Extending System.Formats.Asn1 to use other rules

[shaggygi]

I believe I asked (or searched around) before but couldn't find the answer on how it would be possible to extend System.Formats.Asn1 to use other rules. Meaning, use the API to read/wirte/encode/deocde bytes to and from objects other than the provided 3 types. The BACnet protocol (basic example below) defines the various objects/services and would be great to use this API (or at least utilize most of the provided). It does appear like there are a few items missing like Date, as well.

CCing @bartonjs or @stephentoub to see if they had some options. Thx

BACnetTimeStamp ::= CHOICE {
time [0] Time,
sequence-number [1] Unsigned (0..65535),
datetime [2] BACnetDateTime
}

An example of a context-tagged Boolean is below where it would produce a byte array of 0x2901.

Example: Context-tagged Boolean value
ASN.1 = [2] BOOLEAN
Value = TRUE
Context Tag = 2
Encoded Tag = X'29'
Encoded Data = X'01'

[bartonjs]

In general, the System.Formats.Asn1 library is only providing things from X.680/X.690. Reading a BACnetTimeStamp is outside the scope of what it should perform.

One could make a BACNet library that is powered by, and builds on, System.Formats.Asn1.. but the only things that should obviously be built-in (that is, built into System.Formats.Asn1) are those things defined in the core ITU specifications

The library can even provide extension methods, as appropriate:

public static BACNetTimeStamp ReadBACNetTimeStamp(this AsnReader reader)
{
    ArgumentNullException.ThrowIfNull(reader);

    AsnReader temp = new AsnReader(reader.PeekEncodedValue(), reader.RuleSet);
    AsnTag tag = reader.PeekTag();

    if (tag.TagClass != TagClass.ContextSpecific)
    {
        throw new SomeException();
    }

    AsnReader inner = temp.ReadSequence(tag);

    BACNetTimeStamp timeStamp = tag.TagValue switch
    {
        0 => ReadTime(inner),
        1 => ReadUnsigned(inner),
        2 => ReadBACNetDateTime(inner),
        _ => throw new SomeException(),
    };

    inner.ThrowIfNotEmpty();
    // Everything worked, update the reader's state.
    reader.ReadEncodedValue();
    return timeStamp;
}

[shaggygi]

Thanks @bartonjs for response. It's a shame it doesn't cover this but understand it can't support everything.