TelegramBotClient | Powerful client for Telegram Bot API | REST library

Since it's always null I think that the problem is that you're not creating an instance of your handler. I prepared a demo for you where I did that and it works.

A more orthadox way of handling a DI service with a configuration is to use the IOptions pattern. This way you aren't tightly coupling your startup object with the service. As it stands now, if your configuration changes, you have to modify your startup object.

A way to tackle this and keep your concerns separated, take a look at this:

TelegramBotClientService.cs

The key to your answer is in the resume of your observation when profiling your application's memory: "there appears to have been 1 of each of the objects above created". Since all those objects live inside an infinite application loop, you don't have to worry about their lifetime.
From the code you've posted, the only expensive objects that are created dynamically but won't accumulate during the lifetime of Bot are the exception objects (and their associated call stacks), especially when exceptions are caught by a try-catch.

Assuming that the "Simple Injector" library you are using works properly, there is no reason to doubt the lifetime management being correctly implemented like you did. This means it only depends the way your container is configured.

Right now all your instances have a Transient lifetime, which is the default. It's important to notice this, as it appears you are expecting a Singleton lifetime.
Transient means a new instance for every request opposed to Singleton where the same shared instance is returned for each request. To achieve this behavior you must explicitly register the export with a Singleton lifetime defined:

I solved the problem.

this method should be void and without async. and then we must remove await before bot.DownloadFileAsync

If you read the next paragraph below the html-examples in the documentation, you can see that it says that only the tags mentioned mentioned above are currently supported.

Therefore, I would suggest you use markdown, since there are no restrictions mentioned for this way of parsing. A linebreak in MD is achieved by either two spaces or
. A list in MD can be created by using *, + or - in front of the list's items, for more information refer to this documentation on MD for example.

For the second part of your question, phone numbers get highlighted by the Telegram phone app automatically, but there is no such function for Desktop-clients and no built-in way in the API/libraries. Also, it only works for messages within the range of 0-200 characters, as stated here: How to make phone number a link in Telegram Bot?

I have done in python:

Implementing your own IHostedService would be the best way to go about this. For getting the dbcontext in the service you can use IserviceProvider as your dependency. Serviceprovider will give you the dbcontext. You can configure your custom hosted service to be added as a singleton then. Check this documentation for details:

https://docs.microsoft.com/en-us/dotnet/architecture/microservices/multi-container-microservice-net-applications/background-tasks-with-ihostedservice#implementing-ihostedservice-with-a-custom-hosted-service-class-deriving-from-the-backgroundservice-base-class

^{As described in the comments;}

: terminat ed by other getUpdates request; make sure that only one bot instance is running

Indicates that getUpdates() is being called at the same time.

To prevent this, either stop the other bots, or create a bot that will receive a web-hook HTTP POST request on update.

You should ensure that each message is handled by the bot(s) before you tell Telegram the message has been seen by the getUpdate() offset param.

SendTextMessageAsync returns a Task. Try to await it.