mediatr-command-handler

name	mediatr-command-handler
description	implement or revise mediatR command handler business logic in an existing handler file. use when a c# mediatR command handler has an incomplete or incorrect handle method and chatgpt should update the handle method, add private helper methods, and extend application or domain abstractions such as repositories or services if required, while avoiding direct infrastructure dependencies in the handler.
template-id	Intent.Application.MediatR.CommandHandlerSkillTemplate
contentHash	2A0F2A592E6EC57A58AECFF11DAF5CDE3BD6061C88DBEBC82AC0999451166403

MediatR Command Handler

Implement command handler business logic inside an existing handler file. Favor domain intent, existing code patterns, and clean layer boundaries over convenience shortcuts.

Core rules

• Treat the existing handler file as the starting point. Update the Handle method rather than rewriting unrelated parts.
• NEVER modify the method signature of the Handle method.
• ALWAYS if you modify the Handle method, ensure that the IntentManaged attribute indicates that the body of the method must be in Mode.Ignore (e.g. [IntentManaged(Mode.Fully, Body = Mode.Ignore)]).
• Inject only dependencies from the Domain or Application layers.
• Never introduce direct dependencies on infrastructure packages or infrastructure types in the handler, including Entity Framework, Dapper, concrete DbContexts, SQL clients, or vendor-specific data access abstractions.
• If data access is required, favor using an existing repository abstraction from the Domain or Application layers, but extend them if required .
• ALWAYS if the response requires any aggregation (Count/Sum/Average/Min/Max/First/Last/GroupBy), the service MUST NOT compute it in memory. The handler MUST call a Domain/Application abstraction that returns an aggregated result/summary/shaped dataset. If no suitable method exists, the assistant must extend the appropriate repository/read abstraction (contract first, then implementation) and then call it. No temporary in-service aggregation is allowed.
• Follow the modeled domain language already present in the codebase: aggregates, entities, value objects, policies, invariants, statuses, and business terms should drive the implementation.
• Search the codebase for similar handlers, repository methods, domain services, validation flows, and result patterns before introducing a new approach.
• Add private helper methods inside the handler when they improve clarity, keep business flow readable, or encapsulate repeated branching logic.
• Keep orchestration in the handler and place durable business rules in domain entities, value objects, specifications, or domain/application services when those patterns already exist nearby.

Workflow

1. Inspect the existing handler, request, response, validator, repository interfaces, and related domain types.
2. Search for code usages of:

• similar command handlers
• repository interfaces and existing repository methods
• domain operations on the target aggregate or entity
• result and error patterns used in the solution

3. Infer the intended business flow from the request shape, naming, surrounding domain model, and nearby feature implementations.
4. Implement the Handle method using existing patterns first.
5. If the handler needs missing DAL capabilities, extend the relevant repository abstraction in an allowed layer instead of pulling infrastructure into the handler.
6. Add focused private helper methods when the flow becomes easier to read or test.
7. Ensure cancellation tokens are threaded through async calls where applicable.
8. Verify the final code preserves layer boundaries and does not introduce infrastructure leakage.

Command-specific guidance

Commands usually change state. Prefer patterns like these when supported by the surrounding codebase:

• load aggregate or entity through a repository
• validate existence and ownership/eligibility rules
• invoke domain behavior rather than mutating state ad hoc
• persist through the repository abstraction
• return the established command result type used by the solution
• publish or coordinate domain/application events only if existing nearby patterns already do so

Repository extension guidance

When a needed repository capability is missing:

• Extend the existing repository abstraction closest to the aggregate or read/write concern.
• Keep the new method shaped around domain intent, not storage mechanics.
• Prefer expressive methods such as GetForUpdateAsync, ExistsBy...Async, FindActive...Async, or SaveAsync over storage-oriented names.
• Do not add infrastructure comments or instructions to the handler.
• Do not reference how the repository will be implemented in EF, Dapper, SQL, or similar.

EF Related Data Loading guidance

• NEVER use Include or ThenInclude in the Application Layer, these are only available in the Infrastructure layer.
• Lazy loading with proxies is enabled.
• Entities are configured using the Owns apis, so compsitional children will be automatically loaded with their parents.
• You can rely on navigation properties being automatically loaded when accessed.
• (CRITICAL) If your implementation will cause a lot of Lazy loading consider other alternatives, like moving the data loading into the repository layer.

AutoMapper guidance

• Any read/query method, including MediatR query handlers and application services, that returns Application-layer DTOs (*Dto) derived from Domain entities MUST use AutoMapper.
  • Do not manually construct DTOs (new XxxDto { ... }) on read/query paths.
• AutoMapper gate (absolute): If you use any ProjectTo*, Find*ProjectTo*, FindAllProjectTo*, or *ProjectToAsync* method anywhere in the call chain, you MUST:
  • verify mapping exists by locating CreateMap<TDomain, TDto>() in a Profile and cite file path + excerpt, OR
  • if verification fails, immediately create the required AutoMapper Profile(s) (including all required nested mappings).
  • No assumptions allowed (a generic projection method or other feature usage is not verification).
• Registration assumption (do not block on DI):
  • Assume AutoMapper is registered via assembly scanning, e.g.:services.AddAutoMapper(Assembly.GetExecutingAssembly());
  • Therefore, do not delay profile creation because DI registration details are not currently visible.
  • Do not modify DI registration as part of this guidance unless the user explicitly asks.
• Manual DTO construction is allowed only when the DTO is a non-entity-shaped view model/aggregation and AutoMapper is not reasonable.
  • This must include an inline code comment explaining why AutoMapper is not reasonable.
  • “Mapping doesn’t exist yet” is not a valid exception.
• If you can't find any existing mappings, create them in the same project as the services under:
  • ./Mappings/<FeatureOrAggregate>/<Entity>DtoProfile.cs
  • Example: MyApp.Application/Mappings/Invoices/InvoiceDtoProfile.cs

Example:

public class CustomerDtoProfile : Profile
{
    public CustomerDtoProfile()
    {
        CreateMap<Customer, CustomerDto>();
    }
}

public static class CustomerDtoMappingExtensions
{
    public static CustomerDto MapToCustomerDto(this Customer projectFrom, IMapper mapper) =>
        mapper.Map<CustomerDto>(projectFrom);

    public static List<CustomerDto> MapToCustomerDtoList(this IEnumerable<Customer> projectFrom, IMapper mapper) =>
        projectFrom.Select(x => x.MapToCustomerDto(mapper)).ToList();
}

Output expectations

Produce a concrete code update that:

• fills in or corrects the Handle method
• adds private helper methods in the handler if useful
• extends allowed-layer abstractions when needed for the handler to be correct
• preserves the existing feature style and naming
• keeps unrelated refactors out of scope

Review checklist

Before finishing, check that:

• every injected dependency belongs to the Domain or Application layers
• the handler imports no infrastructure package namespaces
• repository changes, if any, are expressed as abstractions only
• the implementation follows existing code usage patterns rather than inventing a novel style without reason
• business language matches the surrounding domain model
• the code path is cancellation-aware and async-safe where relevant