[Solved] In this test code, why does the component fail to use the mock service? TestBed.configureTestingModule({ declarations: [MyComponent], providers: [{provide: DataService, useClass: RealDataService}] }); | Angular

Angular - Services and Dependency Injection

In this test code, why does the component fail to use the mock service?

TestBed.configureTestingModule({
  declarations: [MyComponent],
  providers: [{provide: DataService, useClass: RealDataService}]
});

ABecause useClass cannot be used in providers

BBecause useClass is set to RealDataService instead of the mock

CBecause declarations array is missing DataService

DBecause DataService is not provided at all

Step-by-Step Solution

Solution:

Step 1: Analyze provider configuration
Provider uses useClass to specify which class to inject for DataService.
Step 2: Identify why mock is not used
Since useClass points to RealDataService, mock is ignored.
Final Answer:
Because useClass is set to RealDataService instead of the mock -> Option B
Quick Check:
Provider useClass must point to mock for testing = D [OK]

Quick Trick: Set useClass to mock service to override real one [OK]

Common Mistakes:

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In this test code, why does the component fail to use the mock service?