Hello,

But getting this error:

The collection has not been initialized. It has not been requested or the request has not been executed. It may need to be explicitly requested.Am I passing wrong parameter in SaveBinaryDirect.

try
            {
                AttachmentCreationInformation at = null;
                foreach (Attachment filename in itemSource.AttachmentFiles)
                {
                    Microsoft.SharePoint.Client.File file = itemSource.ParentList.ParentWeb.GetFileByServerRelativeUrl(filename.ServerRelativeUrl);
                    try
                    {
                        ctxSource.Load(file);
                        ctxSource.ExecuteQuery();
                    }
                    catch (Exception)
                    {
                        
                        //throw;
                    }
                    var imageData = file.OpenBinaryStream();

                    //StreamReader fsReader = new StreamReader(file.OpenBinaryStream());
                    string url = ctxDestination.Url + "Lists/Dokumente1/Attachments/" + lstItem.Id + "/" +file.Name; 
                    Microsoft.SharePoint.Client.File.SaveBinaryDirect(ctxDestination, url, file.OpenBinaryStream().Value, true);
                    //at = new AttachmentCreationInformation { FileName = file.Name, ContentStream = imageData.Value };
                    //lstItem.AttachmentFiles.Add(at);
                }
                lstItem.Update();
                ctxDestination.ExecuteQuery();
            }

Hi,

I hope the below attached thread is same thread what you are trying to discuss here.

https://social.msdn.microsoft.com/Forums/sharepoint/en-US/2fcd4172-366b-464a-b18d-0b583e526222/how-to-upload-attachment-in-sharepoint-list-item?forum=sharepointdevelopment

So you can discuss in the existing thread itself. 

Moderators, please merge these two into one.

Yes, Can you please check the above code what mistake am doing.