Asp.net MVC异步上传问题

Question

We are working on a file upload and have chosen the blueimp plugin. We have this working in the following way.

User selects all files they wish to upload and clicks a button to upload all in one go.

This part works fine and sends all files at once to the server.

The server code sample only is as follows.

public async Task<ActionResult> Upload()
{
    var result = await new Uploader().UploadFile(Request.Files[0]);
    return Json(result);
}
public class Uploader
{
    public async Task<UploadResult> UploadFile(HttpPostedFileBase file)
    {
        //do file processing and save file to disk
        var filerecord = _fileService.GetById(xxxx);
        filerecord.files.Add(new Info { name = file.FileName, size = file.ContentLength });
        //Here for us its Mongo but any not sure what the result would be against another db
        await _fileService.SaveAsync(filerecord);
        return new UploadResult { message = "ok" };
    }
}

//js
    fi.fileupload({
        url: 'uploadfile',
        dataType: 'json',
        autoUpload: false,
        singleFileUploads:false,
        maxNumberOfFiles: maxFiles,
        acceptFileTypes: /(\.|\/)(jpe?g)$/i,
        maxFileSize: 1048576*2, //1MB
        messages: { },
        // Enable image resizing, except for Android and Opera,
        // which actually support image resizing, but fail to
        // send Blob objects via XHR requests:
        disableImageResize: true,
        previewMaxWidth: 50,
        previewMaxHeight: 50,
        previewCrop: false,
        dropZone: $('#dropzone')
    });

    fi.on('fileuploadadd', function (e, data) {

        $('form').submit(function (e) {
            data.submit();
            return false;
        });
});

Just a note that the db in this case is Mongo

The Problem Uploading single files at a time ie select a file click upload all works as expected. Select more than one file and hit upload button all files are uploaded and saved to the disk without an issue. However I would expect the following behaviour

1. Get record from db
2. Add info about the file
3. Save the record

As mentioned this works great with one file at a time but when they all come in at once I am guessing the async behaviour causes the method to loose context of what file its working on as the 3 steps in the debugger seem to be called per file randomly rather than in order.

I have tried various approaches like change the order of the way things happen, also using ConfigureAwait etc.

Any advice would be appreciated.

Answer 1

If i understand your case correctly this code should work for you:

public async Task<ActionResult> Upload()
{
    var files = new HttpPostedFileBase[Request.Files.Count];
    Request.Files.CopyTo(files, 0);
    var tasks = files.Select(f=>new Uploader().UploadFile(f));
    await Task.WhenAll(tasks);
    return Json(tasks.Where(v=>v.Status == TaskStatus.RanToCompletion ).Select(v=>v.Result).ToList());
}