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A set of utilities used to work with Nuqleon data model entity types.

Type system

The DataType base class represents a Data Model compliant data type. The structure of this class hierarchy reflects the supported types, with the different kinds specified in the DataTypeKinds enum. These include:

  • Primitive, e.g. to support int.
  • Array for single-dimensional arrays.
  • Structural for structural types, where properties are modeled as DataProperty.
  • Function for function types, e.g. delegate types in the CLR type system.
  • Expression for expression trees that represent code as data.
  • Quotation for quotations of functions as expression trees.
  • OpenGenericParameter for support of wildcards (e.g. to support binding).

The base class is shown below:

public abstract partial class DataType
    public abstract DataTypeKinds Kind { get; }

    public Type UnderlyingType { get; }

    public virtual DataType Reduce() => this;

    public abstract object CreateInstance(params object[] arguments);

Derived classes exist for all of the different kinds, as listed above. These have specified properties, e.g. an ElementType on the ArrayDataType.

A couple of things are worth noting on the DataType base class:

  • Data model types have underlying CLR types exposed through UnderlyingType.
  • Instances of data model types can be created using a CreateInstance method. This relies on the underlying CLR type; the arguments depends on the kind.
  • Custom data model types are supported (e.g. supporting other types of objects from foreign libraries), which can be Reduced to a standard data model type.

In many cases, data model types are purely used to support expression tree rewrites, and thus provide an intermediate representation to reason over type structures. To enable this functionality, conversions from CLR Types to data model DataTypes are provided through a set of static methods, shown below:

public abstract partial class DataType
    public static DataType FromType(Type type);
    public static DataType FromType(Type type, bool allowCycles);

    public static bool TryFromType(Type type, out DataType result);
    public static bool TryFromType(Type type, bool allowCycles, out DataType result);

    public static void Check(Type type);
    public static void Check(Type type, bool allowCycles);

    public static bool TryCheck(Type type, out ReadOnlyCollection<DataTypeError> errors);
    public static bool TryCheck(Type type, bool allowCycles, out ReadOnlyCollection<DataTypeError> errors);

    public static bool IsStructuralEntityDataType(Type type);
    public static bool IsEntityEnumDataType(Type type);

All of the From* and Check variants take in a CLR Type and perform a conversion to the corresponding DataType. As usual, Try variants return a bool value to indicate success, while other variants throw exceptions. Cycles can occur in nominal types, and can optionally be permitted in data model types as well. However, when doing so, serialization as JSON is not possible because there's no notion of object identity that enables references between instances.

Two additional Is checks can be used to check whether a CLR type is a structural type (i.e. has MappingAttributes applied to its members) or a data model compliant enum, i.e. one where all of the enumeration values have a MappingAttribute.

The rules for conversions of CLR types to DataType are summarized below:

  • PrimitiveDataType:
    • void becomes a PrimitiveDataType with Unit as the underlying type;
    • primitives (integers, bool, char, string, floats, decimal, DateTime[Offset], TimeSpan, Uri, and Guid) map to PrimitiveDataType;
    • enums with a supported primitive underlying types and valid MappingAttribute annotations also map to PrimitiveDataType;
    • nullable primitives and enums, as described above, also map to PrimitiveDataType;
  • ArrayDataType for single-dimensional array-like types with index-based access and elements of a data model type:
    • T[] arrays, IList<T>, IReadOnlyList<T>, and List<T> become ArrayDataTypes;
  • FunctionDataType supports functions with parameter and return types that are data model types:
    • delegate types become FunctionDataTypes;
  • ExpressionDataType supports code-as-data:
    • types derived from Expression (except for Expression<T>) become ExpressionDataTypes;
  • QuotationDataType supports quotations of functions:
    • Expression<T> becomes a QuotationDataType;
  • StructuralDataType:
    • structural types (anonymous types, record types, and data model entities with MappingAttributes) become StrucuturalDataTypes;
    • tuples using System.Tuple<...> types (but not System.ValueTuple<...> for historical reasons) become StructuralDataTypes as well, with properties that have ItemN names.

Note: Record types in this library refer to the notion of record types introduced by Nuqleon.Linq.CompilerServices which long predates the addition of record types in C# 9.0. They are, however, semantically close to C#'s record types, in terms of value equality semantics and their support for construction through property assignments (albeit not constrained to "initialization contexts" as introduced in C# 9.0).

Type visitors

Once a DataType has been obtained, it can be useful to recurse over its structure. This is supported through the use of DataTypeVisitor and DataTypeVisitor<TType, TProperty>. Both visitor types provide protected virtual Visit* methods for the various subclasses that derive from DataType, and a VisitProperty method to handle DataPropertys referenced from StructuralDataTypes.

The non-generic visitor variant simply traverses over the structure of the given DataType and produces a new DataType if any of the recursive visits returns a different instance. This can be used to rewrite data types.

public class DataTypeVisitor
    public virtual DataType Visit(DataType type);

    protected virtual DataType VisitArray(ArrayDataType type);
    protected virtual DataType VisitExpression(ExpressionDataType type);
    protected virtual DataType VisitFunction(FunctionDataType type);
    protected virtual DataType VisitOpenGenericParameter(OpenGenericParameterDataType type);
    protected virtual DataType VisitPrimitive(PrimitiveDataType type);
    protected virtual DataType VisitQuotation(QuotationDataType type);
    protected virtual DataType VisitStructural(StructuralDataType type);
    protected virtual DataType VisitCustom(DataType type);

    protected virtual DataProperty VisitProperty(DataProperty property);

    protected T VisitAndConvert<T>(DataType type);

    protected virtual Type ChangeUnderlyingType(Type type);
    protected virtual MemberInfo ChangeProperty(PropertyInfo property);
    protected virtual MemberInfo ChangeField(FieldInfo field);

    public static ReadOnlyCollection<T> Visit<T>(ReadOnlyCollection<T> nodes, Func<T, T> elementVisitor);

When a Visit method returns a different DataType than the one that was passed in, or when VisitProperty returns a different DataProperty, calls to Change* methods are made to rewrite the UnderlyingType or Property value. If the visitor is used to rewrite data types or its properties, these Change* methods should be overridden. The idea of this type of rewrite is to treat a DataType as a constrained CLR type and to ensure that every rewrite of such a type still has a valid underlying CLR type representation. An example of such a rewrite is to substitute entity types for structurally identical anonymous or record types, e.g. whereby a Person object gets replaced by some <>__AnonymousType1 runtime-generated type, and the Person.Name property gets replaced by the <> property (where the new property name was derived from a MappingAttribute on the original property).

The generic visitor requires two generic arguments to specify the rewrite target for DataType (represented as TType) and for DataProperty (represented as TProperty). This visitor is an abstract class and requires implementation of various Visit and Make methods:

public abstract class DataTypeVisitor<TType, TProperty>
    public virtual TType Visit(DataType type);

    protected virtual TType VisitArray(ArrayDataType type);
    protected abstract TType MakeArray(ArrayDataType type, TType elementType);
    protected abstract TType VisitExpression(ExpressionDataType type);

    protected virtual TType VisitFunction(FunctionDataType type);
    protected abstract TType MakeFunction(FunctionDataType type, ReadOnlyCollection<TType> parameterTypes, TType returnType);

    protected abstract TType VisitOpenGenericParameter(OpenGenericParameterDataType type);

    protected abstract TType VisitPrimitive(PrimitiveDataType type);

    protected virtual TType VisitQuotation(QuotationDataType type);
    protected abstract TType MakeQuotation(QuotationDataType type, TType functionType);

    protected virtual TType VisitStructural(StructuralDataType type);
    protected abstract TType MakeStructural(StructuralDataType type, ReadOnlyCollection<TProperty> properties);

    protected abstract TType VisitCustom(DataType type);

    protected virtual TProperty VisitProperty(DataProperty property);
    protected abstract TProperty MakeProperty(DataProperty property, TType propertyType);

The Visit methods recurse over the structure of non-leaf types (e.g. visiting the element type of an array), and invoke corresponding Make methods to construct the resulting TType given the rewritten child nodes. The same mechanism is used to rewrite properties. An example usage of this visitor is to convert DataType to a corresponding TypeSlim (for use by Bonsai expression trees), or to create a string representation of a type for debugging purposes.


The DataTypeEqualityComparer<T> type implements IEqualityComparer<T> and can be used for any type T that is a valid data model type (i.e. passes DataType.Check(Type) tests). The equality comparer checks for value-based equality by recursing over the structure of the data type while performing pairwise checks of the values passed in. For example, given a class Person:

class Person
    public string Name { get; set; }

    public int Age { get; set; }

the following two values will compare equal:

var people1 = new Person[] { new Person { Name = "Bart", Age = 21 } };
var people2 = new Person[] { new Person { Name = "Bart", Age = 21 } };

That is, the elements in arrays are compared in a pairwise fashion, and properties on objects are compared pairwise as well. Reference equality does not play a role in determining equality. Another way to think about this is whether the corresponding JSON serialized representation of both objects would be identical.

Note: This type of comparer is especially useful when implementing query operators that rely on value-based equality of data model compliant types. For example, an operator such as DistinctUntilChanged relies on correct value-based equality semantics. Note though that runtime-generated record types do have equality semantics built in through an IEquatabl<T> implementation, so the use of a specialized IEqualityComparer<T> is not always needed for such types. (Subtlety can arise when dealing with arrays or lists where equality is based on reference equality of the collections rather than a comparison of the elements.)


The KnownTypeBase<T> base class implements IEquatable<T> in terms of DataTypeEqualityComparer<T> and can be used as the base class for [KnownType]-annotated data types in order to ensure that values have value-based equality. For example, when implementing a WeatherObservable as an ISubscribable<Weather>, it'd make sense to define Weather as follows:

class Weather : KnownTypeBase<Weather>
    public double Temperature { get; set; }

    // Etc.

This ensures that the default equality comparer obtained through EqualityComparer<T>.Default will pick up on the IEquatable<T> implementation provided through KnownTypeBase<T>. This in turn allows operators that rely on value-based equality (such as DistinctUntilChanged) to behave correctly without having to specify custom comparers.

Note: Specifying custom equality comparers throughout the Reaqtor stack is also non-trivial because the IEqualityComparer<T> interface does currently not have a quoted variant in the Reaqtor stack. As such, a user can't refer to a well-known predefined comparer (using a URI to refer to a KnownResource), or use some Create factory method to construct an anonymous comparer based on lambda-based implementations for Equals and GetHashCode. Right now, only "reactive artifact types" are supported throughout the stack, but this set of types could be augmented with things such as IEqualityComparer<T>, IComparer<T>, IScheduler, etc., which may occur as parameters on query operators. This moves the Reaqtor stack closer to a general-purpose "computation space" rather than just a "reactive processing space". In fact, this type of work has been carried out in an internal spin-off project called "ICP" (for IComputationProcessing) as a step up from "IRP" (for IReactiveProcessing), where a computation space is seeded with a set of interfaces whose transitive closure represents the first-class concepts that can cross client and service boundaries through expression shipping (an example of an "ICP" space being a workflow processing system centered around an IControlFlowNode construct). In that worldview, Reaqtor is merely an instantiation of "ICP" for the IReactive* interfaces.

Type substitution

When serializing an expression tree that uses .NET types to represent data types that have a structural nature (i.e. using Mapping attributes on properties), we want to shake off the nominal nature of such types, i.e. the type name and the assembly in which the type was declared. Instead, we want to capture the structure of the type, i.e. the properties and their types. One way to trade nominal types for structural types is by performing an expression tree type substitution step. The base class for such rewrites is the ExpressionEntityTypeSubstitutor abstract class, which relies on an EntityTypeSubstitutor utility.

The EntityTypeSubstitutor is an expression tree visitor that's parameterized on an IDictionary<Type, Type> which specifies the types to be substituted. It relies on two abstract members to be provided by derived types.

public abstract class EntityTypeSubstitutor : TypeSubstitutionExpressionVisitor
    protected EntityTypeSubstitutor(IDictionary<Type, Type> typeMap);

    protected abstract Expression CreateNewExpression(Type type, IDictionary<MemberInfo, Expression> memberAssignments);
    protected abstract object ConvertConstantStructuralCore(object originalValue, StructuralDataType oldDataType, StructuralDataType newDataType);

The CreateNewExpression method is called whenever a NewExpression is encountered whose type is subject to substitution. This enables derived classes to adapt to the construction strategy for the target structural type. For example, anonymous types use constructors but record types use property assignment. The ConvertConstantStructuralCore method is called whenever a ConstantExpression is encountered whose type is subject to substitution. In this case the Value of type System.Object needs to be converted, which is subject to the specific semantics provided by the derived class.

In order to use this functionality, one typically uses a wrapper type called ExpressionEntityTypeSubstitutor, which is an expression tree visitor as well. This type is parameterized on an EntityTypeSubstitutor, as discussed above, and provides the base class facilities to perform rewrites of structural types to anonymous types or record types. In order to carry out a rewrite, the top-level Apply method is used:

public abstract partial class ExpressionEntityTypeSubstitutor
    protected ExpressionEntityTypeSubstitutor(EntityTypeSubstitutor substitutor);

    public virtual Expression Apply(Expression expression);

    protected abstract TypeBuilder GetTypeBuilder(RuntimeCompiler rtc);
    protected abstract void DefineType(RuntimeCompiler rtc, TypeBuilder builder, IEnumerable<KeyValuePair<string, Type>> properties);

Implementations of these abstract methods pick either anonymous types or record types to generate at runtime. This is done by the ExpressionEntityTypeAnonymizer and ExpressionEntityTypeRecordizer derived classes. An example of performing such a rewrite is shown below:

Expression<Func<Person, bool>> f = p => p.Age > 21;

var rw = new ExpressionEntityTypeAnonymizer();
var res = rw.Apply(f);

In the resulting expression, the Person type will have been substituted using an anonymous type that uses the Mapping attribute values for the property names. When passing this expression to a subsequent conversion step to ExpressionSlim, the nominal Person type will have entirely disappeared and the resulting anonymous type will be treated as a structural type. This structural nature will be preserved throughout expression tree serialization and deserialization steps using Bonsai. The final conversion from ExpressionSlim back to Expression can either reconstruct a structurally equivalent type at runtime, or be subject to rewrite steps that perform binding, which may involve substituting the structural type for a known nominal type that has the same structure (e.g. a type marked with the KnownType attribute).

Note: Expression rewriting involving the runtime compilation of new types can be quite expensive. When the only goal is to take the resulting expression tree and serialize it (but not directly evaluate the expression tree), cheaper options are available in the form of expression rewrites in the ExpressionSlim space, as discussed below.

Bonsai utilities

It's very common to substitute nominal data types for structurally equivalent data types prior to expression tree serialization. One way to achieve this is by performing a rewrite in the Expression space, which requires runtime compilation of structural types (e.g. record types). An alternative is to apply this rewrite in the ExpressionSlim space, which can be much cheaper because it doesn't involve runtime compilation and the use of reflection. To support this, we support types such as ExpressionSlimEntityTypeRecordizer that are functionally equivalent to the non-slim variants discussed above.