angular / angular/components

bug(multiple): services provided in root are also provided in NgModules

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area: many P3
Dominant language
TypeScript
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Description

### Is this a regression?

- [ ] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

_No response_

### Description

I am maintaining an Angular library built on top of Angular Material. Among others, this library includes custom components which are to be opened inside a dialog. Here is what the module of such a component looks like:

```typescript
@NgModule({
declarations: [MyComponent],
imports: [MatDialogModule],
export: [MyComponent],
})
export class MyModule {}
```

For applications which use that component in an NgModule, this works fine. However one of the applications is in the progress of migrating to standalone. After making the component standalone and adding `MyModule` to the `imports`, the unit tests involving the `MatDialog` service opening `MyComponent` failed. After investigation, I found out that the instance injected by the component constructor and the instance injected by `TestBed.inject()` are now two separate ones.

I have confirmed with the Angular core team that this is by design, see https://github.com/angular/angular/issues/53929.

The root cause is that `MatDialogModule` has `MatDialog` in its `providers`: https://github.com/angular/components/blob/b8cf1308571b29622422cd56f7f76b7bbc5ba5b1/src/material/dialog/module.ts#L34

IMHO it should be removed from the providers, as the service is `providedIn: 'root'`.

The `MatSnackBar` has the same issue, there might be more affected services in this repo.

### Reproduction

Repo: https://github.com/json-derulo/ng17-dialog-issue
Steps to reproduce:
1. Run `ng test`

### Expected Behavior

There is only one instance of `MatDialog` and the test case in the repro pass

### Actual Behavior

There are multiple instances of `MatDialog` and the test case in the repro fails

### Environment

Angular CLI: 17.0.10
Node: 18.18.2
Package Manager: npm 9.8.1
OS: darwin x64

Angular: 17.0.9
... animations, common, compiler, compiler-cli, core, forms
... platform-browser, platform-browser-dynamic, router

Package Version
---------------------------------------------------------
@angular-devkit/architect 0.1700.10
@angular-devkit/build-angular 17.0.10
@angular-devkit/core 17.0.10
@angular-devkit/schematics 17.0.10
@angular/cdk 17.1.0
@angular/cli 17.0.10
@angular/material 17.1.0
@schematics/angular 17.0.10
rxjs 7.8.1
typescript 5.2.2
zone.js 0.14.3

Contributor guide

Open the contributing guide

Research direction

Start with the linked MatDialogModule definition in src/material/dialog/module.ts and run ng test in the reproduction repository. Check how the MatDialog provider is declared and whether MatSnackBar or other affected services have the same pattern. Done means the reproduction has one MatDialog instance and its test passes.

Written by the indexing model from the issue text.

Assessment

Tech stack
angular, typescript
Domain
frontend, testing
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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