Ever needed a "fire and forget" solution where a client calls a C# WebAPI controller that does something long running, but doesn't really care about the result?

For example, say you have a job scheduling system that runs a series of tasks and one of them is a call to a C# WebAPI service to create something like an export of data from a SQL Server table to a file.

Problem

The table is large and the export will tie up the job scheduling system from running the rest of the tasks in the process, or timeout the job scheduling process. And the result of the WebAPI service doesn't mean anything to the end result of the process, with the WebAPI reporting errors in another way such as by email.

Solution

Make the C# WebAPI report a HttpStatus code of 200 OK and say that the request is complete to the calling client, while still continuing to work on the request in the background.

For this fire and forget functionality, in your WebAPI controller, do this:

public async Task<IActionResult> Post(string id)
{
    try
    {
        return Ok("Request submitted successfully");
    }
    finally
    {
        Response.OnCompleted(async () =>
        {
            await DoProcessing(id);
        });
    }
}

The key is the "Response.OnCompleted" part, which allows your code to execute even after reporting HttpStatus 200 OK to the client.