C# read IFormFile into byte[]

Question

I am trying to read an IFormFile received from a HTTP POST request like this:

 public async Task<ActionResult> UploadDocument([FromForm]DataWrapper data)
    {
        IFormFile file = data.File;
        string fileName = file.FileName;
        long length = file.Length;
        if (length < 0)
            return BadRequest();
        using FileStream fileStream = new FileStream(fileName, FileMode.OpenOrCreate);
        byte[] bytes = new byte[length];
        fileStream.Read(bytes, 0, (int)file.Length);

        ...

    }

but something is wrong, after this line executes:

fileStream.Read(bytes, 0, (int)file.Length);

all of the elements of bytes are zero.

Also, the file with the same name is created in my Visual Studio project, which I would prefer not to happen.

Answer 1

You can't open an IFormFile the same way you would a file on disk. You'll have to use IFormFile.OpenReadStream() instead. Docs here

public async Task<ActionResult> UploadDocument([FromForm]DataWrapper data)
{
    IFormFile file = data.File;

    long length = file.Length;
    if (length < 0)
        return BadRequest();

    using var fileStream = file.OpenReadStream();
    byte[] bytes = new byte[length];
    fileStream.Read(bytes, 0, (int)file.Length);

}

The reason that fileStream.Read(bytes, 0, (int)file.Length); appears to be empty is, because it is. The IFormFile.Filename is the name of the file given by the request and doesn't exist on disk.

Answer 2

Your code's intent seems to be to write to a FileStream, not a byte buffer. What it actually does though, is create a new empty file and read from it into an already cleared buffer. The uploaded file is never used.

Writing to a file

If you really want to save the file, you can use CopyTo :

using(var stream = File.Create(Path.Combine(folder_I_Really_Want,file.FileName))
{
    file.CopyTo(stream);
}

If you want to read from the uploaded file into a buffer without saving to disk, use a MemoryStream. That's just a Stream API buffer over a byte[] buffer. You don't have to specify the size but that reduces reallocations as the internal buffer grows.

Reading into byte[]

Reading into a byte[] through MemoryStream is essentially the same :

var stream = new MemoryStream(file.Length);
file.CopyTo(stream);
var bytes=stream.ToArray();