telegram-bot/modules/StickerTools/Handler.cs

using BotFramework;
using BotFramework.Attributes;
using BotFramework.Enums;
using SixLabors.ImageSharp;
using Telegram.Bot;
using Telegram.Bot.Types;
using Telegram.Bot.Types.Enums;

namespace West.TelegramBot.StickerTools;

public class Handler : BotEventHandler
{
    [Command("png", CommandParseMode.Both)]
    public async Task Sticker2Png()
    {
        var sticker = RawUpdate.Message?.ReplyToMessage?.Sticker;
        if (sticker == null || sticker.IsAnimated || sticker.IsVideo)
        {
            return;
        }
        
        await Bot.SendChatAction(Chat.Id, ChatAction.UploadDocument);
        using var file = new MemoryStream();
        await Bot.GetInfoAndDownloadFile(sticker.FileId, file);
        file.Position = 0;
        
        using var img = await Image.LoadAsync(file);
        await file.DisposeAsync();
        
        using var outfile = new MemoryStream();
        await img.SaveAsPngAsync(outfile);
        outfile.Position = 0;
        await Bot.SendDocument(Chat.Id, new InputFileStream(outfile, "sticker.png"), 
            replyParameters: RawUpdate?.Message?.MessageId);
    }
}