调整JPEG品质时出错：值不能为空参数名称：结构

Question

I have a method where I take an image, make a copy for resizing, and then return it as a byte array. I have attempted to add the ability to adjust the quality of the image per this doc:

http://msdn.microsoft.com/en-us/library/bb882583(v=vs.110).aspx

Image image = Image.FromFile(_filepath);

Image newImage = new Bitmap(newWidth, newHeight);
using (Graphics graphicsHandle = Graphics.FromImage(newImage))
{
    graphicsHandle.InterpolationMode = InterpolationMode.HighQualityBicubic;
    graphicsHandle.DrawImage(image, 0, 0, newWidth, newHeight);
}
MemoryStream memStream = new MemoryStream();

EncoderParameters myEncoderParameters = new EncoderParameters(1);

ImageCodecInfo encoder = ImageCodecInfo.GetImageDecoders().SingleOrDefault(c => c.MimeType == contentType);
System.Drawing.Imaging.Encoder myEncoder = System.Drawing.Imaging.Encoder.Quality;
EncoderParameter myEncoderParameter = new EncoderParameter(myEncoder, 50L);
newImage.Save(memStream, encoder, myEncoderParameters);

return memStream.ToArray();

When I call newImage.Save() , I get this error:

An exception of type 'System.ArgumentNullException' occurred in System.Drawing.dll but was not handled in user code

Message is:

Value cannot be null. Parameter name: structure

Answer 1

MSDN says that you have a ArgumentNullException when calling this Save() overload only if the stream is NULL .

Are you sure that no other (omitted?) code is not nullifying this stream? Are you sure this is the site of the exception?

Answer 2

Forgot a step. I created the EncoderParameter object, but didn't include it in the EncoderParameters collection. The message confused me a bit.

All resolved once I added:

myEncoderParameters.Param[0] = myEncoderParameter;