[Exchange-Online] [EWSMA] [C#] [Windows]: EWS - CreateAttachment problem - The underlying connection was closed: An unexpected error occurred on a send

Question

Hello,
I have a problem with
ExchangeServiceBinding.CreateAttachment(...) method (EWS with Office 365)
When I try to add a small attachment (e.g. 5 MB), then the method works fine.
When I try to add a big attachment (e.g.
12 MB), then the following exception is thrown:
 System.Net.WebException
The underlying connection was closed: An unexpected error occurred on a send.
   at System.Web.Services.Protocols.WebClientProtocol.GetWebResponse(WebRequest request)
   at System.Web.Services.Protocols.HttpWebClientProtocol.GetWebResponse(WebRequest request)
   at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
   at C2EWS.Proxy.ExchangeServiceBinding.CreateAttachment(CreateAttachmentType CreateAttachment1) in d:\source\EWSProxyClasses.cs:line 1471
   at EWSRequest.Program.UploadAttachment(ExchangeServiceBinding esb, ItemIdType parentItemIdType) in d:\source\EWSRequest\Program.cs:line 90
   at EWSRequest.Program.Main(String[] args) in d:\source\EWSRequest\Program.cs:line 59
Inner exception:
 System.IO.IOException
 Unable to write data to the transport connection: An existing connection was forcibly closed by the remote host.
    at System.Net.Sockets.NetworkStream.MultipleWrite(BufferOffsetSize[] buffers)
    at System.Net.Security._SslStream.StartWriting(SplitWritesState splitWrite, SplitWriteAsyncProtocolRequest asyncRequest)
    at System.Net.Security._SslStream.ProcessWrite(BufferOffsetSize[] buffers, SplitWriteAsyncProtocolRequest asyncRequest)
    at System.Net.TlsStream.MultipleWrite(BufferOffsetSize[] buffers)
    at System.Net.Connection.Write(ScatterGatherBuffers writeBuffer)
    at System.Net.ConnectStream.ResubmitWrite(ConnectStream oldStream, Boolean suppressWrite)
Inner exception:
  System.Net.Sockets.SocketException
  An existing connection was forcibly closed by the remote host
     at System.Net.Sockets.Socket.MultipleSend(BufferOffsetSize[] buffers, SocketFlags socketFlags)
     at System.Net.Sockets.NetworkStream.MultipleWrite(BufferOffsetSize[] buffers)
The C# code is the following:
private void UploadAttachment(ExchangeServiceBinding esb, ItemIdType parentItemIdType, string attachmentPath)
// File Attachment
FileAttachmentType fileAttach = new FileAttachmentType();
// Attachment details
byte[] bytes = System.IO.File.ReadAllBytes(attachmentPath);
fileAttach.Content = bytes;
fileAttach.IsInlineSpecified = false;
fileAttach.LastModifiedTimeSpecified = false;
fileAttach.Name = System.IO.Path.GetFileName(attachmentPath);
// Create Attachment
CreateAttachmentType reqCreateAttach = new CreateAttachmentType();
// Add parent Id
reqCreateAttach.ParentItemId = parentItemIdType;
// Add attachment to the query
reqCreateAttach.Attachments = new AttachmentType[1];
reqCreateAttach.Attachments[0] = fileAttach;
// Upload attachment
CreateAttachmentResponseType resp = esb.CreateAttachment(reqCreateAttach);
// Check response errors
CheckErrors(resp);
Could you advise a solution, please?
Robert

Answer 1

Hello,
The maximum size is rather not the problem in this case. As mentioned in my question above, I have a problem uploading messages with 12MB attachments to Office 365 via EWS. According
to the documentation you provided a link to, the message size limit in all plans of Office 365 is 25MB.
When we try the same request using slow network connection (1Mbit/s) there IS problem  - we are NOT able to upload 12 MB attachments.
When we try the same request using faster network connection (10Mbit/s) there is NO problem - we are able to upload 12 MB attachments.
It seems like a kind of timeout - probably from Office365.
Is there a possibility to upload attachment in chunks?
Robert