Resolving InvalidOperationException: A Guide to Dependency Injection in ASP.NET Core

Learn how to fix the `InvalidOperationException` related to service resolution in ASP.NET Core by correctly configuring Dependency Injection.
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This video is based on the question https://stackoverflow.com/q/65886816/ asked by the user 'Mariusmatei' ( https://stackoverflow.com/u/11120282/ ) and on the answer https://stackoverflow.com/a/65887003/ provided by the user 'Jacek' ( https://stackoverflow.com/u/2811109/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: InvalidOperationException: Unable to resolve service for type 'Data.IRestaurantsData' while attempting to activate

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Understanding the InvalidOperationException in ASP.NET Core

If you are new to C# and ASP.NET Core, encountering errors can be quite common, especially when dealing with concepts like Dependency Injection (DI). One frequently seen issue is the InvalidOperationException that indicates your application is unable to resolve a service for a specific type.

In this guide, we'll dissect this error and guide you through a solution to ensure your application runs smoothly.

The Error in Context

In your case, the error reads:

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

This error occurs when the ASP.NET Core DI framework attempts to create an instance of ListModel but fails because it cannot provide an instance of IRestaurantsData.

The Solution: Proper Configuration of Services

To avoid the InvalidOperationException, you need to ensure that your services are registered correctly in the Startup class of your application. Let’s go through this step-by-step:

Step 1: Modify ConfigureServices Method

Open your Startup.cs file and locate the ConfigureServices method. You currently have a part of your service registration nested within a lambda expression for CookiePolicyOptions. Here’s how it looks currently:

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

Step 2: Move Service Registration to Outer Scope

To rectify the issue, you should move the line that registers IRestaurantsData to the outer scope of the ConfigureServices method. Your modified code should look like this:

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

Why This Works

By moving the services.AddSingleton<IRestaurantsData, InMemoryRestaurantsData>() to the outer scope, you're ensuring that this service is registered with the DI container as soon as the application starts. This allows the IRestaurantsData parameter to be injected into your ListModel constructor without any issues.

Conclusion

In summary, correcting the placement of your service registration in Startup.cs is crucial to resolving the InvalidOperationException you're encountering. By following the steps provided, you can ensure that your ASP.NET Core application can leverage Dependency Injection effectively, allowing for smoother execution of your code.

Final Thoughts

As you continue your journey with C# and ASP.NET Core, remember that proper service configuration is key to preventing runtime errors. With practice, you'll develop an intuitive understanding of how Dependency Injection works, setting a solid foundation for your future projects.
This guide serves both as a reference and a guide for fellow developers facing similar issues. If you have any questions or need further clarification, feel free to leave a comment below!

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