Solving the NullInjectorError in Angular: A Guide to Proper Dependency Injection

Discover how to resolve the `NullInjectorError: No provider for service in library` in Angular applications. This guide provides a detailed explanation of dependency injection and offers step-by-step solutions.
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Solving the NullInjectorError in Angular: A Guide to Proper Dependency Injection

When working with Angular, you may encounter a frustrating issue known as NullInjectorError, specifically the message: No provider for Service in library. This error typically arises when Angular's dependency injection system cannot resolve a required service. If you're importing a module from a library into your application and attempt to use a service from that module, but run into this error, you're not alone. Let's take a closer look at this problem and explore how to solve it.

Understanding the Problem

You are likely to run into this issue when you import a module from a library without ensuring that you're using the correct instance of the service. In the scenario presented, the error occurs after the developer attempts to inject the CommonService from a library module into an application component. Here’s the exact error message received:

[[See Video to Reveal this Text or Code Snippet]]

This error suggests that Angular cannot find a provider for CommonService despite it being defined in common.module.ts. Let's investigate the solution.

The Root Cause

The core issue is that the service you're trying to inject (CommonService) is being imported directly from the library's source path rather than being accessed through the library’s public API. By doing so, you're creating a new instance of the service, separate from the one managed by the Angular dependency injection system in common.module.ts.

Step-by-Step Solution

To solve this problem, follow these key steps:

Step 1: Ensure Proper Imports from the Library

Instead of importing CommonService directly with an absolute or relative import path, make sure you're importing it from the public API of the library. In your application’s app.component.ts, change your import statement as follows:

[[See Video to Reveal this Text or Code Snippet]]

By doing this, you're ensuring that the Angular injector can find the correct instance that has been registered in the CommonModule.

Step 2: Check the Module Definition

Your common.module.ts appears to be correctly set up with the provider registered for CommonService. Here’s how the module looks:

[[See Video to Reveal this Text or Code Snippet]]

This setup should work as long as you’re importing CommonService from the public API.

Step 3: Update app.module.ts

Make sure that the CommonModule is imported in your application module (app.module.ts) as follows:

[[See Video to Reveal this Text or Code Snippet]]

This ensures that Angular knows where to find the CommonService provider.

Final Thoughts

After making these adjustments, you should no longer see the NullInjectorError when you inject the CommonService in your AppComponent. Remember, ensuring that services are correctly imported from the library's public API is crucial for Angular's dependency injection to work seamlessly.

Following these steps will help you better manage services and dependencies in Angular, making your application more robust and easier to maintain. Happy coding!

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