A long‑awaited milestone is finally real: generics are supported in .NET nanoFramework, and the public preview starts today! 🥳

This post is intentionally more than an announcement. It’s a behind-the-scenes tour of what it actually took to get here: the metadata and tooling work, the runtime and type system changes, and the many “small” edge cases that turned out not to be small at all. Some parts get unapologetically dense, because generics live right at the heart of the ECMA-335 Common Language Infrastructure (CLI)—the same specification that defines the execution model, metadata, and type system that .NET is built on. And because nanoFramework’s PE/metadata format is an extended subset of ECMA-335 (optimized for constrained devices), we’ll also talk about what had to be added and reshaped so the device can resolve and execute generic code correctly.

TL;DR warning (with 💜): if terms like TypeSpec, MethodSpec, coded indices, token resolution, and generic context propagation sound like a pleasant afternoon, you’re in the right place. If they sound like a tax audit, you may want to skim the headings first—either way, if you’re up to it, let’s go.

Generics in C# (Overview)

For those not familiar with what is generics in C#, here’s a quick overview.

Generics in C# allow you to define classes, methods, and data structures with type parameters, which are placeholders for specific types that are supplied when you use them (Generic type parameters (C# Programming Guide)). This enables type-safe code reuse: you can write a single class or method that works for any data type while still catching type errors at compile time. For example, the .NET List<T> class is generic – you can create a List<int> for integers or a List<string> for strings, and the compiler ensures that only the specified type is used in that list.

Key benefits of generics include:

• Type Safety: They eliminate the need for casting or boxing by preserving type information.
• Reusability: You can implement algorithms once, independently of specific data types, and reuse the code for different types.
• Performance: They reduce runtime overhead by avoiding conversions; e.g., storing value types in a generic collection avoids boxing/unboxing.

Example – Generic Method: Below is a simple generic method that returns the larger of two values of any comparable type (e.g., numbers, strings). The type parameter T must implement IComparable<T> so that they can be compared:

// Generic method to get the maximum of two values
static T Max<T>(T a, T b) where T : IComparable<T>
{
return (a.CompareTo(b) >= 0) ? a : b;
}

// Usage:
int biggerInt = Max(10, 42);         // works with int
string laterString = Max("apple", "zebra"); // works with string

Example – Generic Class: You can also define generic classes. For instance, a minimal generic stack class could be defined as:

public class Stack<T>
{
private T[] _items;
private int _count;
public Stack(int size) => _items = new T[size];
public void Push(T item) => _items[_count++] = item;
public T Pop() => _items[--_count];
}

This Stack<T> can be used as Stack<int> for an integer stack or Stack<string> for a string stack, etc., each providing compile-time type enforcement.

However, due to the constraints of embedded devices, previous versions of .NET nanoFramework did not have support for generics. With this preview, .NET nanoFramework is introducing support for generics, closing the gap with standard C# capabilities. Developers can now write more flexible and reusable code on microcontrollers, just as they would on the full .NET runtime.

Keep in mind that this is a public preview following a closed preview with intensive testing quietly happening in the background. While most generic scenarios are expected to work, there may be limitations or bugs as this feature is brand new.

If you want to know the personal perspective about the details of the development of this feature, please read José Simões blog post about this.

Implementation technical details

As one can imagine, generics support had a huge impact and reach in nanoFramework code base. It touches the type system, assembly declarations, metadata, execution engine, interpreter and debugger. In practical terms, this means everything: firmware, libraries and the Visual Studio extension.

A quick premise: “.NET semantics,” nano-sized metadata

Because .NET nanoFramework is a .NET execution engine, it deliberately honors the same CLI rules that govern C# execution: if code is supported, you should expect the same observable outcome whether it runs on a desktop CLR or on a microcontroller. What changes is not the language contract—it is the packaging and shape of the metadata we deploy to a constrained device.

nanoFramework already shortens and adapts the standard PE/metadata model to keep footprint low and to reduce the amount of runtime crawling the device has to do. Concretely, parts of the PE headers are stripped, metadata tables are limited, indices are constrained, and (importantly) metadata tokens are 16-bit rather than 32-bit—so the IL stream itself differs from desktop .NET.

That approach (keep ECMA 335 semantics, compress the representation) maps perfectly onto what generics demanded next: preserve the exact behavior, but extend the nano metadata model and runtime type system so the device can faithfully reproduce generic type/method behavior without turning boot and token resolution into a heavyweight process.

From a simple C# example to what the runtime must actually do

Let’s use a deliberately small example that exercises both sides of generics: a closed generic type and a closed generic method.

public sealed class Parcel<T>
{
public static int Created;

static Parcel() => Created++;

public Parcel(T value) => Value = value;
public T Value { get; }
}

public static class Ops
{
public static T Echo<T>(T value) => value;
}

public static class Demo
{
public static int Run()
{
var p = new Parcel<int>(42);
return Ops.Echo<int>(p.Value);
}
}

At the C# level, this looks straightforward. At runtime, however, it forces two non-negotiable capabilities:

1. Materialize a closed constructed type: Parcel<int> must become a real runtime type identity (not just “Parcel + int” as a string).
2. Bind and invoke a generic method instantiation: Ops.Echo<int>(…) must resolve to the correct method body with the correct generic arguments applied.

This is where generics stop being “syntax sugar” and become “metadata + type system + execution-flow”.

The IL call that turns into a MethodSpec

A disassembler typically shows something close to the following (simplified for clarity):

.method public static int32 Run() cil managed
{
.maxstack 2

IL_0000: ldc.i4.s   42
IL_0002: newobj     instance void class Parcel`1<int32>::.ctor(int32)
IL_0007: call       int32 class Ops::Echo<int32>(int32)
IL_000C: ret
}

The key detail is the operand behind that call. On desktop .NET, the call operand is a metadata token that might reference a MethodDef, MemberRef, or—when the target is a constructed generic method—a MethodSpec. A MethodSpec is, conceptually, “generic method + instantiation signature”.

ECMA-335 makes the “method + instantiation blob” model explicit: the MethodSpec table stores which generic method is being instantiated (a MethodDefOrRef coded index) and an Instantiation blob containing the signature of that instantiation.

That is exactly the contract you must honor at runtime: when the IL says “call Echo<int>”, the runtime is not merely choosing a method; it is choosing a method plus a specific instantiation context.

And you can see the same model reflected in the metadata writer APIs: MetadataBuilder.AddMethodSpecification(...) takes a handle to the generic method plus a blob handle representing the instantiation (the generic arguments encoded in signature form).

Where TypeSpec comes in

The same story applies to constructed types like Parcel<int>. The TypeSpec table exists to provide a metadata token for a type specified by a signature blob (rather than being a TypeDef/TypeRef), and ECMA explicitly calls out that TypeSpec tokens are used by many IL instructions that consume type tokens (e.g., newarr, box, castclass, sizeof, etc.).

That is why your “tokens become recipes” line is so important: once TypeSpec and MethodSpec exist in the model, resolution can no longer be “token → fully-known type/method.” It becomes “token → signature → bind under context → runtime identity.” we have generics.

Once you accept that both types and methods can be “described by signature blobs,” three metadata concepts become foundational and the metadata “triangle” generics forced us to add these:

• GenericParam: defines the generic parameters owned by a type or method (the “T” itself).
• TypeSpec: represents constructed types (like Parcel<int>) via signature blobs.
• MethodSpec: represents constructed generic methods (like Echo<int>) as a method + instantiation blob.

What had to change in MDP: still compressing, now also preserving generic shape

The Metadata Processor in nanoFramework has always been about two goals: reduce size and reduce complexity for the MCU at runtime. Generics didn’t change those goals—it raised the bar on what must be preserved during compression.

With generics, “dropping metadata you don’t need” becomes much harder, because generic behavior is encoded in the relationships between tables and signature blobs. The MDP now has to:

• carry the additional generic constructs end-to-end (dropping them breaks binding),
• remap and emit references that are frequently TypeSpec/MethodSpec-based, not just TypeDef/MethodDef-based,
• and preserve the spirit of “give the runtime a hand” by keeping enough structure/hints that the device doesn’t need expensive crawling just to answer “what is Parcel<int>?” or “what does Echo<int> mean?”

From the nanoFramework side, this also intersects directly with the constrained PE/metadata format: limited tables, constrained indexes, and 16-bit tokens. Our PE files use both a regular metadata token and a more compact “BinaryToken”, which is part of how you keep the deployed representation efficient on-device.

The type system shift: closed types must be stored and cached as first-class identities

A generic definition (Parcel<T>) is not the thing you instantiate at runtime. What actually participates in execution is a closed constructed type (Parcel<int>), and that closed form needs:

• a stable identity the runtime can compare quickly,
• a link back to the open definition (so you can reuse method bodies and metadata),
• and the concrete generic arguments used to close it.

Without that mapping, the runtime can’t reliably substitute placeholders in signatures, can’t build correct field layouts, and can’t guarantee consistent type identity across the application (which is a correctness issue, not an optimization detail).

This is also where you start to feel the practical consequences of TypeSpec: a TypeSpec is literally “a signature blob plus a token wrapper” — and the runtime must be able to decode it, bind it, and then cache the resulting “closed type” object so the rest of execution can treat it like any other type.

Boot/type-resolution: signatures are now part of loading, not just parsing

Before generics, the loader can largely focus on “known tables” and straightforward linking. With generics, boot-time (or first-use) resolution must also be able to interpret signature blobs that represent:

• constructed types (TypeSpec),
• constructed generic methods (MethodSpec),
• and signatures containing placeholders that only make sense once you know the current type/method instantiation context. we have generics

This is why, architecturally, generics pushes work into the resolution stage rather than just the “metadata read” stage. The runtime must be able to take a “spec” (TypeSpec/MethodSpec), interpret the signature, apply the right context, and then bind the result into a concrete runtime identity that the interpreter and debugger can refer to repeatedly.

Statics: separate static fields and cctors per closed constructed type

This is a place where a generic implementation can be subtly wrong if it shortcuts semantics.

With generics, each distinct closed constructed type has its own static storage, and the static constructor must be tracked per closed type identity — not “once per open generic definition.” Your baseline already states the requirement crisply (and this is exactly the kind of detail worth emphasizing in a technical post). we have generics

Practically, this means the runtime must ensure that Parcel<int> is materialized early enough that:

• its static storage can be allocated under the correct closed-type identity, and
• its .cctor execution state can be tracked correctly, so “cctor runs once” remains true — but “once” is scoped to each constructed type, not to the open definition. we have generics

This also explains your point about execution-flow changes: in “non-generic land,” the engine can often treat statics as a straightforward type-level singleton. In “generic land,” the runtime has to treat statics as per-instantiation, which affects when types are created, when initialization runs, and how first access is detected.

Token resolution and execution flow: why generic context must flow with the call

Once TypeSpec/MethodSpec enter the picture, resolution becomes context-dependent. The runtime must be able to answer:

• “What does this type placeholder mean under the current closed type?”
• “What does this method placeholder mean under the current generic method instantiation?” we have generics

That is why the execution engine must carry and pass type context and method context across calls: callees often need that context to resolve signatures, bind tokens, and interpret generic parameters correctly — especially for nested calls and member access where signatures include generic parameters. we have generics

A very concrete “desktop .NET corroboration” of this requirement shows up in reflection/emit APIs: if you try to resolve a TypeSpec token whose signature contains VAR/MVAR without supplying the necessary generic type and/or method arguments, resolution fails. In other words, the platform itself forces you to provide the same two contexts you’re describing.

Correct resolution of generic tokens is context dependent, so the execution engine must transport context across frames.

What this means inside the interpreter: two mechanical changes you can feel in the runtime

Once generics are in play, the interpreter can no longer treat “a token in IL” as something that always maps directly to a pre-known TypeDef/MethodDef. In practice, token decoding becomes a two-step operation:

1. Decode the operand token from the IL stream (in nanoFramework this is already a compact world, with both a regular MetadataToken and an even more compact BinaryToken).
2. If the token points to a spec rather than a definition (TypeSpec / MethodSpec), the interpreter must follow the token into a signature blob, decode that signature, and only then bind to a concrete runtime type or callable target.

These changes in the metada table even surfaces at the assembly header level: the header marker changes to a v2.0 marker.

The second change: “first-time work” becomes more expensive, so caching becomes non-negotiable

The second mechanical change is where you really earn performance back: generics introduces a lot of first-time work that must be paid once and then amortized.

When the runtime encounters a TypeSpec/MethodSpec for the first time, it may need to:

• materialize a closed constructed type identity (e.g., Parcel<int>),
• bind a MethodSpec to a callable target (generic method + instantiation blob),
• allocate per-instantiation static storage,
• and ensure the .cctor behavior remains faithful to C# rules (“before the first instance is created or any static member is referenced”, and runs once for the relevant type).

This is why the steady-state execution story must be cache-driven. The interpreter cannot afford to repeatedly decode signature blobs, repeatedly substitute generic parameters, or repeatedly rebuild the same closed-type identities on every callsite hit. The hot path needs to look much closer to non-generic code: “resolve once, then reuse handles”.

A useful way to phrase the intent is:

• Cold path (first encounter): decode → substitute → bind → allocate/init as needed → cache
• Hot path (subsequent encounters): reuse cached runtime type/method handles with minimal overhead

That why we have a cache with a virtual methods table.

Why “context” must be carried explicitly in frames and calls

Finally, all of the above only works if resolution is provided the correct context. Desktop .NET makes this requirement explicit in reflection APIs: resolving a TypeSpec or MethodSpec whose signature depends on generic parameters requires you to supply the necessary generic type arguments and/or method arguments; otherwise it throws.

That is the same rule you are implementing in the interpreter: frames and call dispatch must carry a type context and a method context, so any downstream token/signature resolution can correctly substitute VAR / MVAR. This aligns cleanly with nanoFramework’s reduced signature/type model, which explicitly includes Var, MVar, and GenericInst as first-class element types.

Type materialization and caching: how the runtime makes “closed generics” real

At this stage in the story, it helps to be explicit about where nanoFramework draws the line between “desktop .NET compatibility” and “embedded pragmatism”:

• We keep CLI semantics intact (same execution rules, same observable behavior).
• We ship a constrained PE/metadata format that is intentionally smaller and easier to consume on-device (limited tables, limited indices, stripped PE32/COFF header), including the fact that metadata tokens are 16-bit in nanoFramework—so the IL stream is different from desktop .NET.
• In the new PE format v2.0, generics becomes a first-class feature in that constrained format. The AssemblyHeader marker explicitly calls out that NFMRK2 is “version 2.0 (after adding support for generics)”, and the table directory gains GenericParam and MethodSpec, alongside TypeSpec.

We had to have this v2.0 line clearly marked “in the sand” because it forces a runtime capability shift: the interpreter must be able to turn specifications (TypeSpec/MethodSpec + signature blobs) into concrete runtime identities (closed types and callable method instantiations), and then reuse them efficiently.

The key enabling detail is that signatures now contain generic element types. In nanoFramework’s reduced signature type system, v2.0 adds explicit element kinds for:

• Var (generic parameter in a generic type definition),
• MVar (generic parameter in a generic method definition),
• GenericInst (generic type instantiation).

Those two facts—generic element types in signatures and compact coded tokens in IL—drive the rest of the implementation.

Closed type materialization: resolving TypeSpec into a runtime type identity

With generics, a large portion of “type identity” stops being representable as a simple (table, row) pair. A constructed type like Parcel<int> is typically carried via TypeSpec, i.e., a token that points to a signature blob describing the type. The PE v2.0 table layout makes TypeSpec a first-class table, and the format is explicitly built as an “extended subset” of ECMA-335, tuned for constrained systems.

In practice, the runtime’s “TypeSpec resolver” becomes a small engine that does three jobs:

1. Decode the signature blob

• Recognize when the signature is a GenericInst.
• Recursively resolve each argument type (which may itself be another TypeSpec, an array/byref shape, etc.).
• Substitute Var / MVar placeholders when the signature is context-dependent.

1. Bind it to a canonical runtime identity

• Construct a key like: (generic definition handle, list of bound argument type handles) This key is what ensures Parcel<int> is “the same” closed type every time it appears.

1. Cache the result

• The first time you see Parcel<int>, you pay the cost to materialize it.
• Every subsequent time, the type system returns the cached runtime handle without re-decoding the signature.

This caching step is not just an optimization—it’s a correctness enabler. Once you add per-instantiation statics (next section), you need a stable identity for “this exact closed type” so you can hang initialization state and static storage off it.

MethodSpec binding: turning “call Echo<int>” into something callable

A constructed generic method call does not merely reference “a method”; it references “a method + a specific instantiation”. That is what MethodSpec represents in CLI metadata: a row that pairs a MethodDefOrRef coded index with an Instantiation blob (the generic arguments encoded in a signature).

On nanoFramework PE v2.0, MethodSpec appears as a first-class metadata table in the AssemblyHeader’s table directory.

Runtime-wise, the MethodSpec resolver mirrors the TypeSpec resolver:

1. Decode the MethodSpec row

• Resolve the referenced generic method (MethodDef/MemberRef path).
• Decode the instantiation blob into a list of method generic argument types.

1. Apply context-aware substitution

• If any argument type is expressed in terms of Var/MVar, it can only be resolved correctly when you know the current type/method context. nanoFramework’s own type encoding explicitly includes Var and MVar, which is the mechanism the interpreter uses to represent those placeholders.

1. Create a “method instantiation handle” and cache it

• Keyed by something like: (generic method handle, list of method generic argument handles)

At the end of that process, a call instruction no longer just points to “a method body”; it points to “a method body + a binding context”.

The execution-model change: contexts become part of a stack frame

Once Var/MVar exist in signatures, resolution becomes context-dependent by definition. So the interpreter must treat generic context as part of the execution state, not as a one-off detail of metadata parsing. Concretely, that means:

• Every frame associated with a method that lives inside a constructed generic type must carry a type context (the closed type arguments).
• Every call into an instantiated generic method must carry a method context (the closed method arguments).
• Any token/signature resolver that can encounter Var/MVar must accept those contexts as inputs; otherwise, it cannot bind signatures deterministically.

This design also fits naturally with nanoFramework’s compact operand strategy: IL operands often use compact “BinaryTokens” (coded indices) that must be expanded into table + row and then interpreted—sometimes via TypeSpec/MethodSpec signature blobs.

Per-instantiation statics and cctors: where closed type identity becomes mandatory

Now we reach the point where “materialization and caching” stop being architectural neatness and become a strict semantic requirement.

Two C# rules are load-bearing here:

• Each distinct closed constructed type has its own set of static fields.
• A static constructor runs automatically to initialize the type before the first instance is created or any static members declared in that type are referenced, and it runs only once for that type.

Combine those, and the runtime implications are immediate:

• Parcel<int> and Parcel<string> must each have separate static storage.
• Each must have independent “cctor has run” state.
• Therefore the runtime must ensure the closed type is materialized early enough that static storage and initialization tracking attach to the correct closed type identity.

This is also why your earlier point about execution-flow changes matters: non-generic statics can often be treated as “one per TypeDef”. Generic statics must be treated as “one per closed constructed type”, which forces the runtime to allocate and initialize those statics as part of (or immediately after) closed-type materialization, not as a single global singleton attached to the open generic definition.

Resolution flow on nanoFramework: boot-time staging vs first-use binding

With the “spec tokens” (TypeSpec/MethodSpec) and context propagation in place, the remaining work is making resolution predictable and cheap in the steady state. The pattern that emerges is a two-phase design:

• Boot-time staging: build fast indexes and lightweight handles from the metadata tables that are always present.
• First-use binding: only when IL actually touches a TypeSpec/MethodSpec do we decode the signature blob, apply context, and materialize a concrete runtime identity—then cache it aggressively.

This division aligns directly with the nanoFramework PE format goals: keep footprint low, keep tables bounded, and keep runtime crawling minimal.

Boot-time staging: scan, validate, and build lookup surfaces

At boot (or when scanning the Deployment region), the runtime begins with the AssemblyHeader. The header provides verification markers/CRCs and—most importantly for performance—the offsets to every metadata table and blob stream.

A few details here matter for the resolution strategy:

• The PE file starts with the AssemblyHeader at offset 0, and metadata immediately follows it; the device layout is designed for scanning multiple assemblies laid out back-to-back.
• The header contains StartOfTables offsets for each table and blobs, plus an EndOfAssembly offset that makes it cheap to jump to the next assembly when scanning a DAT/Deployment region.
• The header marker explicitly encodes the format version, and NFMRK2 is “version 2.0 (after adding support for generics)”—this our clean “feature gate” for generics-aware resolution.
• PE format “major differences” include: limited metadata tables, stripped PE32/COFF details, constrained indexes, and the fact that metadata tokens are 16-bit.

During this stage, the runtime can build fast, compact lookup structures without fully resolving everything:

1. Table base pointers / spans (TypeDef, MethodDef, FieldDef, TypeRef, MethodRef, TypeSpec, MethodSpec, GenericParam, etc.), derived from StartOfTables.
2. Common coded-index helpers for token decoding (because nanoFramework uses a compact 2-byte “BinaryToken” coded-index scheme following the ECMA-335 convention).
3. Lightweight definition handles for TypeDef/MethodDef that are context-free (these can be registered early because they don’t depend on generic substitution).

This is the point where the runtime becomes “ready to execute” without having to pre-materialize every possible closed type.

Execution-time binding: the token resolver becomes a small “binder”

Once the interpreter starts running IL, token resolution sits on the critical path. nanoFramework’s format makes this more interesting than desktop .NET in two ways:

• Tokens are effectively 16-bit in the IL stream, and many operands use the compact BinaryToken coded-index encoding.
• Some tokens refer to specifications (TypeSpec/MethodSpec) that only become meaningful after decoding a signature blob (and possibly applying generic context).

So the resolver’s job becomes:

1. Decode the operand (often a BinaryToken) into “table kind + index,” using the coded-index tag bits and index bits.
2. Dispatch by table kind:

• TypeDef/TypeRef/MethodDef/MethodRef → mostly direct binding
• TypeSpec/MethodSpec → signature-driven binding path

1. If signature-driven:

• Decode signature elements, including Var, MVar, and GenericInst (these are explicitly part of nanoFramework’s reduced DataType set in PE v2.0).
• Apply the type context and method context to substitute placeholders.

1. Return a bound runtime handle (type identity or callable target).

This is where the earlier “two contexts” requirement becomes operational: the resolver is no longer pure metadata lookup; it is metadata lookup + signature evaluation under context.

Boot vs first-use: what gets resolved when

A practical (and very nanoFramework-friendly) split looks like this:

Resolve eagerly (boot/staging):

• Assembly list + header validation (marker/CRC)
• Table boundaries / offsets
• TypeDef and MethodDef registration for open definitions (including generic type/method definitions)
• Any trivial mappings that don’t require signature evaluation

Resolve lazily (first-use):

• TypeSpec materialization (closed constructed types, including nested instantiations)
• MethodSpec binding (generic method instantiations)
• Any signature-dependent field/method/constraint interpretation that needs context

This split is exactly what our earlier “cold path vs hot path” framing was setting up: signature decoding and substitution is inherently heavier, so one does it only on demand, then cache.

Caching: keying, lifetime, and why invalidation is usually unnecessary

Once you decode a TypeSpec or MethodSpec under a specific context, you want the result to be reusable.

Key choice (typical and effective):

• Closed type cache keyed by: (TypeSpec row or signature blob) + (bound type-argument handles)
• Method instantiation cache keyed by: (MethodSpec row) + (bound method-argument handles)

Lifetime / invalidation:

• In the nanoFramework PE model, assemblies live in a well-defined deployment layout (ROM/FLASH “Deployment region”), and the header has CRCs and a fixed table directory; the runtime is designed to scan and locate assemblies deterministically.
• The runtime does not use versions to resolve references because only one version of an assembly can be loaded at a time, and PE references don’t include versions.

Given those constraints, it is generally safe (design inference) for caches to be process-lifetime: once a spec is bound, it remains valid until reboot/reset because the underlying assembly image does not mutate during execution.

Static initialization under generics: tying caches to “cctor + static storage per closed type”

This is where caching and correctness intersect.

In C#, a static constructor is called automatically before the first instance is created or any static members are referenced, and it is called at most once. Also, the presence of an explicit static constructor prevents the relaxed beforefieldinit behavior, tightening when initialization is permitted to occur.

For generics, the runtime must enforce those rules per closed constructed type identity (e.g., Parcel<int> vs Parcel<string>). That makes the closed-type cache more than a performance structure: it becomes the natural anchor for:

• the “cctor has run” flag, and
• the per-instantiation static field storage.

This is also why PE v2.0 explicitly bounds GenericParam indexing (e.g., GenericParamTableIndex as an 8-bit index as it, by design won’t support more than 255 generic parameters. The system is engineered for predictable, bounded metadata and predictable runtime structures.

How the resolver stays fast: shaping the cold path and hot path

With the pieces above, the runtime can make token resolution behave like this:

• Cold path (first time): decode BinaryToken → follow TypeSpec/MethodSpec → decode signature → substitute Var/MVar → bind → allocate/init statics if required → store in cache.
• Hot path (subsequent times): decode BinaryToken → hit cache → use bound handle directly (minimal overhead).

That is the essential “embedded-friendly” design: correctness is paid once, and the steady-state path is kept close to non-generic execution.

End-to-end walkthrough: resolving a call that targets a MethodSpec

To make this concrete, we will follow a single instruction—call—from the moment the interpreter fetches it, through token decoding, through MethodSpec binding, and into the actual call dispatch.

The C# we start from:

public static class Ops
{
public static T Echo<T>(T value) => value;
}

public static class Demo
{
public static int Run(int x) => Ops.Echo<int>(x);
}

A desktop disassembler typically renders the intent like this:

IL_0000: ldarg.0
IL_0001: call int32 class Ops::Echo<int32>(int32)
IL_0006: ret

The important bit is not the opcode; it is the operand. In generic code, that “method reference” is frequently not a plain MethodDef/MemberRef. It can be a MethodSpec, i.e., a method instantiation.

Step 1: Fetch opcode and read the operand token

In nanoFramework PE files, many IL instructions carry a token operand that points back into metadata. Because nanoFramework constrains metadata table indexes, the interpreter reads a compact operand and then has to determine what it represents.

At this point the resolver has two broad cases:

• Direct reference: token maps straight to MethodDef or MethodRef (fast path).
• Specification reference: token maps to MethodSpec, which requires signature decoding (generic path).

MethodSpec exists as a first-class table in nanoFramework PE format v2.0.

Step 2: Identify “this is a MethodSpec token” and load the MethodSpec row

Once the operand identifies the MethodSpec table (table kind MethodSpec exists in the table-kind enumeration), the runtime retrieves:

• the Method field (which points to the generic method definition/reference), and
• the Signature blob (which encodes the generic arguments of the instantiation).

This shape is mirrored by the official metadata APIs as well: a MethodSpecification exposes a Method handle (MethodDef or MemberRef) plus a Signature blob handle. And when emitting metadata, MetadataBuilder.AddMethodSpecification(...) takes exactly those two things: the generic method entity handle and an instantiation blob that encodes the generic arguments.

So far, we have not resolved anything—we have simply identified that this call site points to “generic method + instantiation blob.”

Step 3: Decode the Method field (MethodDefOrRef coded index)

Inside MethodSpec, the “which method is this an instantiation of?” pointer is not a single table reference; it is a coded index: MethodDefOrRef.

nanoFramework uses the ECMA-335 coded-index convention in its compact BinaryToken representation: for MethodDefOrRef, a 1-bit tag selects between MethodDef and MemberRef (the remaining bits are the row index).

So the interpreter does:

1. Read the coded index.
2. Extract the tag bit.
3. Route to either:

• MethodDef table lookup (method defined in this assembly), or
• MemberRef/MethodRef table lookup (method referenced from elsewhere).

This yields the generic method definition we will be instantiating (in our example: Ops.Echo<T>).

Step 4: Decode the instantiation signature blob

Now we decode the MethodSpec signature blob. Conceptually, this blob is “the generic method arguments” (for Echo<int>, it is a single argument: int32). That is precisely how Microsoft describes it in the metadata writer API: “The instantiation blob encoding the generic arguments of the method.”

On nanoFramework, signature decoding is performed using the interpreter’s reduced DataType encoding. For generics support type system explicitly includes:

• Var: “Generic parameter in a generic type definition”
• MVar: “Generic parameter in a generic method definition”
• GenericInst: “Generic type instantiation”

So the blob decoder walks the encoded types and produces a list of argument type handles. For Echo<int>, the result list is simply [System.Int32]. For more complex cases you may see:

• arguments that are themselves generic instantiations (GenericInst), which requires recursive type materialization, or
• arguments expressed as placeholders (Var/MVar), which cannot be bound without context (next step).

Step 5: Apply generic context substitution (VAR/MVAR binding)

If the instantiation blob contains MVar n, it means “use the n-th generic parameter of the current method.” If it contains Var n, it means “use the n-th generic parameter of the enclosing type.” nanoFramework’s type encoding makes both cases explicit.

This is where the interpreter’s earlier architectural change becomes operational:

• the current frame supplies a type context (closed type arguments), and
• the call site supplies a method context (closed method arguments).

In our running example, the instantiation blob is concrete (int32), so substitution is trivial. But this step is mandatory in the general case because MethodSpec signatures and nested signatures can contain Var/MVar placeholders.

Step 6: Bind and cache the instantiated method target

At this point the runtime has:

• the underlying generic method definition/reference (resolved from MethodDefOrRef), and
• the fully bound list of instantiation argument type handles (post-substitution).

Now it can create (or reuse) an internal “instantiated method handle” keyed by something like:

(generic method identity, bound method-argument list)

Caching here is non-negotiable for two reasons:

1. Signature decoding is significantly more expensive than direct MethodDef lookup.
2. Without caching, a hot callsite would repeatedly rebuild the same instantiation.

This is fully aligned with why nanoFramework is an “extended subset” of ECMA-335: keep metadata compact, avoid repeated runtime crawling, and pay the signature/instantiation costs once.

Step 7: Dispatch the call and propagate the method context to the callee

Finally, the interpreter dispatches the call:

• pushes a new frame,
• associates the callee with the instantiated method handle,
• and attaches the method context so that any further token/signature resolution inside the callee can correctly resolve MVar (and Var, if the callee is within a constructed type).

This is the point where MethodSpec stops being “metadata structure” and becomes “execution reality”: the callee is invoked with the correct binding information to preserve CLI semantics.

Closing the generic world: how the Metadata Processor decides what to emit (and why)

One question inevitably follows the “new tables + new resolution rules” story:* if the runtime can’t discover or manufacture new generic instantiations on-device, how do we ensure every required TypeSpec/MethodSpec is already present in the .pe?*

The answer is that the Metadata Processor (MDP) performs a deterministic “closure” over everything an application can reach, and then emits only that reachable set into the nanoFramework PE format.

Why a “closure” is necessary on nanoFramework?

nanoFramework’s PE format is an extended subset of ECMA-335, shaped by MCU constraints: metadata tables are limited, indexes are bounded, and tokens are smaller.

Those constraints are precisely why we cannot rely on runtime “late discovery” patterns that desktop environments can sometimes tolerate. On nanoFramework, the runtime must be able to resolve everything from the deployed metadata, quickly and predictably. That means MDP must ensure that every generic instantiation used by the program is already materialized in metadata form.

Also worth highlighting: MDP isn’t an optional tool in this pipeline—it’s the post-build transformer that takes the Roslyn-produced assembly and converts it into the nanoFramework .pe representation the device runs.

And what “closure” means in practice for generics?

For non-generic code, “reachable metadata” is mostly about ensuring referenced types/methods/fields make it into the reduced PE tables. For generics, there’s an additional rule:

Every closed generic type and every instantiated generic method that the IL can reach must have a metadata representation.

That representation is exactly what TypeSpec and MethodSpec provide:

• TypeSpec gives a token for a type described by a signature blob (which is how constructed types are represented).
• MethodSpec ties a generic method (a MethodDef/MemberRef) to an instantiation signature (the generic arguments at the call site).

And this begs the question: how MDP discovers the required TypeSpec/MethodSpec rows?

The key is that MDP crawls the IL and the metadata references, building an instantiation graph.

Concretely, MDP scans for the constructs that imply generic instantiations:

1. Member references whose “parent” is a TypeSpec In nanoFramework PE, many IL references use “coded indices” (compact 2-byte BinaryTokens). The MemberRefParent coded index explicitly includes TypeSpec as one of its possible parents, and MethodDefOrRef can be a MemberRef. Translation: if IL references a method/field on a constructed type, the containing type is very often a TypeSpec, and MDP must carry that forward.
2. Method calls that go through MethodSpec A generic method invocation that supplies concrete type arguments needs a MethodSpec row (“which method” + “which instantiation signature”).
3. Nested/composed generic signatures Generic instantiations rarely appear “flat.” A constructed type can contain other constructed types in its signature (e.g., Bucket<List<int>>-style nesting), and method/field references can embed these composite shapes.

This is where the closure work becomes recursive: discover one TypeSpec, then also discover the types used inside its signature, and the member refs that depend on it, and the method specs that reference those…

So, why crawling IL/member refs is essential? This can be condensed in the following bullet points:

• If we don’t crawl MethodSpec, we’ll miss generic method instantiations, and we’ll fail at runtime when a call site needs an instantiation context that isn’t in the PE.
• If we don’t expand nested TypeSpec, we’ll emit a top-level constructed type token but miss its inner type arguments (and any dependent member references).
• If we don’t filter aggressively, we inflate metadata—which is dangerous given the constrains on the storage space that the PE is expected to run.

This is a determinist process (and that matters) in the sense that:

• MDP does not “guess.” It follows the references the compiled IL contains (plus whatever additional roots nanoFramework declares as required for execution/debugging).
• The output is deterministic for a given input assembly set. That determinism is what lets the runtime stay simple and fast: it resolves tokens against tables that already contain the closed types/methods the program can actually reach.

This is also why the PE format leans so hard on compact tokens/coded indices: the runtime benefits from pre-shaped metadata, and the MDP is where the “heavy lifting” belongs.

Virtual dispatch and generic interfaces: making ***callvirt ***work with TypeSpec/MethodSpec

So far, we have focused on call (often via MethodSpec) and newobj (often involving TypeSpec). To make the generics implementation feel complete, it is worth explicitly covering virtual dispatch—because the most interesting cases in real applications happen through callvirt, especially when interfaces are involved.

The baseline: what callvirt really means in the CLI

callvirt selects the target based on the runtime type of the instance, not just the compile-time type. It is the instruction used for virtual dispatch and is also commonly emitted for non-virtual instance calls (with the same “choose based on runtime type” semantics).

In CLI terms, virtual methods form an override chain, controlled by rules such as newslot vs overriding and explicit overrides via MethodImpl (.override).

How the type system “builds slots” (virtual) and “builds maps” (interfaces)

At load/type-construction time, the runtime builds two related structures:

1. Virtual method slots (vtable-like layout) Virtual methods participate in an inheritance hierarchy; newslot forces a new virtual member, while non-newslot methods can override inherited virtuals.
2. Interface dispatch tables (interface → candidate implementations) ECMA-335 specifies that the VES constructs an interface table (per interface method, a list of candidate implementations) on the open form of the class, then uses a defined algorithm at invocation time to pick the correct target. Importantly for generics, the invocation algorithm explicitly says it performs lookup starting from the runtime class and substitutes generic arguments specified on the invoking class.

This is exactly where nanoFramework’s earlier design decision (“carry generic context in frames/calls”) becomes necessary: interface/virtual dispatch may require matching signatures after substituting VAR/MVAR and applying the constructed type’s generic arguments.

The three generic scenarios that matter

1) The target method is on a constructed generic type

When you callvirt a method declared on (or overridden by) a constructed generic type (e.g., Bucket<int>), the runtime still follows normal virtual dispatch rules—choose the most derived applicable implementation based on the runtime instance type. Where generics changes the mechanics is in how the call site identifies the member: in generic contexts, metadata often anchors member references to a constructed declaring type, and the CLI model allows overriding/explicit mapping using constructs expressed with TypeSpec (see .override syntax referencing TypeSpec::MethodName).

2) The interface is generic

For a generic interface (e.g., IFoo<T>), the dispatch problem is not just “find a method named M”; it is “find the implementation of IFoo<T>::M for this particular instantiation of the interface/type.”

ECMA-335 makes this concrete:

• The VES constructs the interface table on the open type.
• When an interface method is invoked, the runtime begins at the runtime class and uses that interface table while substituting generic arguments from the invoking class.
• Ordering can matter when multiple implementations exist due to different type parameters; the spec explicitly notes that declaration order can affect which implementation is selected.

That “substitute generic arguments” clause is the key detail to state explicitly in your post, because it ties the CLI dispatch rules directly to nanoFramework’s requirement to carry and apply the type context during dispatch.

3) A generic method is invoked through an interface reference

This is the “stacked” case: you have interface dispatch and a method instantiation.

Two things are worth stating succinctly:

• Interface methods are virtual/abstract by nature, so they are invoked via the callvirt mechanism, and their resolution follows the interface table algorithm (including generic argument substitution).
• When the target method is itself a generic method instantiation, the instantiation is represented by MethodSpec (“which generic method is being instantiated”) plus an instantiation signature (“with which generic arguments”).

In other words: for “generic method through interface,” nanoFramework must do two bindings correctly and in the right order: interface-target selection (using the runtime type + substituted type arguments), and then method instantiation binding (MethodSpec + method generic arguments). Both steps depend on having the correct generic context available at the point of dispatch.

New core types and interfaces added to support generics

To make generics useful in real code — not just something you can declare — the runtime and base class libraries