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WARNING: this is a proof-of-concept and not ready for production

Moldinium

Moldinium2

--- the Moldinium Sample App bound against different UI technologies

The Blazor WebAssembly version of the sample app is hosted here.

Introduction

Moldinium allows you to write code in interfaces such as these:

interface Job
{
    CancellationToken Ct { get; init; }

    Boolean IsRunning { get; set; }

    Int32 Progress { get; set; }

    async Task Run() { /* ... */ }
}

and then creates class implementations for them at runtime through dynamic type creation.

For xaml consumption, those can have INotifyPropertyChanged automatically implemented, eg. to fire when Progress is set to a new value.

For Blazor consumption, those can have a different notification mechanism implemented that allows Blazor components to be written like this:

@inherits LocalComponentBase

<div class="progress" style="height: 4px;">
    <div class="progress-bar" role="progressbar" style="width: @Job.Progress%;"></div>
</div>

@code {
    [Parameter]
    public Job Job { get; set; } = null!;
}

And again, every time Progress changes, this Blazor component re-renders. Take out the reference to Progress and it will no longer re-render.

The same magic that makes this possible is also improving on usage in the xaml world: A job collection may be dependent on all the IsRunning fields of its children:

interface JobCollection
{
    IList<Job> Jobs { get; set; }

    Boolean HaveRunningChildren => Jobs.Any(j => j.IsRunning);
}

In a xaml environment, the created class that implements INotifyPropertyChanged will fire it for HaveRunningChildren the moment it needs to be re-evaluated due to changes of IsRunning in one of its children. In fact, the value of HaveRunningChildren will be cached and multiple reads will retrieve the cached value until a dependent value change invalidates this cache.

This is called value dependency tracking and is a form of state management that has been implemented in the JavaScript world a number of times, but there was none in .NET until now as far as I know.

Since you can't create interfaces with new and don't have access to the implementations that will be created at runtime, Moldinium also provides a simple dependency injection system that allows you to create these types:

interface JobCollection
{
    // Moldinium injects this delegate and calling it will get you a new
    // Job with the passed token set to its Ct property
    Func<CancellationToken, Job> NewJob { get; init; }

    // ...
}

Since interfaces don't have constructors, we use init setters to mark dependencies.

Rationale

It all started with dependency tracking, which I loved since KockoutJS. In JavaScript, it's also the concept behind MobX and VueJS. It helps significantly with UI development by shifting the burden of tracking what needs updating in the presence of changing sources to a generic solution.

However, it requires that all access to trackable variables and computations are wrapped by the dependency tracking framework. Without some form of type creation or proxying this results in ugly application code. MobX, for example, can use JavaScript proxies and needs React components to be wrapped by one of its framework facilities. KnockoutJS and VueJS hide this behind their templating engines.

This is why I created the dynamic type creation part of Moldinium which can bake this logic into the properties for the user and allow clean and simple-looking application code. It looks even cleaner than MobX does in React.

Once dynamic type creation was in the picture, one needs dependency injection to resolve the implementation types that are only present at runtime: new no longer works.

Moldinium's dependency injection can perhaps be replaced by an existing one, but I had some strong opinions on dependency injection anyway - see the relevant notes on implementation below for more on that.

Then, since all properties need to be virtual to allow redefining, I wondered if one wouldn't be better off using only interfaces to write application code right away.

Sample application

There's a all-in-one-file sample application using only interfaces and records. It comes in a class library that doesn't depend on anything besides .NET, not even on Moldinium: Dependency injection purists will rejoice.

This class library is used by three different projects which each provide a UI for it: One for WPF, one for Blazor WebAssembly and a stateless one using ASP.NET MVC.

All depend on Moldinium and ask it to instantiate the application. The WPF one asks for tracking with notifying (through INotifyPropertyChanged), while the Blazor one asks only for tracking. Instead, the Blazor application has all its components derive indirectly from MoldiniumComponentBase which makes the magic work (see the implementation notes about dependency tracking below for details).

The ASP.NET one is content without any notification mechanism: The entities will behave without any magic applied to them. This version of the sample app, however, does something interesting with its default collection to support its multi-threaded environment, see the section below.

Collections and default values

The Moldinium type creator also allows ensuring defaults to non-nullable properties of certain types. After the common case of Strings (which are then defaulted to the empty string), this is mostly a concern with collections.

Besides INotifyPropertChanged, there's also INotifyCollectionChanged for collections, which xaml-based UIs want to see lest they either don't update their list views or recreate all item components when the whole collection instance changes.

Blazor is more forgiving in the latter case as it reconciles anyway, but the former case still necessitates a collection type that can be tracked.

.NET itself provides ObservableCollection<> which is only sufficient if no dependency tracking is desired, but, unfortunately, it doesn't implement IList<>.

Moldinium provides LiveList, which implements INotifyCollectionChanged, but also implements IList<> and has a derived implementation that can be tracked at well.

Since the ASP.NET sample app has to expect multiple threads accessing the sample app concurrently, it provides its own implementation: ConcurrentList, which is a very simple implementation that should be thread-safe. (The sample app itself isn't really thread-safe, but it's good enough for demonstration purposes.)

Default configurations

The entry to your Moldinium-created types are extension methods such as ServiceCollection.AddSingletonMoldiniumRootModel<YourRootType> which take a configuration builder. On resolving the root type instance, it will be instantiated with all the dependencies of the then-available IServiceProvider also provided.

The most important option of the builder is to select a mode from which the implementations of the created types as well as the defaults for collection types derive.

There are four modes:

	Basic	Notifying only	Tracking only	Notif. + Track.
IList<> default	List<>	LiveList<>**	LiveList<>	LiveList<>
ICollection<> default	List<>	ObservableCollction<>	LiveList<>	LiveList<>
Computations cached	no	no	yes	yes
Thread-safe	yes*	yes*	no	no
Uerful for	stateless work	very little	Blazor	XAML

*as long as your app is thread-safe and you use thread-safe implementations for IList<> and ICollection<>

**only because ObservableCollection<> doesn't implement IList<>

Moldinium gives created types the same name as their corresponding interface, but they live in a runtime assembly with a name that shows them to be such created types. For example, types from the ASP.NET Sample end up in an assembly named MoldiniumTypes.Basic.ConcurrentList, showing what to expect from the types therein.

Moldinium standard semantics for interface implementations

Here's an overview of Moldiniums standard semantics:

interface MyInterface
{
    // A potentially trackable variable, defaults to "" because not nullable
    String Name { get; set; }

    // A potentially cached computation because property and implemented
    String UpperCaseName => Name.ToUpper();

    // Also a potentially cached computation but it would also invalidate on writing
    Int32 WritableComputation { get { /* ... */ } set { /* ... */ } }

    String Unimplemented { get; } // error: no implementation

    // Methods are never cached
    String GetUpperCaseName() => Name.ToUpper();

    // Optional dependency
    ADependency1? OptionalDependency { get; init; }

    // Essential dependency, doesn't need a default either because it
    // is expected to be initialized
    ADependency2 RequiredDependency { get; init; }
}

Note that we assume that this is the interface to create a type from, ie. the moldinium type. Of course, a base interface can declare the Unimplemented property and a derived interface can still have a class type created if it implements this property.

A new language?

One could interpret this new style of coding as being a "new language" that has been "discovered within C#", somewhat like JSON was discovered within JavaScript. The analogy isn't perfect as JSON throws away the vast majority of JavaScript whereas this new language throws away only a very small part (the classes and their constructors).

I couldn't think of a great name yet. Contenders are "c minus" (which may already be taken) and "calm c".

The "new language" perspective gives some justification for breaking the C# style guide of naming interfaces only with a prefixed "I". Instead, we're prefixing interfaces with an "I" if they still play the role of interfaces. If they play the role of concrete types, they lose the "I".

Outlook

Maturity and State of the Library

This library is a proof-of-concept and not ready for production. There's a substantial amount of details that are not yet implemented properly and what's there is likely quite buggy.

One example would be that while the dependency injection component tells apart optional dependencies from essential (required) ones, it does so only for init setters. A factory declaration like Func<Dep1, Dep2?, Foo> will not allow you to pass null for the second dependency at runtime. For this to work, some work on nullability would have to be done, as the information about nullability is stored in a complicated fashion and lives in unexpected places (see here). For example, the one for the factory above would live on the property where it's injected. A facility that should help with this, NullabilityInfoContext, is not available in Blazor WebAssembly (strangely it is when I run it locally - I noticed the problem only after I deployed the sample to Azure).

Then there is "an icky part of the bakery" I talk about below in the notes on implementation.

What to expect

It's unlikely I can afford to bring this library to a really mature state, let alone maintain it after that.

I document the in my opinion quite elegant design below in part so that someone with more free time has something to build on in the following sections.

Also note that after those, there are some notes on further features I would regard as essential for practical use.

Notes on implementation

Separation of Concerns

The three components baking (type creation), tracking and injecting do not depend on each other and could as well each have their own library. The source layout is this:

Common -utilities usable by all three components
Components
- Baking
- Tracking
- Injection
Combined -putting the three components together

Dependency Tracking

Dependency tracking requires a common tracking repository and having all computations (eg. the implementation of HaveRunningChildren in the introduction) executed within an evaluation scope in that repository.

The repository executes the computation and when, during, the computation accesses a trackable (eg. IsRunning), this trackable tells the repository about it being read. After the computation is completed, the repository now does not only know the result, but also which trackables have been read - those are the ones the computation depends on: If one of those changes, the computation changes, ie. HaveRunningChildren invalidates.

In the current Moldinium implementation, the repository is a static singleton. This is how it's done in JavaScript and while it's not an issue there, in .NET it would be an issue for Blazor Server where multiple circuits need to be separated. A future version should do this properly.

In any case, only one thread must be evaluating anything at a time for each repository and that's a conceptual limitation. A future version should guard against misuse here.

There are some further features that MobX does and Moldinium doesn't yet do, as they are not strictly necessary for a proof-of-concept. Those are transactions and early recomputation (computed - ie. implemented - properties re-evaluate their cache even before anything tries to read them).

The former is mostly an optimization and the latter improves debugging.

Dynamic Type Creation

Moldinium's type creator is called the bakery.

The bakery creates types using System.Reflection.Emit and does some CIL weaving.

Instead of monolithically doing just what is needed for this proof-of-concept, the bakery is very configurable for a variety of purposes. In particular, it does not know about the mode (basic, notifying, etc.) discussed above and instead is configured by getting a default provider as well as a number of implementation and wrapper structs.

It creates a type by looking at a number of interfaces to implement and separately considers properties, events and methods. Each of these can in principle be requiring an implementation (if they are not yet implemented by an interface) or a wrapper (if they are).

The implementation and wrapper structs are then baked into the resulting type as fields. Those structs are provided for each of the modes talked about above. The following is a wrapper struct for wrapping an implemented property to implement INotifyPropertyChanged:

public struct NotifyingComputedPropertyImplementation<Value, Container>
    : INotifyingComputedPropertyImplementation<Value, Container, NotifyingPropertyMixin>
    where Container : class
{
    // Just to make the bakery happy
    public Boolean BeforeGet() => true;

    public void AfterSet(Container container, ref NotifyingPropertyMixin mixin)
    {
        mixin.NotifyPropertyChanged(container);
    }

    // Presumably the value hasn't changed if the setter threw;
    // the return value indicates that the exception should be rethrown
    public Boolean AfterErrorSet() => true;
}

(taken from here)

The newly created type gets a field of this struct for each such property and one field of type NotifyingPropertyMixin (also a struct) shared by those former fields.

As you see, the bakery itself therefore does not proxy anything and the constructed type does not necessarily require heap allocations except the one for itself.

The interface the struct above derives from is defined like this:

public interface INotifyingComputedPropertyImplementation<
    [TypeKind(ImplementationTypeArgumentKind.Value)] Value,
    [TypeKind(ImplementationTypeArgumentKind.Container)] Container,
    [TypeKind(ImplementationTypeArgumentKind.Mixin)] Mixin
> : IPropertyWrapperImplementation
    where Container : class
{
    Boolean BeforeGet();

    void AfterSet(Container container, ref Mixin mixin);
    Boolean AfterErrorSet();
}

When the bakery receives the struct implementation or wrapper type for a property, it analyzes this interface first to understand what the types on its methods are supposed to mean. It then creates code for the getters and setters of the wrapped property on the created type with CIL weaving. This code calls them with the given parameters according to the interface definition. The methods that implement the wrapping code, such as AfterSet, must have one of some pre-defined names that imply their purpose, but their parameters can be any, as long they are declared this way.

The moment when looking at this interface is also when the bakery realizes that NotifyingPropertyMixin actually is a mixin (again because of the type attribute) and that its interface (INotifyPropertyChanged) should become an interface of the created type and all the properties, events and methods of that interface also need implementing.

This design allows defining type creation with clear separation of concerns and an easy way to provide additional custom wrappers and implementations for properties.

Unfortunately, multiple wrappers are not yet implemented, so it's currently not easy to do so in combination with the ones needed for tracking and notifying (unless you replicate their code in your implementation and wrapper). This would obviously be very useful and may, again, come in a later version.

Details about the implementation and wrapper structs

The structs are expected to have a certain set of expected names. For properties, an implementation is expected to have a single one for each property method:

struct MyPropertyImplementation<..., T, ...> : ...
{
    public void Init(...);
    public T Get(...);
    public void Set(...);
}

The init method is used to set default values and is called from the created type's constructor. Wrappers are more complex:

struct MyPropertyWrapper<...> : ...
    public Boolean BeforeGet(...);
    public void AfterGet(...);
    public Boolean AfterErrorGet(...);

    // same for the setter

The return value for Before methods indicates whether the wrapped method should be called. A cache, for instance, may choose not to.

The return value for AfterError methods indicates whether the exception should be rethrown.

For events, the picture is analogous, with Add and Remove instead of Get and Set. For non-special methods there are only wrappers and the methods are only called Before, After and AfterErrror.

Again, the method parameters themselves can be configured freely with the interface definition the implementation or wrapper derives from. The ImplementationTypeArgumentKinds that are supported are:

Value: Property type (only property structs)
Handler: Event handler type (only event structs)
Return: Method return type (only method structs)
Exception: Exception type for the AfterError* methods, should be constrained to Sytem.Exception
Container: The baked type
Mixin: A mixin

The first three are conceptually similar and must be taken by value in implementations and by ref in wrappers.

Although this design would also allow multiple mixins to be used, this isn't currently allowed.

While you can't debug the generated IL code, you can debug those implementation and wrapper structs when they live in your own assemblies or you disable "just my code".

An icky part of the bakery

Due to a limitation in .NET's reflection APIs I had to work around, there's a messy part that is likely buggy and definitely unable to handle all desired cases.

When a class method implements a base method, the reflection API allows to extract this information with the MethodInfo.BaseMethod property. Interfaces can now also implement other interface methods:

public interface IBaseInterface
{
    public String Name { get; }
}

public interface IImplementingInterface : IBaseInterface
{
    String IBaseInterface.Name => "foo";
}

As in the case of classes, the information appears in the disassembly as an override declaration on the method body:

.method private hidebysig specialname virtual final 
        instance string  IBaseInterface.get_Name() cil managed
{
  .override IBaseInterface::get_Name
  // Code size       6 (0x6)
  .maxstack  8
  IL_0000:  ldstr      "foo"
  IL_0005:  ret
} // end of method IImplementingInterface::IBaseInterface.get_Name

However, there's no way to get at this information using reflection.

Moldinium works around this by collecting all the methods together that have the same name and signature and then see if there's a unique implementation. Since SignatureHelper is also not available on Blazor WebAssembly there's a lot of code that is reproducing .NET runtime logic, which is probably quite buggy.

The current implementation has also more limitations than it needs to, but I'd rather see the .NET folk fix this little oversight in the reflection APIs than work on this further.

You can't implement all baked classes in C#

Interestingly, the baked types can be of a form that you can't implement purely in C# (without again using reflection helpers anyway).

In the example of the previous section, the Name getter may have to be wrapped, ie. the baked type also has a Name getter that must call down to IImplementingInterface.Name's getter. At the same time, the Name of the baked type must again implement IBaseInterface.Name.

This requires that the baked type's function must call IImplementingInterface.Name's getter non-virtually. (A virtual call would be an infinite recursion.)

For classes, you can call a base's methods non-virtually in C# by using base.Name, but for interfaces, there is no such thing (and there's no unique base, so the syntax would have to be something like base(IImplementingInterface).Name if it was to exist in C#).

On the CLR level, however, you can just call the method non-virtually like any other.

(If it wasn't private, that is. In fact, the dynamic assembly has to be also annotated with an undocumented IgnoresAccessChecksToAttribute to allow calling those methods; another thing that had cost me days to figure out.)

Dependency Injection

Nested-scopes IoC Containers

Most IoC containers have three different modes of registering types in three different modes: singleton, scoped and transient.

They also do lifetime management.

I think this design can be improved by thinking of these three modes as nested scopes living in a tree, the root scope containing the singletons and the transient object living in their own leave scopes.

Classical IoC containers offer those three modes mostly so that the dependencies for each of those modes can be checked at container validation time, which is usually when the program starts or when tests are run. Another reason is so that code that creates new scopes don't need a dependency on the IoC container. It follows that lifetime management must be done by the container: if it creates the instances on each level, it needs to dispose of them too.

If those two initial issues (dependency on the container and early validation) are deemed irrelevant, one could just create a new container at the time a nested scope is needed, inject new "singletons", and resolve again there.

Moldinium instead allows resolving factories that can be used to create subscopes and provide new dependencies for them in a manner that still allows early validation.

Take the following example:

interface App
{
    Func<CancellationToken, Job> NewJob { get; init; }

    async Task Run(CancellationToken ct)
    {
        await using var job = NewJob(ct);

        await job.Run();
    }
}

interface Job : IAsyncDisposable
{
    Func<RequestConfig, Request> NewRequest { get; init; }

    async Task Run()
    {
        var request = NewRequest(new RequestConfig(new Uri("...")));

        await request.FetchContent();
    }

    // Our job needs this for mysterious reasons
    async ValueTask IAsyncDisposable.DisposeAsync() { /* ... */ }
}

record RequestConfig(Uri Url);

interface Request
{
    RequestConfig Config { get; init; }

    CancellationToken Ct { get; init; }

    HttpClient HttpClient { get; init; }

    async Task FetchContent() { /* ... */ }
}

The top scope App is resolved by the application setup code, likely in Program.cs.

First Moldinium validates the dependencies:

App needs a factory that creates Jobs but for those it provides CancellationTokens
Job needs a factory that creates Requests, but for those it provides RequestConfigs
Request gets a CancellationToken from two scopes up and now has all except HttpClient, which we assume was provided by the root setup

By making the factories be typed on the dependencies that will be provided, Moldinium can check all that at validation time before any instances are created.

I also see no reason for lifetime management to be part of the container with this design: All object creation is done only when calling factory functions, so it's now the user that creates. The user should then dispose as well.

This simplifies the matter considerably: Lifetime management is far from trivial, especially when you consider IAsyncDisposable and exception handling during cleanup. That can now be done in the usual C# fashion using usings and await usings and debugged accordingly.

The Dependency Report

Moldinium can give a dependency report. For the example above that is:

    - Bakery resolved fei`App
      - InitSetter resolved feb`App
        - Activator resolved veb`App
          - AcceptRootType resolved teb`App
        - Factory resolved foc`Func<CancellationTokenJob>
    
    - subscope for foc`Func<CancellationTokenJob>
      - Bakery resolved fei`Job
        - InitSetter resolved feb`Job
          - Activator resolved veb`Job
            - AcceptRootType resolved teb`Job
          - Factory resolved foc`Func<RequestConfigRequest>
    
      - subscope for foc`Func<RequestConfigRequest>
        - Bakery resolved fei`Request
          - InitSetter resolved feb`Request
            - Activator resolved veb`Request
              - AcceptRootType resolved teb`Request
            - ServiceProvider resolved foc`HttpClient
            - FactoryArguments resolved foc`RequestConfig
            - FactoryArguments resolved fes`CancellationToken

You can nicely see how the subscopes nest here, and how arguments are used for resolving dependencies.

The prefixes (fei, veb, etc.) have the following meaning:

first character is the runtime maturity:
- t: type without instance
- v: uninitialized instance (virgin)
- f: finished / externally initialized (init-setters set)
the second character can be e for essential (required) and o for optional
the third character is a property of the type itself:
- b: baked class (taken from an attribute the bakery puts on the type)
- c: class or record
- i: interface
- s: struct

The first two of those are an additional disambiguator of the dependency. Something can depend on a type or a finished instance - those are not the same thing.

The last of those is a property of the type itself and is just there for the benefit for this report.

Dependency providers and the resolution process

The injection system resolves by going from a root dependency asking a number of dependency providers if they can resolve the dependency.

The names in the report are the class names of such providers, minus the DependencyProvider suffix. So, Bakery is the BakeryDependencyProvider and it's configured to provide a f*i* for any f*b* that counts as a molidinum type (you configure that with IdentifyMoldiniumTypes on the configuration builder).

The InitSetter then says it's able to provide a finished instance if it has a freshly created instance and some further dependencies coming from the properties that need to be set.

The Activator can provide such a virgin instance provided the type itself is known, which we assume for the root types of all scopes and is provided by AcceptRootType.

The root types are those you explicitly hooked up with one of the Add*MoldiniumRoot and also those returned by factories (they are the roots of subscopes).

The Factory is responsible for resolving factory delegates. It's special in that it marks the returned resolution as requiring a subscope, which the dependency injection system handles after the current scope is completely resolved.

The Factory also provides an additional dependency provider for the new subscope, the Arguments that will provide the dependencies injected through the arguments of the delegate.

After the dependency injection system has resolved everything for the current scope, it goes to the recorded subscopes and recursively starts again.

This whole logic lives in the Scope class which represents a scope at validation time and also forms a tree with its subscopes.

There's also RuntimeScope, which is the representation of an instantiated scope. There will be as many instances of RuntimeScope for each Scope as there were calls to the factory that represents the scope (and usually only one for the root scope).

Further essential features not yet implemented

Internal Dependency Resolution, Entity Framework and Deserializers

Besides maturity, one of the main limitations of the current implementation is that it can't be used with deserializers: Those do reflection themselves. Upon trying to create an object for a property they will complain that they don't know what type to use when that property's type is just an interface.

This could be solved by making the baked type implement the interfaces privately and instead expose the properties with different types: The baked types.

Then, the only remaining issue is that these types need to be default-constructible. They technically already are, but a deserializer will not have their init setters initialized by using the correct dependency injection.

While the model types typically used in these scenarios may not really need proper dependency injection, there is one dependency that is needed: That of the tracking repository (once it's no longer static).

To make this work, I'm eyeing ambient storage: .NET has the wonderful ability to allow methods to be called with a call context that can store stuff. It's like thread-local storage, but also works across asynchronous chains of execution.

Before calling the deserializer, the dependency injection system would be installed in ambient storage and then retrieved by the constructor of a baked type to do internal dependency resolution. This would allow dependency injection to work in these scenarios and also make the baked type's instance have the correct tracking repository.

Another issue is type hierarchies: Deserializers require a single-base hierarchy for polymorphic deserialization and, currently, baked types don't derive from other baked types at all. This could be done though, but of course requires the user to somehow sometimes distinguish a unique base.

On the plus side, such libraries are generally willing to use an existing collection instance in properties typed as IList<> or ICollection<>. So, at least there is no additional trouble on that front.

Entity Framework

Entity Framework is even more important than deserializers as entities are more often bound against in UIs. It's similar in that it also wants to create its entity types. It has one major additional problem: Fluent configuration.

The best solution that can be implemented with reasonable effort would offer the user to configure like this:

using Outer = NamespaceWhereTheModelsLive;

namespace NamespaceWhereTheModelsLive;

// ...

public interface IMyInterfaceDbContext<
    EntityType1,
    EntityType1
>
    where EntityType1 : Outer.EntityType1
    where EntityType2 : Outer.EntityType2
{
    DbSet<EntityType1> Entities1 { get; set; }
    DbSet<EntityType2> Entities2 { get; set; }

    void OnModelCreating(ModelBuilder builder)
    {
        builder.Entity<EntityType1>(b => b.HasKey(e => e.Id));
    }
}

Moldinium could then provide a baked implementation for IMyInterfaceDbContext simply by providing the correct arguments to the type parameters. The result should be good enough for EF, but I haven't tried.

This does require some repetition of type names, obviously, but at least it's only one place per context that is ugly.

Another option would be to write a wrapper around EF's ModelBuilder. This would be very specific to EF and may require maintenance when EF's ModelBuilder changes.

Access control

Interfaces can't have private members (except private method implementations), but they can have protected members.

Moldinium should allow that. I'm thinking about using protected init setters to mark internal dependency resolution (see the section above).

Dependency Tracking

I already mentioned supporting Blazor Server by making the repository non-static and have it injected.

The tracking system is also quite wasteful as it needs each trackable to have its own object. This can lead to an explosion of heap allocations in a complex application. It would be better if all trackables on a single moldinium model share a single heap instance to communicate with the repository.

The tracking system could also do with a lot more tools guarding against misuse and diagnostic facilities, such as a dependency report similar to that the dependency injection system has.

Dependency Injection

Already mentioned is the weakness regarding nullable types for factories.

There's another issue that users likely desire: To globally define additional dependencies provided for the created type of a specific factory without having to specify those as argument types or passing them at the factory call.

This is the way scoped dependencies are set up in most IoC containers, and it should be an option here as well.

The user can declare a factory type explicitly as a delegate:

public delegate MySubscopRootType MySubscope( /* ... */ );

public interface SomeType
{
    MySubscope CreateMySubscopeRootType { get; init; }

    // ...
}

Then, it should be allowed to register additional dependencies on the application root identifying the subscope with this delegate type in the same way other IoC containers allow it.