通过 Web API 返回图像时出现问题

我遇到了一个奇怪的问题，通过 web api 返回图像，我在应用程序中的几个地方这样做没有失败，但这给我带来了问题，任何帮助将不胜感激。

这行得通

  Bitmap bitmap = getImage();
        MemoryStream bitmapStream = new MemoryStream();
        bitmap.Save("C:''test.png", System.Drawing.Imaging.ImageFormat.Png);
        if (bitmap != null)
        {
            if (Request.Headers.IfModifiedSince.HasValue)
            {
                // The file has not been modified since the browser cached it.
                return new HttpResponseMessage(HttpStatusCode.NotModified);
            }
            HttpResponseMessage response = new HttpResponseMessage(HttpStatusCode.OK);
            response.Content = new StreamContent(new FileStream("C:''test.png", FileMode.Open));
            response.Content.Headers.ContentType = new MediaTypeHeaderValue("image/png");
            return response;
        }
        else
        {
            return new HttpResponseMessage(HttpStatusCode.NotFound);
        }

虽然这没有

 Bitmap bitmap = getImage();
        MemoryStream bitmapStream = new MemoryStream();
        bitmap.Save(bitmapStream, System.Drawing.Imaging.ImageFormat.Png);
        if (bitmap != null)
        {
            if (Request.Headers.IfModifiedSince.HasValue)
            {
                // The file has not been modified since the browser cached it.
                return new HttpResponseMessage(HttpStatusCode.NotModified);
            }
            HttpResponseMessage response = new HttpResponseMessage(HttpStatusCode.OK);
            response.Content = new StreamContent(bitmapStream);
            response.Content.Headers.ContentType = new MediaTypeHeaderValue("image/png");
            return response;
        }
        else
        {
            return new HttpResponseMessage(HttpStatusCode.NotFound);
        }

我真的需要在内存中执行此操作，

而不是将临时文件保存到驱动器，但是由于某种原因，当我在内存中执行此操作时，结果是一个空文件 0 字节，我确实尝试手动设置内容长度，但随后文件根本不下载。