Diagnostics

Compiler diagnostics emitted by DesignPatterns source generators and analyzers. IDs are defined in DesignPatterns.Diagnostics.DiagnosticIds.

Help links in the IDE point to this page (#dp### anchors).

Singleton (DP001–DP002)

ID	Severity	When
DP001	Error	[GenerateSingleton] target is not partial
DP002	Error	Invalid singleton target (must be sealed class with parameterless ctor)

Strategy (DP003–DP007)

ID	Severity	When
DP003	Error	Duplicate strategy key
DP004	Error	Implementation does not match strategy contract
DP005	—	(reserved / handler overlap — see DP005 under Chain)
DP006	Info	Implementation type not registered with [RegisterStrategy]
DP007	Error	Strategy implementation missing parameterless constructor

Chain of Responsibility (DP005, DP008–DP009, DP024)

ID	Severity	When
DP005	Error	Duplicate [HandlerOrder] value
DP008	Error	Handler does not implement the pipeline contract
DP009	Error	Handler missing parameterless constructor
DP024	Info	Handler type not registered with [HandlerOrder]

Composite (DP010–DP015)

ID	Severity	When
DP010	Error	Duplicate composite part key
DP011	Error	Unknown parent key
DP012	Error	Cycle in composite tree
DP013	Error	Part does not match node contract
DP014	Error	Part missing parameterless constructor
DP015	Error	Registry holder missing [CompositeBuildable]

Decorator (DP016–DP019)

ID	Severity	When
DP016	Error	Duplicate decorator order
DP017	Error	Decorator does not match contract
DP018	Error	Type does not implement IDecorator<T>
DP019	Error	Decorator missing parameterless constructor

Factory Registry (DP020–DP023)

ID	Severity	When
DP020	Error	Duplicate factory key
DP021	Error	Factory does not match product contract
DP022	Error	Factory missing parameterless constructor
DP023	Info	Product type not registered with [RegisterFactory]

Registry key (DP025)

ID	Severity	When
DP025	Info	String literal key passed to a generated Strategy or Factory registry lookup is not among registered keys

Where it applies: Get / TryGet / Create / TryCreate on IStrategyRegistry<,> / IFactoryRegistry<,> when the key argument is a string literal (not a {Contract}Keys constant).

Message: includes the contract type and the list of registered keys.

Fix: prefer {Contract}Keys.* constants. The CorrectRegistryKey CodeFix suggests the closest registered key when the typo is near a known key.

See also Registry key conventions.

Plugin assemblies (DP033)

ID	Severity	When
DP033	Error	The same strategy key for the same contract is registered in more than one referenced provider assembly

Where it applies: host (or any) compilations that reference multiple plugin provider assemblies contributing [RegisterStrategy] / [RegisterStrategy<TContract>] for the same contract with an identical key string.

Message: includes the key, contract type, and conflicting assembly names.

Fix: use distinct keys per provider assembly, or reference only one provider per contract dimension. Duplicate keys within a single assembly remain DP003 (generator).

See Plugin assemblies sample, the main repo PluginAssemblies.md, and Configuration.

State transition table (DP026–DP031)

ID	Severity	When
DP026	Error	Duplicate (from state, trigger) edge
DP027	Error	[Transition] state value is not a declared enum member
DP028	Error	[Transition] trigger value is not a declared enum member
DP029	Error	[StateMachine] Initial is not a declared state enum member
DP030	Error	Holder class is not static partial
DP031	Info	State enum member never appears as a [Transition] source (terminal/reserved hint)

See State transition table.

State guard (DP032, DP034–DP035)

ID	Severity	When
DP032	Error	[Transition(Guard = nameof(Method))] guard method not found on holder class
DP034	Error	Guard method is not static (defensive — C# compiler rejects instance methods on static classes first)
DP035	Error	Guard method has wrong signature (must be static bool Method(TState, TTrigger))

State literal edge (DP036)

ID	Severity	When
DP036	Info	TryTransition called with literal (state, trigger) arguments that do not match any declared [Transition] edge

State entry/exit actions (DP037–DP039)

ID	Severity	When
DP037	Error	[Transition(OnEnter/OnExit = nameof(Method))] action method not found on holder class
DP038	Error	Action method is not static (unreachable in practice — CS0708 fires first; retained for completeness)
DP039	Error	Action method has wrong signature (must be static void Method(TState, TState, TTrigger) or static ValueTask Method(TState, TState, TTrigger, CancellationToken))

Composite DI + Visitor (DP040–DP041)

ID	Severity	When
DP040	Error	BuildRoot(IServiceProvider) called but a composite node is not registered in the container
DP041	Info	Generated I{Contract}NodeVisitor does not cover all node types (reserved — the C# compiler enforces full coverage via CS0535 when implementing the generated interface)

Decorator DI + async (DP042–DP043)

ID	Severity	When
DP042	Error	Async decorator method signature is incorrect (DecorateAsync must return ValueTask<TService>)
DP043	Error	Async decorator cannot be resolved from DI when using Build(IServiceProvider, core)

EventAggregator (DP044–DP046)

ID	Severity	When
DP044	Info	Type implements IEventHandler<T> but is not marked with [RegisterEventHandler]
DP045	Error	Duplicate [RegisterEventHandler] on the same handler for the same event type
DP046	Error	Type marked with [RegisterEventHandler<T>] but does not implement IEventHandler<T>

Strategy guard (DP047–DP049)

ID	Severity	When
DP047	Error	[RegisterStrategy(Guard = nameof(Method))] guard method not found on strategy class
DP048	Error	Guard method is not static
DP049	Error	Guard method has wrong signature (must be static bool Method(TKey))

Handler guard (DP050–DP052)

ID	Severity	When
DP050	Error	[HandlerOrder(Guard = nameof(Method))] guard method not found on handler class
DP051	Error	Guard method is not static
DP052	Error	Guard method has wrong signature (must be static bool Method(TContext))

Factory async + pooling (DP053–DP055)

ID	Severity	When
DP053	Error	[RegisterFactory(IsAsync = true)] but factory does not implement IAsyncFactory<T>
DP054	Error	PoolSize is negative (must be ≥ 0; 0 disables pooling)
DP055	Warning	PoolSize > 1024 (may cause excessive memory usage)

State hierarchy (DP056–DP059)

ID	Severity	When
DP056	Error	A state hierarchy contains a cycle
DP057	Error	[StateParent] references a state that is not a member of the state enum
DP058	Error	[StateParent] declares a state as its own parent
DP059	Info	A declared parent state has no children and no outgoing transitions

DI lifetime (DP060–DP062)

ID	Severity	When
DP060	Warning	A generated registry lifetime exceeds the implementation lifetime and may capture a shorter-lived service
DP061	Info	An implementation lifetime exceeds the registry lifetime and may cause unnecessary instances
DP062	Warning	A Singleton service depends on a Scoped or Transient service

Composite tree schema (DP063–DP065)

ID	Severity	When
DP063	Warning	A Composite tree exceeds the MaxDepth declared by [CompositeSchema]
DP064	Error	A Composite child type is not listed in the parent’s AllowedChildTypes
DP065	Warning	A Composite contract exceeds the MaxNodes declared by [CompositeSchema]

Singleton lifecycle (DP066–DP071)

ID	Severity	When
DP066	Warning	A Singleton factory delegate resolves a Scoped or Transient service and captures it
DP067	Error	A GenerateSingleton asynchronous initializer is missing, non-static, or has an invalid Task/ValueTask signature
DP068	Warning	A generated singleton is also registered as a DI Singleton, creating two lifecycle owners
DP069	Info	A non-thread-safe generated singleton contains mutable instance state
DP070	Info	Mutable static singleton state was detected; prefer an immutable Lazy-backed singleton or DI
DP071	Warning	Mutable static singleton state is also registered as a DI Singleton

Code fixes

DesignPatterns.CodeFixes ships inside the Skymly.DesignPatterns meta package (analyzers/dotnet/cs). Selected diagnostics offer one-click fixes in the IDE (requires C# Workspaces):

Diagnostic	Code fix (summary)
DP006	Add [RegisterStrategy("key", typeof(TContract))]
DP023	Add [RegisterFactory("key", typeof(TContract))]
DP024	Add [HandlerOrder(order, typeof(TContext))]
DP025	Replace literal with nearest registered key
DP001	Add partial modifier
DP015	Add [CompositeBuildable]
DP004 / DP013 / DP017 / DP021	Add contract implementation
DP007 / DP009 / DP014 / DP019 / DP022	Add parameterless constructor

Generator errors (DP001–DP005, DP007–DP022) are fixed by correcting attributes or types; no CodeFix is provided for duplicate-key errors.

Category prefix: DesignPatterns.