I also tested a lot of different ways before solved this. And I didn´t find any working solution described. My solution is currently based on the .NET Core 3.0 framework.

reader.readToEnd() seamed like a simple way, even though it compiled, it throwed an runtime exception required me to use async call. So instead I used ReadToEndAsync(), however it worked sometimes, and sometimes not. Giving me errors like, cannot read after stream is closed. The problem is that we cannot guarantee that it will return the result in the same thread (even if we use the await). So we need some kind of callback. This solution worked for me.

[Route("[controller]/[action]")]
public class MyController : ControllerBase
{

    // ...

    [HttpPost]
    public async void TheAction()
    {
        try
        {
            HttpContext.Request.EnableBuffering();
            Request.Body.Position = 0;
            using (StreamReader stream = new StreamReader(HttpContext.Request.Body))
            {
                var task = stream
                    .ReadToEndAsync()
                    .ContinueWith(t => {
                        var res = t.Result;
                        // TODO: Handle the post result!
                    });

                // await processing of the result
                task.Wait();
            }
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Failed to handle post!");
        }
    }

I also wanted to read the Request.Body without automatically map it to some action parameter model. Tested a lot of different ways before solved this. And I didn´t find any working solution described here. This solution is currently based on the .NET Core 3.0 framework.

reader.readToEnd() seamed like a simple way, even though it compiled, it throwed an runtime exception required me to use async call. So instead I used ReadToEndAsync(), however it worked sometimes, and sometimes not. Giving me errors like, cannot read after stream is closed. The problem is that we cannot guarantee that it will return the result in the same thread (even if we use the await). So we need some kind of callback. This solution worked for me.

[Route("[controller]/[action]")]
public class MyController : ControllerBase
{

    // ...

    [HttpPost]
    public async void TheAction()
    {
        try
        {
            HttpContext.Request.EnableBuffering();
            Request.Body.Position = 0;
            using (StreamReader stream = new StreamReader(HttpContext.Request.Body))
            {
                var task = stream
                    .ReadToEndAsync()
                    .ContinueWith(t => {
                        var res = t.Result;
                        // TODO: Handle the post result!
                    });

                // await processing of the result
                task.Wait();
            }
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Failed to handle post!");
        }
    }