Uploading and Downloading large files in ASP.NET Core 3.1?

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If you have files that large, never use byte[] or MemoryStream in your code. Only operate on streams if you download/upload files.

You have a couple of options:

  • If you control both client and server, consider using something like tus. There are both client- and server-implementations for .NET. This would probably the easiest and most robust option.
  • If you upload large files with the HttpClient, simply use the StreamContent class to send them. Again, don’t use a MemoryStream as source, but something else like a FileStream.
  • If you download large files with the HttpClient, it is important to specify the HttpCompletionOptions, for example var response = await httpClient.SendAsync(httpRequest, HttpCompletionOption.ResponseHeadersRead). Otherwise, the HttpClient would buffer the entire response in memory. You can then process the response file as a stream via var stream = response.Content.ReadAsStreamAsync().

ASP.NET Core specific advice:

  • If you want to receive files via HTTP POST, you need to increase the request size limit: [RequestSizeLimit(10L * 1024L * 1024L * 1024L)] and [RequestFormLimits(MultipartBodyLengthLimit = 10L * 1024L * 1024L * 1024L)]. In addition, you need to disable the form value binding, otherwise the whole request will be buffered into memory:
   [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
   public class DisableFormValueModelBindingAttribute : Attribute, IResourceFilter
   {
       public void OnResourceExecuting(ResourceExecutingContext context)
       {
           var factories = context.ValueProviderFactories;
           factories.RemoveType<FormValueProviderFactory>();
           factories.RemoveType<FormFileValueProviderFactory>();
           factories.RemoveType<JQueryFormValueProviderFactory>();
       }

       public void OnResourceExecuted(ResourceExecutedContext context)
       {
       }
   }
  • To return a file from a controller, simple return it via the File method, which accepts a stream: return File(stream, mimeType, fileName);

A sample controller would look like this (see https://docs.microsoft.com/en-us/aspnet/core/mvc/models/file-uploads?view=aspnetcore-3.1 for the missing helper classes):

private const MaxFileSize = 10L * 1024L * 1024L * 1024L; // 10GB, adjust to your need

[DisableFormValueModelBinding]
[RequestSizeLimit(MaxFileSize)]
[RequestFormLimits(MultipartBodyLengthLimit = MaxFileSize)]
public async Task ReceiveFile()
{
    if (!MultipartRequestHelper.IsMultipartContentType(Request.ContentType))
        throw new BadRequestException("Not a multipart request");

    var boundary = MultipartRequestHelper.GetBoundary(MediaTypeHeaderValue.Parse(Request.ContentType));
    var reader = new MultipartReader(boundary, Request.Body);

    // note: this is for a single file, you could also process multiple files
    var section = await reader.ReadNextSectionAsync();

    if (section == null)
        throw new BadRequestException("No sections in multipart defined");

    if (!ContentDispositionHeaderValue.TryParse(section.ContentDisposition, out var contentDisposition))
        throw new BadRequestException("No content disposition in multipart defined");

    var fileName = contentDisposition.FileNameStar.ToString();
    if (string.IsNullOrEmpty(fileName))
    {
        fileName = contentDisposition.FileName.ToString();
    }

    if (string.IsNullOrEmpty(fileName))
        throw new BadRequestException("No filename defined.");

    using var fileStream = section.Body;
    await SendFileSomewhere(fileStream);
}

// This should probably not be inside the controller class
private async Task SendFileSomewhere(Stream stream)
{
    using var request = new HttpRequestMessage()
    {
        Method = HttpMethod.Post,
        RequestUri = new Uri("YOUR_DESTINATION_URI"),
        Content = new StreamContent(stream),
    };
    using var response = await _httpClient.SendAsync(request);
    // TODO check response status etc.
}

In this example, we stream the entire file to another service. In some cases, it would be better to save the file temporarily to the disk.

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