Resolving the System.InvalidOperationException in ASP.NET Core: Service Activation Error

Learn how to fix the `System.InvalidOperationException` in ASP.NET Core by resolving service dependencies for controllers effectively.
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This video is based on the question https://stackoverflow.com/q/70204929/ asked by the user 'Olugbenga' ( https://stackoverflow.com/u/17480273/ ) and on the answer https://stackoverflow.com/a/70205409/ provided by the user 'Nkosi' ( https://stackoverflow.com/u/5233410/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Troubleshooting System.InvalidOperationException in ASP.NET Core: A Comprehensive Guide

In ASP.NET Core development, encountering exceptions can be a common hurdle. One such error that developers often face is the System.InvalidOperationException, which states: "Unable to resolve service for type while attempting to activate." This error typically occurs during the service activation process, which is essential for Dependency Injection in ASP.NET Core applications.

In this guide, we'll explore a specific instance of this error, understand its causes, and walk through the steps to resolve it effectively.

Understanding the Problem

You might encounter an error when making a GET request to your API, similar to the following:

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

This indicates that the Dependency Injection container could not find the registered service while trying to create an instance of the MerchantController. Let's examine what might be causing this issue.

The Core Issue

When looking at the code, we see that in the MerchantController, the constructor is expecting a MerchantService type:

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

However, in the Startup.cs file, the service is registered as an interface (IMerchantService):

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

This mismatch between the expected type in the controller and the registered type in the service container is the root cause of the InvalidOperationException.

The Solution

To resolve this issue, we need to ensure that the controller's constructor accepts the same type that has been registered in the Dependency Injection container. Below are the steps to fix this error:

Step 1: Modify the Controller Constructor

Change the constructor of the AdminController so that it accepts the IMerchantService interface instead of the MerchantService implementation:

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

Step 2: Verify Service Registration

Next, ensure that your service registration in Startup.cs is correct, which should already be set properly if you’ve followed conventional practices:

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

Step 3: Test Your Application

After making these changes, rebuild your application and re-run the request in Postman or another API testing tool. The error should no longer occur, and your request should return the expected results.

Conclusion

Dependency Injection is a powerful feature in ASP.NET Core that allows for better code separation and testing. However, it requires careful type management to avoid common pitfalls, such as the System.InvalidOperationException encountered in this scenario. By ensuring that the types declared in your controllers match those registered in your service container, you can maintain a cleaner, more maintainable codebase.

With this understanding, you're now equipped to troubleshoot similar issues in the future. Happy coding!

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