Save animated gif using System.Drawing

// Gdi+ constants absent from System.Drawing.
const int PropertyTagFrameDelay = 0x5100;
const int PropertyTagLoopCount = 0x5101;
const short PropertyTagTypeLong = 4;
const short PropertyTagTypeShort = 3;

// ...

var gifEncoder = GetEncoder(ImageFormat.Gif);
// Params of the first frame.
var encoderParams1 = new EncoderParameters(1);
encoderParams1.Param[0] = new EncoderParameter(Encoder.SaveFlag, (long)EncoderValue.MultiFrame);
// Params of other frames.
var encoderParamsN = new EncoderParameters(1);
encoderParamsN.Param[0] = new EncoderParameter(Encoder.SaveFlag, (long)EncoderValue.FrameDimensionTime);
// Params for the finalizing call.
var encoderParamsFlush = new EncoderParameters(1);
encoderParamsFlush.Param[0] = new EncoderParameter(Encoder.SaveFlag, (long)EncoderValue.Flush);

// PropertyItem for the frame delay (apparently, no other way to create a fresh instance).
var frameDelay = (PropertyItem)FormatterServices.GetUninitializedObject(typeof(PropertyItem));
frameDelay.Id = PropertyTagFrameDelay;
frameDelay.Type = PropertyTagTypeLong;
// Length of the value in bytes.
frameDelay.Len = Bitmaps.Count * 4;
// The value is an array of 4-byte entries: one per frame.
// Every entry is the frame delay in 1/100-s of a second, in little endian.
frameDelay.Value = new byte[Bitmaps.Count * UintBytes];
// E.g., here, we're setting the delay of every frame to 1 second.
var frameDelayBytes = BitConverter.GetBytes((uint)100);
for (int j = 0; j < Bitmaps.Count; ++j)
    Array.Copy(frameDelayBytes, 0, frameDelay.Value, j * UintBytes, UintBytes);

// PropertyItem for the number of animation loops.
var loopPropertyItem = (PropertyItem)FormatterServices.GetUninitializedObject(typeof(PropertyItem));
loopPropertyItem.Id = PropertyTagLoopCount;
loopPropertyItem.Type = PropertyTagTypeShort;
loopPropertyItem.Len = 1;
// 0 means to animate forever.
loopPropertyItem.Value = BitConverter.GetBytes((ushort)0);

using (var stream = new FileStream("animation.gif", FileMode.Create))
{
    bool first = true;
    Bitmap firstBitmap = null;
    // Bitmaps is a collection of Bitmap instances that'll become gif frames.
    foreach (var bitmap in Bitmaps)
    {
        if (first)
        {
            firstBitmap = bitmap;
            firstBitmap.SetPropertyItem(frameDelay);
            firstBitmap.SetPropertyItem(loopPropertyItem);
            firstBitmap.Save(stream, gifEncoder, encoderParams1);
            first = false;
        }
        else
        {
            firstBitmap.SaveAdd(bitmap, encoderParamsN);
        }
    }
    firstBitmap.SaveAdd(encoderParamsFlush);
}

// ...

private ImageCodecInfo GetEncoder(ImageFormat format)
{

    ImageCodecInfo[] codecs = ImageCodecInfo.GetImageDecoders();
    foreach (ImageCodecInfo codec in codecs)
    {
        if (codec.FormatID == format.Guid)
        {
            return codec;
        }
    }
    return null;
}