Fix using HttpClient as a singleton

[timothy-b]

I appreciate the transition to using HttpClient in this commit (thanks @GregoryConrad!) , but there's a known problem with using a singleton HttpClient: it won't respect DNS changes made over its lifetime. According to the following documentation, the current best-practice for managing HttpClients over the lifetime of long-running apps is by using an IHttpClientFactory.

• https://docs.microsoft.com/en-us/dotnet/architecture/microservices/implement-resilient-applications/use-httpclientfactory-to-implement-resilient-http-requests
• https://docs.microsoft.com/en-us/aspnet/core/fundamentals/http-requests?view=aspnetcore-5.0

So, rather than constructing a HttpClient here, I suspect we should perhaps pass in a Func<HttpMessageHandler, HttpClient> to the class's constructor, and store the resulting client as an instance member.

This concern is blocking me from upgrading Vantiv.CnpSdkForNet in my team's project. So, I'd be happy to open a PR and implement my suggested changes, but since this would be my first time contributing to this repo, I'd appreciate it if someone would give me the thumbs-up before I got started.

[VantivSDK]

Hi timothy, thanks for helping us with your suggestion. Please feel free to apply your suggestion in PR. Once you are ready with PR, we will perform a few additional tests at our end. And then we will accept and release a new version with your changes.

[andrebts]

@timothy-b Please refer to this twilio/twilio-csharp#529

[mattisking]

So, I'm not planning any kind of immediate PR because my branch intentionally breaks some functionality (mostly around logging to just use ILogger) but this is basically one of a couple options for what you want to do:

I'm moving to my version internally on a project but the lifting is done here if you are interested, including updating all the tests, etc. I added a sample for using Dependency Injection. My sample is still .Net Core 3.1 and could be moved to .Net 6 but we haven't made that move yet.

https://github.com/mattisking/cnp-sdk-for-dotnet

I am willing to do some more work on it, it might just take a little time as I'm swamped at work.

[mgungorchamp]

Use IHttpClientFactory to implement resilient HTTP requests

Don't use singleton!
https://learn.microsoft.com/en-us/dotnet/architecture/microservices/implement-resilient-applications/use-httpclientfactory-to-implement-resilient-http-requests