Problem

I want to return a file in my ASP.Net Web API Controller, but all my approaches return the HttpResponseMessage as JSON.

Code so far

public async Task DownloadAsync(string id)
{
    var response = new HttpResponseMessage(HttpStatusCode.OK);
    response.Content = new StreamContent({{__insert_stream_here__}});
    response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
    return response;
}

When I call this endpoint in my browser, the Web API returns the HttpResponseMessage as JSON with the HTTP Content Header set to application/json.

Ответы (6)

If this is ASP.net-Core then you are mixing web API versions. Have the action return a derived IActionResult because in your current code the framework is treating HttpResponseMessage as a model.

[Route("api/[controller]")]
public class DownloadController : Controller {
    //GET api/download/12345abc
    [HttpGet("{id}")]
    public async Task Download(string id) {
        Stream stream = await {{__get_stream_based_on_id_here__}}

        if(stream == null)
            return NotFound(); // returns a NotFoundResult with Status404NotFound response.

        return File(stream, "application/octet-stream"); // returns a FileStreamResult
    }    
}

Note:

The framework will dispose of the stream used in this case when the response is completed. If a using statement is used, the stream will be disposed before the response has been sent and result in an exception or corrupt response.

You can return FileResult with this methods:

1: Return FileStreamResult

    [HttpGet("get-file-stream/{id}"]
    public async Task DownloadAsync(string id)
    {
        var fileName="myfileName.txt";
        var mimeType="application/...."; 
        Stream stream = await GetFileStreamById(id);

        return new FileStreamResult(stream, mimeType)
        {
            FileDownloadName = fileName
        };
    }

2: Return FileContentResult

    [HttpGet("get-file-content/{id}"]
    public async Task DownloadAsync(string id)
    {
        var fileName="myfileName.txt";
        var mimeType="application/...."; 
        byte[] fileBytes = await GetFileBytesById(id);

        return new FileContentResult(fileBytes, mimeType)
        {
            FileDownloadName = fileName
        };
    }

Following is the basic example of returning file (e.g Image file) in .NET Core Web API:

No Image found

Below is the code for returning File from controller to view. Following is Action method which will return file:

    [Route("api/[controller]")]
    public class DownloadController : Controller
    {
        //GET api/download/123
        [HttpGet]
        public async Task RenderImage(string userId)
        {
            //get Image file using _fileservice from db
            var result = await _fileService.getFile(userId);

            if (result.byteStream == null)
                return NotFound();

            return File(result.byteStream, result.ContentType, result.FileName);
        }
    }

Note:

Our file should be first converted into byte[] and then saved in database as varbinary(max) in order to retrieve.

FileStreamResult works for me. and File is not an IActionResult. I don't know how it can work.

ASP.NET 5 WEB API & Angular 12

You can return a FileContentResult object (Blob) from the server. It'll not get automatically downloaded. You may create an anchor tag in your front-end app programmatically and set the href property to an object URL created from the Blob by the method below. Now clicking on the anchor will download the file. You can set a file name by setting the 'download' attribute to the anchor as well.

downloadFile(path: string): Observable {
        return this._httpClient.post(`${environment.ApiRoot}/accountVerification/downloadFile`, { path: path }, {
            observe: 'response',
            responseType: 'blob'
        });
    }

saveFile(path: string, fileName: string): void {
            this._accountApprovalsService.downloadFile(path).pipe(
                take(1)
            ).subscribe((resp) => {
                let downloadLink = document.createElement('a');
                downloadLink.href = window.URL.createObjectURL(resp.body);
                downloadLink.setAttribute('download', fileName);
                document.body.appendChild(downloadLink);
                downloadLink.click();
                downloadLink.remove();
            });
            
        }

Backend

[HttpPost]
[Authorize(Roles = "SystemAdmin, SystemUser")]
public async Task DownloadFile(FilePath model)
{
    if (ModelState.IsValid)
    {
        try
        {
            var fileName = System.IO.Path.GetFileName(model.Path);
            var content = await System.IO.File.ReadAllBytesAsync(model.Path);
            new FileExtensionContentTypeProvider()
                .TryGetContentType(fileName, out string contentType);
            return File(content, contentType, fileName);
        }
        catch
        {
            return BadRequest();
        }
    }

    return BadRequest();

}

Here is a simplistic example of streaming a file:

using System.IO;
using Microsoft.AspNetCore.Mvc;
[HttpGet("{id}")]
public async Task Download(int id)
{
    var path = "";
    var stream = File.OpenRead(path);
    return new FileStreamResult(stream, "application/octet-stream");
}

Note:

Be sure to use FileStreamResult from Microsoft.AspNetCore.Mvc and not from System.Web.Mvc.

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