Marker Interface

Definition

A Marker Interface is an interface with no methods (or minimal properties) that signals that a type possesses a specific behavior or characteristic. Framework components detect these interfaces via reflection and apply the associated behavior.

Diagram

classDiagram
    class ISoftDeletable {
        <<marker>>
        +IsDeleted : bool
        +DeletedAt : DateTimeOffset?
        +DeletedBy : string?
    }

    class IMultiTenant {
        <<marker>>
        +TenantId : Guid?
    }

    class IActive {
        <<marker>>
        +IsActive : bool
    }

    class IDomainEvent {
        <<marker>>
    }

    class IIntegrationEvent {
        <<marker>>
    }

    class IUserFriendlyException {
        <<marker>>
    }

    class IHasErrorCode {
        <<marker>>
        +ErrorCode : string
    }

    note for ISoftDeletable "SoftDeleteInterceptor<br/>Query filter"
    note for IMultiTenant "AuditedEntityInterceptor<br/>Query filter"
    note for IDomainEvent "Local queue Wolverine"
    note for IIntegrationEvent "Outbox Wolverine"
    note for IUserFriendlyException "Message exposed to client"

Implementation in Granit

Entity markers

Interface	File	Detected by
ISoftDeletable	src/Granit.Core/Domain/ISoftDeletable.cs	SoftDeleteInterceptor, ApplyGranitConventions()
IMultiTenant	src/Granit.Core/Domain/IMultiTenant.cs	AuditedEntityInterceptor, ApplyGranitConventions()
IActive	src/Granit.Core/Domain/IActive.cs	ApplyGranitConventions()

Event markers

Interface	File	Detected by
IDomainEvent	src/Granit.Core/Events/IDomainEvent.cs	Wolverine routing — local queue
IIntegrationEvent	src/Granit.Core/Events/IIntegrationEvent.cs	Wolverine routing — transport/Outbox

Exception markers

Interface	File	Detected by
IUserFriendlyException	src/Granit.Core/Exceptions/IUserFriendlyException.cs	GranitExceptionHandler — message exposed to client
IHasErrorCode	src/Granit.Core/Exceptions/IHasErrorCode.cs	GranitExceptionHandler — error code in ProblemDetails
IHasValidationErrors	src/Granit.Core/Exceptions/IHasValidationErrors.cs	GranitExceptionHandler — field errors in extensions

Idempotency markers

Interface	File	Detected by
IIdempotencyMetadata	src/Granit.Idempotency/Abstractions/IIdempotencyMetadata.cs	IdempotencyMiddleware — enables idempotency on the endpoint

Rationale

Markers allow applying cross-cutting behaviors (audit, filtering, routing) declaratively, without coupling entities to infrastructure frameworks. An entity implementing ISoftDeletable automatically gets soft delete and the query filter — no additional code required.

Usage example

// The entity declares its characteristics via markers
public sealed class MedicalRecord : FullAuditedEntity, IMultiTenant, IActive
{
    public Guid? TenantId { get; set; }       // <- IMultiTenant
    public bool IsActive { get; set; } = true; // <- IActive
    // ISoftDeletable is inherited from FullAuditedEntity

    public string Diagnosis { get; set; } = string.Empty;
}

// The framework detects markers and automatically applies:
// - Query filter: WHERE IsDeleted=false AND IsActive=true AND TenantId=@tid
// - Audit interceptor: CreatedAt/By, ModifiedAt/By, TenantId
// - Soft delete interceptor: DELETE -> UPDATE IsDeleted=true