status">Current count: @currentCount</p>
   
   <button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
   <button class="btn btn-secondary" @onclick="RefreshPage">Refresh</button>
   
   @code {
       private int currentCount = 0;
   
       [Parameter]
       [EditorRequired]
       public int IncrementAmount { get; set; }
   
       private void IncrementCount()
       {
           currentCount += IncrementAmount;
       }
   
       private void RefreshPage()
       {
           NavigationManager.Refresh();
       }
   }

3. Update Home.razor:

   @page "/"
   
   <PageTitle>Home</PageTitle>
   
   <h1>Hello, world!</h1>
   
   <p>Page load count: @pageLoadCount</p>
   
   <CounterButton @key="0" @rendermode="InteractiveWebAssembly" IncrementAmount="@pageLoadCount" />
       
   @code {
       private static int pageLoadCount = 0;
   
       protected override void OnInitialized()
       {
           pageLoadCount++;
       }
   }

4. Run the app using dotnet run
5. Click "Click me" and then click "Refresh" and observe that the counter retains its state
6. Publish the app using dotnet publish
7. Run the published app
8. Click "Click me" and then click "Refresh" and observe that the counter loses its state

Other notes

This appears to be a trimming issue, since doing the following solves the problem:

1. Update the .Client.csproj file to include:

   <ItemGroup>
     <TrimmerRootDescriptor Include="Roots.xml" />
   </ItemGroup>

2. Add a Roots.xml file to the .Client folder:

   <linker>
     <assembly fullname="Microsoft.AspNetCore.Components.WebAssembly" preserve="all">
     </assembly>
   </linker>

.NET SDK Version: 8.0.100

Thanks for contacting us.

We're moving this issue to the .NET 9 Planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s).
If we later determine, that the issue has no community involvement, or it's very rare and low-impact issue, we will close it - so that the team can focus on more important and high impact issues.
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