Uploading files

First, read the documentation on sending files.

Upload local file

To upload local file open stream and call one of the file-sending functions:

await using Stream stream = System.IO.File.OpenRead(@"../hamlet.pdf");
Message message = await botClient.SendDocumentAsync(
    chatId: chatId,
    document: new InputOnlineFile(content: stream, fileName: "hamlet.pdf"),
    caption: "The Tragedy of Hamlet,\nPrince of Denmark");

Be aware of limitation for this method - 10 MB max size for photos, 50 MB for other files.

Upload file by file identifier

If the file is already stored somewhere on the Telegram servers, you don't need to reupload it: each file object has a FileId property. Simply pass this FileId as a parameter instead of uploading. There are no limits for files sent this way.

var fileId = update.Message.Photo.Last().FileId;
Message message = await botClient.SendPhotoAsync(
    chatId: chatId,
    photo: fileId);

Upload by URL

Provide Telegram with an HTTP URL for the file to be sent. Telegram will download and send the file. 5 MB max size for photos and 20 MB max for other types of content.

Message message = await botClient.SendPhotoAsync(
    chatId: chatId,
    photo: new InputOnlineFile("https://cdn.pixabay.com/photo/2017/04/11/21/34/giraffe-2222908_640.jpg"));