Reading excel attachment data in C# from raw MIME message posted by SendGrid

Question

I have SendGrid posting incoming emails as raw MIME messages to my C# API. Here is an example of the raw payload. I want to be able to open the excel attachments from the incoming MIME message and read the row and column data.

Once the API receives the post, I use MimeKit to process the attachments as follows (assume we are only dealing with .xlsx attachments here):

using (var emailSream = GenerateStreamFromString(emailString))
{
    var msg = MimeMessage.Load(emailSream);

    foreach (var attachment in msg.Attachments)
    {
        //  I want to be able to read the columns and rows of the excel sheet here.  
        //  I am already able to skip over any non-excel type attachments without issue.
    }
}

Where GenerateStreamFromString is defined as:

private Stream GenerateStreamFromString(string s)
{
    MemoryStream stream = new MemoryStream();
    StreamWriter writer = new StreamWriter(stream);
    writer.Write(s);
    writer.Flush();
    stream.Position = 0;
    return stream;
}

I have tried loading the attachment stream into Excel Data Reader:

using (var attachmentStream = ((MimePart)attachment).ContentObject.Stream)
{
    IExcelDataReader reader = ExcelReaderFactory.CreateOpenXmlReader(attachmentStream);
    //  Reader has "Cannot Find Central Directory" error.
}

As well as through EPPlus

var attachmentStream = ((MimePart)attachment).ContentObject.Stream
using (ExcelPackage package = new ExcelPackage(attachmentStream))
{
    //  Errors are thrown before we get here.
}

But both packages throw errors, which leads me to believe I am not getting the data stream properly from the MIME message.

Any help or thoughts are greatly appreciated, thanks in advance.

score 3 · Accepted Answer · answered Sep 09 '16 at 00:19

Doh! Needed to decode the attachment (SendGrid even provided the encoding information for me, I just didn't listen well):

using (var attachmentStream = new MemoryStream())
{
    ((MimePart)attachment).ContentObject.DecodeTo(attachmentStream);
    attachmentStream.Position = 0;
    IExcelDataReader reader = ExcelReaderFactory.CreateOpenXmlReader(attachmentStream);
}

Problem solved!

Reading excel attachment data in C# from raw MIME message posted by SendGrid

1 Answers1