从 stream 在 c# 中创建缩略图

Question

经过 3 个小时的调试和搜索，我有了这个。

        public Stream ResizeImageToThumbnail(Stream imageStream, int width)
        {
            var image = Image.FromStream(imageStream);

            var height = (width * image.Height) / image.Width;
            var thumbnail = image.GetThumbnailImage(width, height, null, IntPtr.Zero);

            using var thumbnailStream = new MemoryStream();
            thumbnail.Save(thumbnailStream, ImageFormat.Jpeg);
            return thumbnailStream;
        }

问题是，它返回异常

ArgumentException：参数无效。

在

var image = Image.FromStream(imageStream);

参数为file.OpenFileStream(); 其中file是IFormFile 。

我没主意了。

编辑

要求的代码：

foreach (var item in model.UploadedImages) //item = IFormFile, image only allowed from HTML's end
{
   using var ms = item.OpenReadStream();
   _service.AttachImage(newId, ms, item.FileName);
   ms.Position = 0;
   _service.AttachThumb(newId, ms, item.FileName); //cannot access a closed stream exception
}

编辑 2

AttachThumb 使用封闭的 stream，返回异常（检查注释）。 ResizeImageToThumbnail似乎返回关闭的 stream。

public void AttachThumb(Guid id, Stream imageStream, string imageName)
        {
            Post post = GetPost(id);

            ObjectId imageId = _gridFS.UploadFromStream(imageName, ResizeImageToThumbnail(imageStream, 640)); //cannot use closed stream

            post.ImagesThumbs.Add(imageId.ToString());

            var filter = Builders<Post>.Filter.Eq(x => x.Id, id);
            var update = Builders<Post>.Update.Set("ImagesThumbs", post.ImagesThumbs);
            _posts.UpdateOne(filter, update);
        }

        public Stream ResizeImageToThumbnail(Stream imageStream, int width)
        {
            var image = Image.FromStream(imageStream);

            var height = (width * image.Height) / image.Width;
            var thumbnail = image.GetThumbnailImage(width, height, null, IntPtr.Zero);

            using var thumbnailStream = new MemoryStream();
            thumbnail.Save(thumbnailStream, ImageFormat.Jpeg);
            return thumbnailStream;
        }

Answer 1

首先将文件内容复制到 stream 中，然后使用该 stream。 在两者之间倒带 stream。 请注意，两个 Attach* 方法不得异步访问 stream。 否则，您必须先创建第二个内存副本。

foreach (var item in model.UploadedImages)
{
    // Copy content in to stream
    using (MemoryStream stream = new MemoryStream())
    {
        item.CopyTo(stream);

        // Rewind and use
        stream.Position = 0;
        _service.AttachImage(newId, stream, item.FileName);
        
        // Rewind and use
        stream.Position = 0;
        _service.AttachThumb(newId, stream, item.FileName);
    }
}

---

正如用户 Jimi 所指出的，您必须从ResizeImageToThumbnail方法中删除using子句。 否则，该方法返回一个已经释放的MemoryStream ：

public Stream ResizeImageToThumbnail(Stream imageStream, int width)
{
    var image = Image.FromStream(imageStream);

    var height = (width * image.Height) / image.Width;
    var thumbnail = image.GetThumbnailImage(width, height, null, IntPtr.Zero);

    // Bad: using var thumbnailStream = new MemoryStream();
    // Remove using
    var thumbnailStream = new MemoryStream(); 
    thumbnail.Save(thumbnailStream, ImageFormat.Jpeg);
    return thumbnailStream;
}

然后将 stream 放在外面：

public void AttachThumb(Guid id, Stream imageStream, string imageName)
{
    Post post = GetPost(id);

    using var thumbnailStream = ResizeImageToThumbnail(imageStream, 640);
    // rewind stream
    thumbnailStream.position = 0;
    ObjectId imageId = _gridFS.UploadFromStream(imageName, thumbnailStream); 

    ...

---

您可以通过不让ResizeImageToThumbnail方法创建 stream 而是在外部创建 stream 并将其作为参数传递给ResizeImageToThumbnail来改进此设计。 所以ResizeImageToThumbnail的调用者然后负责创建和处置 stream。