Return file in ASP.Net Core Web API

You can return FileResult with this methods:

1: Return FileStreamResult

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

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

Here is a simplistic example of streaming a file:

using System.IO;
using Microsoft.AspNetCore.Mvc;

[HttpGet("{id}")]
public async Task<FileStreamResult> Download(int id)
{
    var path = "<Get the file path using the ID>";
    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.

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<any> {
        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<IActionResult> 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();

}

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

<img src="@Url.Action("RenderImage", new { id = id})" alt="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<IActionResult> 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.

add builder.Services.AddSingleton(); in Program.cs

    [HttpGet("{fileId}")]
    public ActionResult GetFile(string fileId)
    {
        string pathToFile = "test.rar";
        if (!System.IO.File.Exists(pathToFile))
        {
            return NotFound();
        }

        if(!_fileExtensionContentTypeProvider.TryGetContentType(pathToFile,
            out var ContentType))
        {
            ContentType = "application/octet-stream";
        }
        var byets=System.IO.File.ReadAllBytes(pathToFile);
        return File(byets, ContentType, Path.GetFileName(pathToFile));
    }
}

For anyone wondering how it is being done with Results<> instead of IActionResult.

public async Task<Results<FileStreamHttpResult, NotFound>> Download(string id)
{
    Stream stream = await GetMyStreamAsync();

    if(stream == null)
        return TypedResults.NotFound();

    return TypedResults.File(stream, contentType: "application/octet-stream", fileDownloadName: "filename.ext");
}