如何使用C＃正确创建缩略图？

Question

I have a console application written using C# on the top of Core .NET 2.2 framework.

I want to create asynchronous Task that would write a full-size image to storage. Additionally, the process will need to create a thumbnail and write it to the default storage.

Follow is the method that processes the logic. I documented each line to explain that I believe is happening

// This method accepts FileObject and returns a task
// The generated task will write the file as is to the default storage
// Then it'll create a thumbnail of that images and store it to the default storage
public async Task ProcessImage(FileObject file, int thumbnailWidth = 250)
{
    // The name of the full-size image
    string filename = string.Format("{0}{1}", file.ObjectId, file.Extension);

    // The name along with the exact path to the full-size image
    string path = Path.Combine(file.ContentId, filename);

    // Write the full-size image to the storage
    await Storage.CreateAsync(file.Content, path)
                 .ContinueWith(task =>
    {
        // Reset the stream to the beginning since this will be the second time the stream is read
        file.Content.Seek(0, SeekOrigin.Begin);

        // Create original Image
        Image image = Image.FromStream(file.Content);

        // Calulate the height of the new thumbnail
        int height = (thumbnailWidth * image.Height) / image.Width;

        // Create the new thumbnail
        Image thumb = image.GetThumbnailImage(thumbnailWidth, height, null, IntPtr.Zero);

        using (MemoryStream thumbnailStream = new MemoryStream())
        {
            // Save the thumbnail to the memory stream
            thumb.Save(thumbnailStream, image.RawFormat);

            // The name of the new thumbnail
            string thumbnailFilename = string.Format("thumbnail_{0}", filename);

            // The name along with the exact path to the thumbnail
            string thumbnailPath = Path.Combine(file.ContentId, thumbnailFilename);

            // Write the thumbnail to storage
            Storage.CreateAsync(thumbnailStream, thumbnailPath);
        }

        // Dispose the file object to ensure the Stream is disposed
        file.Dispose();
        image.Dispose();
        thumb.Dispose();
    });
}

Here is my FileObject if needed

public class FileObject : IDisposable
{
    public string ContentId { get; set; }
    public string ObjectId { get; set; }
    public ContentType ContentType { get; set; }
    public string Extension { get; set; }
    public Stream Content { get; set; }
    private bool IsDisposed;

    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (IsDisposed)
            return;

        if (disposing && Content != null)
        {
            Content.Close();
            Content.Dispose();
        }

        IsDisposed = true;
    }
}

The above code writes the correct full-size image to the storage drive. It also writes the thumbnail to storage. However, the thumbnail is always corrupted. In other words, the generated thumbnail file is always written with 0 bytes.

How can I correctly create my thumbnail from file.Content stream after writing the same stream to the storage?

Answer 1

I figured out the cause of the issue. For some reason the line thumb.Save(thumbnailStream, image.RawFormat); position the thumbnailStream at the end and when writing to the storage nothing gets written

Fixing that issues was to reset the seek position after writing to the stream like this

    using (MemoryStream thumbnailStream = new MemoryStream())
    {
        // Save the thumbnail to the memory stream
        thumb.Save(thumbnailStream, image.RawFormat);

        // Reset the seek position to the begining
        thumbnailStream.Seek(0, SeekOrigin.Begin);

        // The name of the new thumbnail
        string thumbnailFilename = string.Format("thumbnail_{0}", filename);

        // The name along with the exact path to the thumbnail
        string thumbnailPath = Path.Combine(file.ContentId, thumbnailFilename);

        // Write the thumbnail to storage
        Storage.CreateAsync(thumbnailStream, thumbnailPath);
    }

I am not sure what is the benefit that is gained when thumb.Save(...) does not reset the position to 0 after copying into a new stream! I just feel that it should be doing that since it will always write a new stream not appending to an existing one.