We have C# worker app on our server that already uses the Atlassian SDK. There are external libraries for Slack as well to send messages but could it be done through the JIRA API?
Welcome to the Community! Interesting question. Having a notification go through Jira probably makes sense as part of an issue transition/update. We have an Atlassian-built integration for Jira Server that simplifies the actual Slack notifications, based on Jira issue changes. If you're doing something else in Jira at the same time, this is probably the way you want to go.
Example of Jira Server for Slack configuration based on issue changes
There are some additional apps on the marketplace if you find this way interesting but need it configured a little differently.
I'd probably steer away from trying to tap into Jira APIs for the sole purpose of sending Slack messages. C# isn't a language I've seen people use in Slack API boilerplate examples, but I'm sure the configurations you've already found are probably simpler than trying to trigger that through Jira (without doing an issue action). There's also the additional complexity/failpoint to consider; if Jira is down for some reason (during an upgrade for example), you don't want to be relying on a service that arbitrarily is not responding to your C# app.
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