BlazorWinFormsApp and BlazorWpfApp throw NullReferenceException when using ExampleJsInterop in .NET 6 Preview 3

[Jinjinov]

Describe the bug

If ExampleJsInterop is called from WinForms or from WPF it throws a NullReferenceException

To Reproduce

In WinForms:

• open https://github.com/dotnet/aspnetcore/tree/main/src/Components/WebView/Samples/BlazorWinFormsApp

• Form1.cs, add WebviewAppShared.ExampleJsInterop exampleJsInterop; to class Form1

• add exampleJsInterop.Prompt("Hello world!"); to button1_Click

• add serviceCollection.AddScoped<WebviewAppShared.ExampleJsInterop>(); below var serviceCollection = new ServiceCollection();

• replace blazorWebView1.Services = serviceCollection.BuildServiceProvider(); with

    IServiceProvider serviceProvider = serviceCollection.BuildServiceProvider();
    blazorWebView1.Services = serviceProvider;
    exampleJsInterop = serviceProvider.GetRequiredService<WebviewAppShared.ExampleJsInterop>();
  

• run app

• click on Click to see counter value

In WPF:

• open https://github.com/dotnet/aspnetcore/tree/main/src/Components/WebView/Samples/BlazorWpfApp

• MainWindow.xaml.cs, add WebviewAppShared.ExampleJsInterop exampleJsInterop; to class MainWindow

• add exampleJsInterop.Prompt("Hello world!"); to Button_Click

• add serviceCollection.AddScoped<WebviewAppShared.ExampleJsInterop>(); below var serviceCollection = new ServiceCollection();

• replace Resources.Add("services", serviceCollection.BuildServiceProvider()); with

    IServiceProvider serviceProvider = serviceCollection.BuildServiceProvider();
    Resources.Add("services", serviceProvider);
    exampleJsInterop = serviceProvider.GetRequiredService<WebviewAppShared.ExampleJsInterop>();
  

• run app

• click on Check counter

Exceptions (if any)

System.NullReferenceException: 'Object reference not set to an instance of an object.'

Further technical details

• ASP.NET Core version: 16.10.127.22515
• The IDE (VS / VS Code/ VS4Mac) you're running on, and its version: Visual Studio 2019 - Version 16.10.0 Preview 1.0
• Include the output of dotnet --info

.NET SDK (reflecting any global.json):
 Version:   6.0.100-preview.3.21202.5
 Commit:    aee38a6dd4

Runtime Environment:
 OS Name:     Windows
 OS Version:  6.3.9600
 OS Platform: Windows
 RID:         win81-x64
 Base Path:   C:\Program Files\dotnet\sdk\6.0.100-preview.3.21202.5\

Host (useful for support):
  Version: 6.0.0-preview.3.21201.4
  Commit:  236cb21e3c

.NET SDKs installed:
  3.1.406 [C:\Program Files\dotnet\sdk]
  5.0.200-preview.21077.7 [C:\Program Files\dotnet\sdk]
  5.0.201 [C:\Program Files\dotnet\sdk]
  6.0.100-preview.3.21202.5 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 3.1.7 [C:\Program Files\dotnet\shared\Microsoft.AspNe
tCore.App]
  Microsoft.AspNetCore.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.AspNe
tCore.App]
  Microsoft.AspNetCore.App 3.1.12 [C:\Program Files\dotnet\shared\Microsoft.AspN
etCore.App]
  Microsoft.AspNetCore.App 3.1.13 [C:\Program Files\dotnet\shared\Microsoft.AspN
etCore.App]
  Microsoft.AspNetCore.App 5.0.2 [C:\Program Files\dotnet\shared\Microsoft.AspNe
tCore.App]
  Microsoft.AspNetCore.App 5.0.4 [C:\Program Files\dotnet\shared\Microsoft.AspNe
tCore.App]
  Microsoft.AspNetCore.App 6.0.0-preview.3.21201.13 [C:\Program Files\dotnet\sha
red\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 3.1.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.
App]
  Microsoft.NETCore.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.
App]
  Microsoft.NETCore.App 3.1.12 [C:\Program Files\dotnet\shared\Microsoft.NETCore
.App]
  Microsoft.NETCore.App 3.1.13 [C:\Program Files\dotnet\shared\Microsoft.NETCore
.App]
  Microsoft.NETCore.App 5.0.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.
App]
  Microsoft.NETCore.App 5.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.
App]
  Microsoft.NETCore.App 6.0.0-preview.3.21201.4 [C:\Program Files\dotnet\shared\
Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.W
indowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.12 [C:\Program Files\dotnet\shared\Microsoft.
WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.13 [C:\Program Files\dotnet\shared\Microsoft.
WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.2 [C:\Program Files\dotnet\shared\Microsoft.W
indowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.4 [C:\Program Files\dotnet\shared\Microsoft.W
indowsDesktop.App]
  Microsoft.WindowsDesktop.App 6.0.0-preview.3.21201.3 [C:\Program Files\dotnet\
shared\Microsoft.WindowsDesktop.App]

To install additional .NET runtimes or SDKs:
  https://aka.ms/dotnet-download

[mkArtakMSFT]

Thanks for contacting us. Please share the full exception details including the stack trace. That will simplify our investigation.

Thanks for contacting us.
We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. Because it's not immediately obvious that this is a bug in our framework, we would like to keep this around to collect more feedback, which can later help us determine the impact of it. 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.
To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.

[Jinjinov]

Please share the full exception details including the stack trace.

@mkArtakMSFT

exception:

System.NullReferenceException: 'Object reference not set to an instance of an object.'
This exception was originally thrown at this call stack:
    [External Code]
    WebviewAppShared.ExampleJsInterop.Prompt(string) in ExampleJsInterop.cs

details:

$exception	{"Object reference not set to an instance of an object."}	System.NullReferenceException
Data	{System.Collections.ListDictionaryInternal}	System.Collections.IDictionary {System.Collections.ListDictionaryInternal}
HResult	-2147467261	int
HelpLink	null	string
InnerException	null	System.Exception
Message	"Object reference not set to an instance of an object."	string
Source	"Microsoft.AspNetCore.Components.WebView"	string
StackTrace	"
   at Microsoft.AspNetCore.Components.WebView.Services.WebViewJSRuntime.BeginInvokeJS(Int64 taskId, String identifier, String argsJson, JSCallResultType resultType, Int64 targetInstanceId)\r\n
   at Microsoft.JSInterop.JSRuntime.InvokeAsync[TValue](Int64 targetInstanceId, String identifier, CancellationToken cancellationToken, Object[] args)\r\n
   at Microsoft.JSInterop.JSRuntime.<InvokeAsync>d__15`1.MoveNext()\r\n
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()\r\n
   at WebviewAppShared.ExampleJsInterop.<Prompt>d__2.MoveNext() in ExampleJsInterop.cs:line 26"	string
TargetSite	{Void BeginInvokeJS(Int64, System.String, System.String, Microsoft.JSInterop.JSCallResultType, Int64)}	System.Reflection.MethodBase {System.Reflection.RuntimeMethodInfo}
Static members		
Non-Public members	

[Eilon]

@TanayParikh - is this related to the issue you're working on for JS Interop in BlazorWebView?

[TanayParikh]

This may be related, I noticed we aren't providing the appropriate JS/Dotnet object reference json converters during IPC sending/receiving. Currently working on a fix for that. Will try repro-ing this to see if it gets fixed through those changes, and update this issue accordingly.

[TanayParikh]

I noticed we aren't providing the appropriate JS/Dotnet object reference json converters during IPC sending/receiving. Currently working on a fix for that.

Turned out to be a non-issue, so this issue doesn't seem to be related to my changes in JS Interop.

[Eilon]

I tried this with the Photino.NET sample in the repo and it works. But WPF isn't working. Will investigate more.

[Eilon]

For some reason in WPF (and probably WinForms), we get more than 1 instance of WebViewJSRuntime, and the "later" ones are not properly initialized, meaning that they don't have an ipcSender, so they null-ref.

[Eilon]

Just guessing, but it appears that because the WebViewJSRuntime is a scoped service, that for some reason a new instance is being created from a new scope, and that new instance is not properly initialized.

So why did the Photino.NET sample work? Because the sample code was different. In my Photino.NET repro I was using @inject RazorClassLibrary1.ExampleJsInterop exampleJsInterop from one of the Razor components to call the JS code. That presumably uses the correct scope. But in the WPF repro the code is getting the ExampleJsInterop service from the "WPF" code, and I guess that's using a different scope.

[Eilon]

@SteveSandersonMS / @javiercn - I'd love your input on this one. I'm guessing that this behavior might be by-design, but then I'm not sure the right way to build this sample.

I sense that I want to somehow "run some .NET code as if it's in the context ("scope") of the code in the Razor component" and thus share the same DI scope, and thus get the same instances of services that are available within that scope.

[SteveSandersonMS]

@Eilon The slightly awkward thing is that, each time the page inside the BlazorWebView reloads, we set up a new scope. We have to do that because (for example) you have a new JS context with no pre-existing state so we have to reset everything to do with it. It's like reloading the page with WebAssembly or Server - those create new DI contexts too.

So given this, if some code outside that system asks for a scope, which scope does it get? One that might become obsolete later if the page is reloaded? Kind of dangerous, but represents the truth about what's going on.

We could address this simply by exposing some IServiceProvider? CurrentPageServices { get; set; } property from WebViewManager whose value may be null (if no page has yet loaded) or may change (if the page navigates) and then trust the developer to understand this and not store a reference to the instance anywhere. TBH this is probably not too bad really.

Alternatively we could have some kind of RunAsync(Func<IServiceProvider, Task>) method on WebViewManager that's some kind of combo of "dispatch" and "give me the current service provider" to encourage people to treat it as a transient thing. Not sure people will follow the reasoning though.

What do you think?

[Eilon]

@SteveSandersonMS yes the RunAsync sounds pretty neat, because it should always work (right?). The problem with CurrentPageServices is that I feel that there's no good way to predict when it's valid and non-null (right?).

I'm basically thinking about how I would want to write the sample code in the app in a way that is predictably correct, and I think something like:

void Button_Click()
{
    blazorWebView1.RunAsync(async (services, more_context_needed(?)) => {
        var js = services.Get(...);
        await js.DoStuff();
    });
}

I wonder if there's any other context that we would want to provider? Or, if the entire purpose of this is to provide the scoped services, then logically that's the only context that's needed, so it would be sufficient.