ref struct ASN.1 Decoder type generation

[vcsjones]

Some explanation to the changes

The XSLT now has an emitType to define whether a struct, ref struct, or both are emitted. The default is just the struct as it is today.

The "default DER init" moved to its own file-scoped type in the generated file so that both the ref struct and the struct can share the default init fields. This keeps source file and assembly size down.

The struct-based emit is largely unchanged, except for using the file default DER init.

The ref struct implementation basically does what you would think. Since it is a ref struct, it prefers to work over spans, and allows us to expose things like ReadOnlySpan<byte> of contents.

To support ref structs referring to ref structs, there is a new field called valueTypeName that can be applied to asn:AsnType which the ref struct emitter will use. If it is not present, then it is decoded as raw bytes.

Since ReadOnlySpan<byte> cannot be Nullable<T>, optional types have a corresponding Has-prefixed boolean field. Like HasParameters for a field called Parameters.

The trickiest part was how to handle collections. We can't have arrays, lists, etc. of ReadOnlySpan<byte>. Instead, the new emitter creates a Get<Name> method that returns a ref struct enumerator that on-the-fly decodes. This is a slight behavior deviation from the struct based one that eagerly decodes things in to an array. So exceptions may not happen during the call to Decode and will be deferred to the enumeration.

To demonstrate the utility of this, I changed two existing places to use the new ref struct approach.

1. PublicKey, which is a SubjectPublicKeyInfoAsn to ValueSubjectPublicKeyInfoAsn
2. CertificateRequest, and all of its supporting types. This one is more complex because it involves collections, etc.

These changes were largely successful, but unfortunately resulted in some duplication of "helpers" for now which may or may not go away if we move more things to the ref struct.

The overall result is positive: No more use of PointerMemoryManager in these two locations.

Support for Encode is not yet implemented. That can be a follow up if it is deemed worthwhile.

[Copilot]

Pull request overview

This PR is a proof-of-concept update to the ASN.1 asnxml/XSLT code generator to optionally emit span-based ref struct decoders (Value* types) alongside the existing struct decoders, and it migrates PublicKey and CertificateRequest parsing to use the new Value* decoders to avoid PointerMemoryManager.

Changes:

• Extend asn.xslt + schema annotations (emitType, valueTypeName, valueName) to generate Value* ref struct decoders and lazy collection enumerators.
• Refactor generated “default DER init” constants into file static class Shared* helpers so both struct and ref struct decoders can share defaults.
• Switch PublicKey and CertificateRequest.Load decoding paths to use ValueSubjectPublicKeyInfoAsn, ValueCertificationRequestInfoAsn, etc.

Reviewed changes

Copilot reviewed 29 out of 29 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/PublicKey.cs	Switch SPKI decode to ValueSubjectPublicKeyInfoAsn and remove pointer-based memory manager usage.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/CertificateRequest.Load.cs	Switch PKCS#10 parsing to Value* decoders and lazy attribute/value enumeration.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/Asn1/TbsCertificateAsn.xml.cs	Move default DER init to SharedTbsCertificateAsn.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/Asn1/CertificationRequestInfoAsn.xml.cs	Add ValueCertificationRequestInfoAsn with span fields + lazy GetAttributes.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/Asn1/CertificationRequestInfoAsn.xml	Enable emitType="both" and map nested types via valueTypeName / valueName.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/Asn1/BasicConstraintsAsn.xml.cs	Move default DER init to SharedBasicConstraintsAsn.
src/libraries/System.Security.Cryptography.Pkcs/src/System/Security/Cryptography/Pkcs/Asn1/Rfc3161TstInfo.xml.cs	Move defaults to SharedRfc3161TstInfo and add ValueRfc3161TstInfo.
src/libraries/System.Security.Cryptography.Pkcs/src/System/Security/Cryptography/Pkcs/Asn1/Rfc3161TstInfo.xml	Enable emitType="both" and map MessageImprint via valueTypeName.
src/libraries/System.Security.Cryptography.Pkcs/src/System/Security/Cryptography/Pkcs/Asn1/Rfc3161TimeStampReq.xml.cs	Move default DER init to SharedRfc3161TimeStampReq.
src/libraries/System.Security.Cryptography.Pkcs/src/System/Security/Cryptography/Pkcs/Asn1/MessageImprint.xml.cs	Add ValueMessageImprint span-based decoder.
src/libraries/System.Security.Cryptography.Pkcs/src/System/Security/Cryptography/Pkcs/Asn1/MessageImprint.xml	Enable emitType="both".
src/libraries/System.Security.Cryptography.Pkcs/src/System/Security/Cryptography/Pkcs/Asn1/EssCertIdV2.xml.cs	Move default DER init to SharedEssCertIdV2.
src/libraries/Common/src/System/Security/Cryptography/Asn1/asn.xslt	Core generator changes: emitType, Value* decoding, Shared defaults, lazy collection enumerators.
src/libraries/Common/src/System/Security/Cryptography/Asn1/X509ExtensionAsn.xml.cs	Move defaults to SharedX509ExtensionAsn and add ValueX509ExtensionAsn.
src/libraries/Common/src/System/Security/Cryptography/Asn1/X509ExtensionAsn.xml	Enable emitType="both".
src/libraries/Common/src/System/Security/Cryptography/Asn1/SubjectPublicKeyInfoAsn.xml.cs	Add ValueSubjectPublicKeyInfoAsn.
src/libraries/Common/src/System/Security/Cryptography/Asn1/SubjectPublicKeyInfoAsn.xml	Enable emitType="both" and map Algorithm via valueTypeName.
src/libraries/Common/src/System/Security/Cryptography/Asn1/PssParamsAsn.xml.cs	Move defaults to SharedPssParamsAsn and add ValuePssParamsAsn.
src/libraries/Common/src/System/Security/Cryptography/Asn1/PssParamsAsn.xml	Enable emitType="both" and map nested alg IDs via valueTypeName.
src/libraries/Common/src/System/Security/Cryptography/Asn1/PssParamsAsn.manual.cs	Add ValuePssParamsAsn.GetSignaturePadding helper.
src/libraries/Common/src/System/Security/Cryptography/Asn1/Pkcs12/MacData.xml.cs	Move default DER init to SharedMacData.
src/libraries/Common/src/System/Security/Cryptography/Asn1/Pbkdf2Params.xml.cs	Move default DER init to SharedPbkdf2Params.
src/libraries/Common/src/System/Security/Cryptography/Asn1/OaepParamsAsn.xml.cs	Move default DER init to SharedOaepParamsAsn.
src/libraries/Common/src/System/Security/Cryptography/Asn1/GeneralSubtreeAsn.xml.cs	Move default DER init to SharedGeneralSubtreeAsn.
src/libraries/Common/src/System/Security/Cryptography/Asn1/AttributeAsn.xml.cs	Add ValueAttributeAsn span-based decoder + lazy GetAttrValues.
src/libraries/Common/src/System/Security/Cryptography/Asn1/AttributeAsn.xml	Enable emitType="both" and name the value enumerator via valueName.
src/libraries/Common/src/System/Security/Cryptography/Asn1/AlgorithmIdentifierAsn.xml.cs	Add ValueAlgorithmIdentifierAsn span-based decoder.
src/libraries/Common/src/System/Security/Cryptography/Asn1/AlgorithmIdentifierAsn.xml	Enable emitType="both".
src/libraries/Common/src/System/Security/Cryptography/Asn1/AlgorithmIdentifierAsn.manual.cs	Add ValueAlgorithmIdentifierAsn helpers (Equals, null-parameter checks).

Comments suppressed due to low confidence (1)

src/libraries/Common/src/System/Security/Cryptography/Asn1/asn.xslt:1519

• Same as the typed enumerator above: this Enumerator constructor only assigns _reader when encoded is non-empty. Initialize _reader on all paths (e.g., _reader = default; before the if) to avoid a definite-assignment compile error in generated code.

            internal ref struct Enumerator
            {
                private ValueAsnReader _reader;
                private ReadOnlySpan<byte> _current;

                internal Enumerator(ReadOnlySpan<byte> encoded, AsnEncodingRules ruleSet)
                {
                    if (!encoded.IsEmpty)
                    {
                        ValueAsnReader outerReader = new ValueAsnReader(encoded, ruleSet);
                        _reader = outerReader.Read<xsl:value-of select="$collNoun"/>(<xsl:call-template name="MaybeImplicitCall0"/>);
                    }

                    _current = default;
                }

---

You can also share your feedback on Copilot code review. Take the survey.

[Copilot]

Pull request overview

Copilot reviewed 29 out of 29 changed files in this pull request and generated 2 comments.

Comments suppressed due to low confidence (1)

src/libraries/Common/src/System/Security/Cryptography/Asn1/asn.xslt:1516

• Same definite-assignment issue here: _reader is only assigned inside the if (!encoded.IsEmpty) block. Ensure _reader is assigned on all paths (e.g., _reader = default at the start of the constructor).

                internal Enumerator(ReadOnlySpan<byte> encoded, AsnEncodingRules ruleSet)
                {
                    if (!encoded.IsEmpty)
                    {
                        ValueAsnReader outerReader = new ValueAsnReader(encoded, ruleSet);
                        _reader = outerReader.Read<xsl:value-of select="$collNoun"/>(<xsl:call-template name="MaybeImplicitCall0"/>);
                    }

---

You can also share your feedback on Copilot code review. Take the survey.

[bartonjs]

Nice. Your treating it as a separate mode and just duplicating everything has drawbacks (more xslt to maintain), but overcame a lot of the limitations I ran into when doing this previously (and trying to just be one or the other).

CertificateRequest.Load.cs and PublicKey.cs both generally look good.

OTOH, the work CertificateRequest does before enumerating represents more throwaway allocatey work... on the other, you've gotten rid of allocating the lists on success. So, seems like a net positive. (Slightly sad to lose Length, but, oh well...)

[Copilot]

Pull request overview

Copilot reviewed 30 out of 30 changed files in this pull request and generated 3 comments.

---

You can also share your feedback on Copilot code review. Take the survey.

[Copilot]

Pull request overview

Copilot reviewed 30 out of 30 changed files in this pull request and generated 3 comments.

---

You can also share your feedback on Copilot code review. Take the survey.