Convert from HttpResponseMessage to IActionResult in .NET Core

You can return using hardset status codes like Ok(); or BadRequest();

Or return using a dynamic one using

StatusCode(<Your status code number>,<Optional return object>);

This is using Microsoft.AspNetCore.Mvc

Below is this.StatusCode spelled out a little more:

/* "this" comes from your class being a subclass of Microsoft.AspNetCore.Mvc.ControllerBase */
StatusCodeResult scr = this.StatusCode(200);
/* OR */
Object myObject = new Object();
ObjectResult ores = this.StatusCode(200, myObject);
return scr; /* or return ores;*/

This is much easier to accomplish in .NET v5 (erstwhile .NET Core v5). The ControllerBase class which is found in Microsoft.AspNetCore.Mvc.Core DLL contains scores of methods which return objects which inherit from IActionResult. IActionResult is the replacement for HttpResponseMessage. I ported below web API method which used to return a file. It was based on .NET Framework v4.6.

[HttpGet]
public IHttpActionResult GetFooBar()
{
    var fileContentByteArray = foodBarDomainService.GetExeByteArray();
    var result = new HttpResponseMessage(HttpStatusCode.OK)
    {
        Content = new ByteArrayContent(fileContentByteArray)
    };
    result.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
    {
        FileName = "FooBar.exe"
    };
    result.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
    return ResponseMessage(result);
}

Here is the code which I ported to .NET v5:

[HttpGet("GetFooBar", Name = "GetFooBar")]
public IActionResult GetFooBar(string token)
{
    var fileContentByteArray = foodBarDomainService.GetExeByteArray();
    return File(fileContentByteArray, "application/octet-stream", "FooBar.exe");
}

Here is the complete list of all the methods which return a class that implementins IActionResult interface to serve a specific purpose:

public virtual AcceptedResult Accepted(Uri uri, [ActionResultObjectValue] object value);
public virtual AcceptedResult Accepted(string uri, [ActionResultObjectValue] object value);
public virtual AcceptedResult Accepted(Uri uri);
public virtual AcceptedResult Accepted([ActionResultObjectValue] object value);
public virtual AcceptedResult Accepted();
public virtual AcceptedResult Accepted(string uri);
public virtual AcceptedAtActionResult AcceptedAtAction(string actionName, string controllerName, object routeValues, [ActionResultObjectValue] object value);
public virtual AcceptedAtActionResult AcceptedAtAction(string actionName, object routeValues, [ActionResultObjectValue] object value);
public virtual AcceptedAtActionResult AcceptedAtAction(string actionName);
public virtual AcceptedAtActionResult AcceptedAtAction(string actionName, string controllerName);
public virtual AcceptedAtActionResult AcceptedAtAction(string actionName, [ActionResultObjectValue] object value);
public virtual AcceptedAtActionResult AcceptedAtAction(string actionName, string controllerName, [ActionResultObjectValue] object routeValues);
public virtual AcceptedAtRouteResult AcceptedAtRoute(string routeName);
public virtual AcceptedAtRouteResult AcceptedAtRoute(object routeValues, [ActionResultObjectValue] object value);
public virtual AcceptedAtRouteResult AcceptedAtRoute(string routeName, object routeValues, [ActionResultObjectValue] object value);
public virtual AcceptedAtRouteResult AcceptedAtRoute(string routeName, object routeValues);
public virtual AcceptedAtRouteResult AcceptedAtRoute([ActionResultObjectValue] object routeValues);
public virtual BadRequestResult BadRequest();
public virtual BadRequestObjectResult BadRequest([ActionResultObjectValue] object error);
public virtual BadRequestObjectResult BadRequest([ActionResultObjectValue] ModelStateDictionary modelState);
public virtual ChallengeResult Challenge(params string[] authenticationSchemes);
public virtual ChallengeResult Challenge();
public virtual ChallengeResult Challenge(AuthenticationProperties properties);
public virtual ChallengeResult Challenge(AuthenticationProperties properties, params string[] authenticationSchemes);
public virtual ConflictResult Conflict();
public virtual ConflictObjectResult Conflict([ActionResultObjectValue] object error);
public virtual ConflictObjectResult Conflict([ActionResultObjectValue] ModelStateDictionary modelState);
public virtual ContentResult Content(string content);
public virtual ContentResult Content(string content, MediaTypeHeaderValue contentType);
public virtual ContentResult Content(string content, string contentType, Encoding contentEncoding);
public virtual ContentResult Content(string content, string contentType);
public virtual CreatedResult Created(Uri uri, [ActionResultObjectValue] object value);
public virtual CreatedResult Created(string uri, [ActionResultObjectValue] object value);
public virtual CreatedAtActionResult CreatedAtAction(string actionName, string controllerName, object routeValues, [ActionResultObjectValue] object value);
public virtual CreatedAtActionResult CreatedAtAction(string actionName, object routeValues, [ActionResultObjectValue] object value);
public virtual CreatedAtActionResult CreatedAtAction(string actionName, [ActionResultObjectValue] object value);
public virtual CreatedAtRouteResult CreatedAtRoute(object routeValues, [ActionResultObjectValue] object value);
public virtual CreatedAtRouteResult CreatedAtRoute(string routeName, object routeValues, [ActionResultObjectValue] object value);
public virtual CreatedAtRouteResult CreatedAtRoute(string routeName, [ActionResultObjectValue] object value);
public virtual FileStreamResult File(Stream fileStream, string contentType, string fileDownloadName);
public virtual VirtualFileResult File(string virtualPath, string contentType, string fileDownloadName, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag, bool enableRangeProcessing);
public virtual VirtualFileResult File(string virtualPath, string contentType, string fileDownloadName, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag);
public virtual VirtualFileResult File(string virtualPath, string contentType, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag, bool enableRangeProcessing);
public virtual VirtualFileResult File(string virtualPath, string contentType, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag);
public virtual VirtualFileResult File(string virtualPath, string contentType, string fileDownloadName, bool enableRangeProcessing);
public virtual VirtualFileResult File(string virtualPath, string contentType, string fileDownloadName);
public virtual VirtualFileResult File(string virtualPath, string contentType, bool enableRangeProcessing);
public virtual VirtualFileResult File(string virtualPath, string contentType);
public virtual FileStreamResult File(Stream fileStream, string contentType, string fileDownloadName, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag, bool enableRangeProcessing);
public virtual FileStreamResult File(Stream fileStream, string contentType, string fileDownloadName, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag);
public virtual FileStreamResult File(Stream fileStream, string contentType, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag, bool enableRangeProcessing);
public virtual FileStreamResult File(Stream fileStream, string contentType, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag);
public virtual FileStreamResult File(Stream fileStream, string contentType, string fileDownloadName, bool enableRangeProcessing);
public virtual FileStreamResult File(Stream fileStream, string contentType);
public virtual FileContentResult File(byte[] fileContents, string contentType, string fileDownloadName, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag, bool enableRangeProcessing);
public virtual FileContentResult File(byte[] fileContents, string contentType, string fileDownloadName, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag);
public virtual FileContentResult File(byte[] fileContents, string contentType, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag, bool enableRangeProcessing);
public virtual FileContentResult File(byte[] fileContents, string contentType, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag);
public virtual FileContentResult File(byte[] fileContents, string contentType, string fileDownloadName, bool enableRangeProcessing);
public virtual FileContentResult File(byte[] fileContents, string contentType, string fileDownloadName);
public virtual FileContentResult File(byte[] fileContents, string contentType, bool enableRangeProcessing);
public virtual FileContentResult File(byte[] fileContents, string contentType);
public virtual FileStreamResult File(Stream fileStream, string contentType, bool enableRangeProcessing);
public virtual ForbidResult Forbid(params string[] authenticationSchemes);
public virtual ForbidResult Forbid();
public virtual ForbidResult Forbid(AuthenticationProperties properties, params string[] authenticationSchemes);
public virtual ForbidResult Forbid(AuthenticationProperties properties);
public virtual LocalRedirectResult LocalRedirect(string localUrl);
public virtual LocalRedirectResult LocalRedirectPermanent(string localUrl);
public virtual LocalRedirectResult LocalRedirectPermanentPreserveMethod(string localUrl);
public virtual LocalRedirectResult LocalRedirectPreserveMethod(string localUrl);
public virtual NoContentResult NoContent();
public virtual NotFoundObjectResult NotFound([ActionResultObjectValue] object value);
public virtual NotFoundResult NotFound();
public virtual OkResult Ok();
public virtual OkObjectResult Ok([ActionResultObjectValue] object value);
public virtual PhysicalFileResult PhysicalFile(string physicalPath, string contentType, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag);
public virtual PhysicalFileResult PhysicalFile(string physicalPath, string contentType, string fileDownloadName, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag, bool enableRangeProcessing);
public virtual PhysicalFileResult PhysicalFile(string physicalPath, string contentType, string fileDownloadName, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag);
public virtual PhysicalFileResult PhysicalFile(string physicalPath, string contentType, DateTimeOffset? lastModified, EntityTagHeaderValue entityTag, bool enableRangeProcessing);
public virtual PhysicalFileResult PhysicalFile(string physicalPath, string contentType, string fileDownloadName, bool enableRangeProcessing);
public virtual PhysicalFileResult PhysicalFile(string physicalPath, string contentType, string fileDownloadName);
public virtual PhysicalFileResult PhysicalFile(string physicalPath, string contentType, bool enableRangeProcessing);
public virtual PhysicalFileResult PhysicalFile(string physicalPath, string contentType);
public virtual ObjectResult Problem(string detail = null, string instance = null, int? statusCode = null, string title = null, string type = null);
public virtual RedirectResult Redirect(string url);
public virtual RedirectResult RedirectPermanent(string url);
public virtual RedirectResult RedirectPermanentPreserveMethod(string url);
public virtual RedirectResult RedirectPreserveMethod(string url);
public virtual RedirectToActionResult RedirectToAction(string actionName, string controllerName, string fragment);
public virtual RedirectToActionResult RedirectToAction(string actionName, string controllerName, object routeValues);
public virtual RedirectToActionResult RedirectToAction(string actionName, string controllerName, object routeValues, string fragment);
public virtual RedirectToActionResult RedirectToAction(string actionName);
public virtual RedirectToActionResult RedirectToAction(string actionName, object routeValues);
public virtual RedirectToActionResult RedirectToAction();
public virtual RedirectToActionResult RedirectToAction(string actionName, string controllerName);
public virtual RedirectToActionResult RedirectToActionPermanent(string actionName, string controllerName, object routeValues, string fragment);
public virtual RedirectToActionResult RedirectToActionPermanent(string actionName, string controllerName, object routeValues);
public virtual RedirectToActionResult RedirectToActionPermanent(string actionName, string controllerName, string fragment);
public virtual RedirectToActionResult RedirectToActionPermanent(string actionName, string controllerName);
public virtual RedirectToActionResult RedirectToActionPermanent(string actionName, object routeValues);
public virtual RedirectToActionResult RedirectToActionPermanent(string actionName);
public virtual RedirectToActionResult RedirectToActionPermanentPreserveMethod(string actionName = null, string controllerName = null, object routeValues = null, string fragment = null);
public virtual RedirectToActionResult RedirectToActionPreserveMethod(string actionName = null, string controllerName = null, object routeValues = null, string fragment = null);
public virtual RedirectToPageResult RedirectToPage(string pageName);
public virtual RedirectToPageResult RedirectToPage(string pageName, string pageHandler, object routeValues, string fragment);
public virtual RedirectToPageResult RedirectToPage(string pageName, string pageHandler, string fragment);
public virtual RedirectToPageResult RedirectToPage(string pageName, string pageHandler, object routeValues);
public virtual RedirectToPageResult RedirectToPage(string pageName, string pageHandler);
public virtual RedirectToPageResult RedirectToPage(string pageName, object routeValues);
public virtual RedirectToPageResult RedirectToPagePermanent(string pageName, string pageHandler, string fragment);
public virtual RedirectToPageResult RedirectToPagePermanent(string pageName);
public virtual RedirectToPageResult RedirectToPagePermanent(string pageName, object routeValues);
public virtual RedirectToPageResult RedirectToPagePermanent(string pageName, string pageHandler);
public virtual RedirectToPageResult RedirectToPagePermanent(string pageName, string pageHandler, object routeValues, string fragment);
public virtual RedirectToPageResult RedirectToPagePermanentPreserveMethod(string pageName, string pageHandler = null, object routeValues = null, string fragment = null);
public virtual RedirectToPageResult RedirectToPagePreserveMethod(string pageName, string pageHandler = null, object routeValues = null, string fragment = null);
public virtual RedirectToRouteResult RedirectToRoute(string routeName);
public virtual RedirectToRouteResult RedirectToRoute(string routeName, object routeValues);
public virtual RedirectToRouteResult RedirectToRoute(string routeName, string fragment);
public virtual RedirectToRouteResult RedirectToRoute(string routeName, object routeValues, string fragment);
public virtual RedirectToRouteResult RedirectToRoute(object routeValues);
public virtual RedirectToRouteResult RedirectToRoutePermanent(string routeName, string fragment);
public virtual RedirectToRouteResult RedirectToRoutePermanent(string routeName, object routeValues);
public virtual RedirectToRouteResult RedirectToRoutePermanent(string routeName, object routeValues, string fragment);
public virtual RedirectToRouteResult RedirectToRoutePermanent(string routeName);
public virtual RedirectToRouteResult RedirectToRoutePermanent(object routeValues);
public virtual RedirectToRouteResult RedirectToRoutePermanentPreserveMethod(string routeName = null, object routeValues = null, string fragment = null);
public virtual RedirectToRouteResult RedirectToRoutePreserveMethod(string routeName = null, object routeValues = null, string fragment = null);
public virtual SignInResult SignIn(ClaimsPrincipal principal, string authenticationScheme);
public virtual SignInResult SignIn(ClaimsPrincipal principal, AuthenticationProperties properties);
public virtual SignInResult SignIn(ClaimsPrincipal principal, AuthenticationProperties properties, string authenticationScheme);
public virtual SignInResult SignIn(ClaimsPrincipal principal);
public virtual SignOutResult SignOut();
public virtual SignOutResult SignOut(AuthenticationProperties properties);
public virtual SignOutResult SignOut(params string[] authenticationSchemes);
public virtual SignOutResult SignOut(AuthenticationProperties properties, params string[] authenticationSchemes);
public virtual StatusCodeResult StatusCode([ActionResultStatusCode] int statusCode);

You should be able to migrate your existing code into ASP.NET Core 2.x by using Microsoft.AspNetCore.Mvc.WebApiCompatShim nuget package.

Then you may do one of the following:

1 use ResponseMessageResult

return new ResponseMessageResult(result);

2 inherit your Controller from ApiController and leave your code as is:

public class MyController : ApiController 
{
    public IActionResult Get()
    {
        // other code...
        HttpResponseMessage result = await client.SendAync(request);
        if (result.StatusCode != HttpStatusCode.OK)
        {
            IActionResult response = ResponseMessage(result);
            return response;
        }
        // other code...
    }    
}

More information inside the official documentation