How to solve InvalidOperationException for constructors using HttpClientFactory in C#

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A few days ago I was preparing the demo for a new article. The demo included a class with an IHttpClientFactory service injected into the constructor. Nothing more.

Then, running the application (well, actually, executing the code), this error popped out:

System.InvalidOperationException: A suitable constructor for type ‘X’ could not be located. Ensure the type is concrete and all parameters of a public constructor are either registered as services or passed as arguments. Also ensure no extraneous arguments are provided.

How to solve it? It’s easy. But first, let me show you what I did in the wrong version.

Setting up the wrong example

For this example, I created an elementary project.
It’s a .NET 7 API project, with only one controller, GenderController, which calls another service defined in the IGenderizeService interface.

IGenderizeService is implemented by a class, GenderizeService, which is the one that fails to load and, therefore, causes the exception to be thrown. The class calls an external endpoint, parses the result, and then returns it to the caller:

Finally, I’ve defined the services in the Program class, and then I’ve specified which is the base URL for the HttpClient instance generated in the GenderizeService class:

That’s it! Can you spot the error?

2 ways to solve the error

The error was quite simple, but it took me a while to spot:

In the constructor I was injecting an IHttpClientFactory:

while in the host definition I was declaring an HttpClient for a specific class:

Apparently, even if we’ve specified how to create an instance for a specific class, we could not build it using an IHttpClientFactory.

So, here are 2 ways to solve it.

Use named HttpClient in HttpClientFactory

Named HttpClients are a helpful way to define a specific HttpClient and use it across different services.

It’s as simple as assigning a name to an HttpClient instance and then using the same name when you need that specific client.

So, define it in the Startup method:

and retrieve it using CreateClient:

💡 Quick tip: define the HttpClient names in a constant field shared across the whole system!

Inject HttpClient instead of IHttpClientFactory

The other way is by injecting an HttpClient instance instead of an IHttpClientFactory.

So we can restore the previous version of the Startup part:

and, instead of injecting an IHttpClientFactory, we can directly inject an HttpClient instance:

We no longer need to call _httpClientFactory.CreateClient because the injected instance of HttpClient is already customized with the settings we’ve defined at Startup.

Further readings

I’ve briefly talked about HttpClientFactory in one article of my C# tips series:

🔗 C# Tip: use IHttpClientFactory to generate HttpClient instance | Code4IT

And, more in detail, I’ve also talked about one way to mock HttpClientFactory instances in unit tests using Moq:

🔗 How to test HttpClientFactory with Moq | Code4IT

Finally, why do we need to use HttpClientFactories instead of HttpClients?

🔗 Use IHttpClientFactory to implement resilient HTTP requests | Microsoft Docs

Wrapping up

Yes, it was that easy!

We received the error message

A suitable constructor for type ‘X’ could not be located.

because we were mixing two ways to customize and use HttpClient instances.

But we’ve only opened Pandora’s box: we will come back to this topic soon!

For now, Happy coding!



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