Question: Have you ever tried to decode the Thread-Index header field?

[Sicos1977]

I have a use case where I want to use the Thread-Index header to link messages together (Yes I know there is also the References field but that one is not always filled in my case). I tried decoded this header AQHbJet7Z+efu/5M5UWYnpinBaQePrKfAKzegAAO5bCAAAHygIAAD3LwgAG3uyCAAAECjYAXUgfggASoxyCAAAqegIADX0fwgAFtahCAAAThwIAAAMtwgAAAupCAAAEUEIAAImAggAAHlkCAAC0xcA== with some example code that I found on the internet.

        try
        {
            Raw = threadIndex;

            var bytes = Convert.FromBase64String(threadIndex);

            // Thread index length should be 22 plus extra 5 bytes per reply
            if (bytes.Length < 22 || (bytes.Length - 22) % 5 != 0)
                return;

            Id = new Guid(bytes.Skip(6).Take(16).ToArray());
            var childBlockCount = (bytes.Length - 22) / 5;

            if (childBlockCount == 0)
                Date = DateTime.FromFileTimeUtc(bytes.Skip(1).Take(6).Select(b => (long)b).Aggregate((l1, l2) => (l1 << 8) + l2) << 16).ToLocalTime();
            else
            {
                Date = DateTime.FromFileTimeUtc(bytes.Take(6).Select(b => (long)b).Aggregate((l1, l2) => (l1 << 8) + l2) << 16).ToLocalTime();
                Dates = [Date];

                for (var i = 0; i < childBlockCount; i++)
                {
                    var childTicks = bytes.Skip(22 + i * 5).Take(4).Select(b => (long)b).Aggregate((l1, l2) => (l1 << 8) + l2) << 18;
                    childTicks &= ~((long)1 << 50);
                    Date = Date.AddTicks(childTicks);
                    if (i < childBlockCount - 1) Dates.Add(Date);
                }
            }
        }
        catch 
        {
            // Ignore
        }

With some headers this works but most of the times I just get dated in the year 1830 or in 1601 so something is different.

I also tried following this documentation --> https://learn.microsoft.com/en-us/office/client-developer/outlook/mapi/tracking-conversations?redirectedfrom=MSDN ... but still no luck.

internal ThreadIndex(string threadIndex)
{
    try
    {
        Raw = threadIndex;

        // Decode base64 string to bytes
        var bytes = Convert.FromBase64String(threadIndex);

        // Validate minimum length (22 bytes for the header block)
        if (bytes.Length < 22 || (bytes.Length - 22) % 5 != 0)
            return;

        // Parse the header block
        if (bytes[0] != 1) // Reserved byte must be 1
            return;

        // Extract and compute the date from the FILETIME format in the header
        var headerTicks = bytes.Skip(1).Take(5).Select(b => (long)b).Aggregate((l1, l2) => (l1 << 8) + l2) << 16;
        Date = DateTime.FromFileTimeUtc(headerTicks).ToLocalTime();

        // Extract GUID from the header block
        Id = new Guid(bytes.Skip(6).Take(16).ToArray());

        // Prepare the list of dates
        Dates = new List<DateTime> { Date };

        // Parse child blocks (if present)
        var childBlockCount = (bytes.Length - 22) / 5;
        for (var i = 0; i < childBlockCount; i++)
        {
            // Extract each child block (5 bytes)
            var childBytes = bytes.Skip(22 + i * 5).Take(5).ToArray();

            // Extract the first bit (time encoding strategy)
            var strategyBit = (childBytes[0] & 0b10000000) != 0;

            // Extract the 31 bits for time difference
            var timeDiff = ((childBytes[0] & 0b01111111) << 24)
                         | (childBytes[1] << 16)
                         | (childBytes[2] << 8)
                         | childBytes[3];

            if (strategyBit)
                timeDiff <<= 23; // High-precision, shift low 23 bits
            else
                timeDiff <<= 18; // Low-precision, shift low 18 bits

            // Mask out the 50th bit to handle the encoding properly
            timeDiff &= ~((long)1 << 50);

            // Compute the child date using the time difference
            var childDate = Date.AddTicks(timeDiff);
            Dates.Add(childDate);
        }
    }
    catch
    {
        // Ignore exceptions for invalid input
    }
}

So I was wondering if you ever tried to do something like this with MimeKit?

[Sicos1977]

Note: I found your ThreadIndex implementation in mimekit but that also gives random junk dates when trying to decode the threadindex string in the post above.

[Sicos1977]

For the moment I just gave up, I think the documentation on the Microsoft site is missing something and without knowing what it seems impossible to crack this egg.

[jstedfast]

I don't think I've ever tried to decode it, but now you have me interested haha.

[Sicos1977]

Well give it a try, it drove me nuts this week because every time I thought I had everything working another thread-index failed with weird dates. For some reason I sometimes have to get the first 6 bytes from the header for a correct date time even when the specifications say that the first byte is reserved and always 1. In another situation I had to skip the first byte (specification) and then take the next 6 instead of 5....

I also had problems with the child blocks. When the header date was correct and there are for example 16 child block most of the blocks give correct dates but than a few of them give a date in the future ... like 2025 instead of 2024 ... so weird.

So that is why I think something is missing in the documentation on the Microsoft site. I also searched Google and asked ChatGPT but even that came up with not always working code. Al so other people seem to have the same problems that I ran into.

Also tried to find some C++ of working code in another language (so that I could port it) .... but al so no luck