.NET CQRS API Development Skill

Purpose

This skill guides the specification and creation of .NET APIs using the CQRS pattern with MediatR, FluentValidation, and Carter modules. It provides a structured approach to building feature-based, vertically-sliced APIs with robust validation and clear separation of concerns.

Core Principles

Architectural Philosophy

• Vertical Slice Architecture: Features are self-contained units organized by business capability
• CQRS Separation: Commands (write operations) and Queries (read operations) are explicitly separated
• Result Pattern: All operations return Result<T> for explicit success/failure handling
• Functional Composition: Prefer declarative, immutable approaches over imperative mutations

Technology Stack

• MediatR: Implements mediator pattern for decoupling endpoint handlers from business logic
• FluentValidation: Provides declarative, strongly-typed validation rules
• Carter: Enables minimal API-style endpoint definition with module organization
• Mapster (inferred): Used for object mapping (.Adapt<T>() pattern)

Project Structure

/Features
  /{FeatureName}
    - Add{Feature}.cs          # Command endpoint + handler
    - Update{Feature}.cs       # Command endpoint + handler
    - Get{Feature}.cs          # Single item query endpoint + handler
    - Get{Feature}s.cs         # Collection query endpoint + handler
    - {Feature}AddRequest.cs   # Command DTO
    - {Feature}UpdateRequest.cs # Command DTO
    - {Feature}Response.cs     # Query response DTO

Feature Specification Process

Step 1: Define the Feature Domain

Before writing code, establish:

1. Feature Name: Clear, business-focused identifier (e.g., "Order", "Customer", "Invoice")
2. Business Context: What problem does this feature solve?
3. Operations Needed: Which CQRS operations are required?
   • Create (Add)
   • Read Single (Get)
   • Read Collection (GetAll/List)
   • Update
   • Delete (not in templates, but may be needed)
4. Domain Model: What properties define this entity?
5. Validation Rules: What business rules must be enforced?
6. Authorization Requirements: Who can access these endpoints?

Step 2: Design the Data Contract

Define three types of DTOs:

Request DTOs (for commands):

• Contain only the data needed to perform the operation
• Exclude computed or system-managed fields (IDs, timestamps)
• Should be immutable or follow init-only patterns

Response DTOs (for queries):

• May inherit from domain models
• Include data needed by consumers
• Can include computed/derived properties

Domain Models (internal):

• Represent the entity in the data layer
• Should not be directly exposed via API

Step 3: Specify Validation Rules

For each operation, define:

• Required fields and null checks
• Range validations (GreaterThan, LessThan)
• Format validations (Regex patterns, date formats)
• Business rule validations (cross-field rules)
• Custom validation logic requirements

Step 4: Define Routing Strategy

Establish URL patterns following RESTful conventions:

• Collection routes: /{feature-plural} or /Features/{FeatureName}
• Item routes: /{feature-plural}/{id} or /Features/{FeatureName}/{id}
• Consider route parameters vs query strings
• Maintain consistency across features

Implementation Templates

Command Pattern (Add/Create)

Use when: Creating new entities

Structure:

namespace {ProjectName}.Features.{FeatureName};

// 1. Carter Endpoint Module
public class Add{Feature}Endpoint : ICarterModule {
    public void AddRoutes(IEndpointRouteBuilder app) {
        app.MapPost("{route}", async ({Feature}AddRequest request, ISender sender) => {
            var command = new Add{Feature}.Command { {Feature} = request };
            var result = await sender.Send(command);
            
            if (result.IsFailure) {
                return Results.BadRequest(result.Error);
            }
            
            return Results.Ok(result.Value);
        })
        .Produces<int>() // Or appropriate return type
        .RequireAuthorization("{PolicyName}")
        .WithMetadata(new RouteMetadata { Tags = new[] { "{TagName}" } });
    }
}

// 2. Command Definition
public static partial class Add{Feature} {
    public class Command : IRequest<Result<int>> { // Or TResult
        public {Feature}AddRequest? {Feature} { get; set; }
    }
    
    // 3. Validation Rules
    public class Validator : AbstractValidator<Command> {
        public Validator() {
            RuleFor(x => x.{Feature})
                .NotNull()
                .WithMessage("No {feature} data provided.");
            
            // Add specific field validations
            RuleFor(x => x.{Feature}!.PropertyName)
                .GreaterThan(0)
                .WithMessage("PropertyName cannot be zero.");
        }
    }
    
    // 4. Command Handler
    internal sealed partial class {Feature}Handler : IRequestHandler<Command, Result<int>> {
        private readonly ILogger<{Feature}Handler> _logger;
        private readonly IValidator<Command> _validator;
        private readonly I{Feature}Data _data;
        
        public {Feature}Handler(
            IValidator<Command> validator,
            ILogger<{Feature}Handler> logger,
            I{Feature}Data data
        ) {
            _validator = validator;
            _logger = logger;
            _data = data;
        }
        
        public async Task<Result<int>> Handle(Command request, CancellationToken cancellationToken) {
            // Validate
            var validationResult = _validator.Validate(request);
            if (!validationResult.IsValid) {
                return Result.Failure<int>(new Error("Add{Feature}.Validation", validationResult.ToString()));
            }
            
            try {
                // Map and execute
                {Feature}Model new{Feature} = request.{Feature}!.Adapt<{Feature}Model>();
                var {feature} = await _data.Add{Feature}(new{Feature});
                
                if ({feature} is null) {
                    return Result.Failure<int>(new Error("Add{Feature}", "Failed to add {feature}"));
                }
                
                return {feature}.ID;
            } catch (Exception ex) {
                return Result.Failure<int>(new Error("Add{Feature}.Exception", ex.Message));
            }
        }
    }
}

Key Patterns:

• Endpoint delegates to MediatR command via ISender
• Validation happens in handler, not endpoint
• Result pattern for explicit error handling
• Exception handling wraps unexpected failures
• Returns entity ID on success

Query Pattern (Get Single)

Use when: Retrieving a specific entity by identifier

Structure:

namespace {ProjectName}.Features.{FeatureName};

// 1. Carter Endpoint Module
public class Get{Feature}Endpoint : ICarterModule {
    public void AddRoutes(IEndpointRouteBuilder app) {
        app.MapGet("{route}/{id:int}", async (int id, ISender sender) => {
            var query = new Get{Feature}.{Feature}Query { ID = id };
            var result = await sender.Send(query);
            
            if (result.IsFailure) {
                return Results.NotFound(result.Error);
            }
            
            return Results.Ok(result.Value);
        })
        .Produces<{Feature}Response>()
        .RequireAuthorization("{PolicyName}")
        .WithMetadata(new RouteMetadata { Tags = new[] { "{TagName}" } });
    }
}

// 2. Query Definition
public static class Get{Feature} {
    public class {Feature}Query : IRequest<Result<{Feature}Response>> {
        public int ID { get; set; } = 0;
        // Additional filter properties
    }
    
    // 3. Validation Rules
    public class Validator : AbstractValidator<{Feature}Query> {
        public Validator() {
            RuleFor(x => x.ID)
                .GreaterThan(0)
                .WithMessage("ID cannot be zero.");
        }
    }
    
    // 4. Query Handler
    internal sealed class Handler : IRequestHandler<{Feature}Query, Result<{Feature}Response>> {
        private readonly ILogger<Handler> _logger;
        private readonly IValidator<{Feature}Query> _validator;
        private readonly I{Feature}Data _data;
        
        public Handler(
            ILogger<Handler> logger,
            IValidator<{Feature}Query> validator,
            I{Feature}Data data
        ) {
            _logger = logger;
            _validator = validator;
            _data = data;
        }
        
        public async Task<Result<{Feature}Response>> Handle({Feature}Query request, CancellationToken cancellationToken) {
            // Validate
            var validationResult = _validator.Validate(request);
            if (!validationResult.IsValid) {
                return Result.Failure<{Feature}Response>(new Error("Get{Feature}.Validation", validationResult.ToString()));
            }
            
            // Retrieve and map
            var {feature} = await _data.Get{Feature}(request.ID);
            
            if ({feature} is null) {
                return Result.Failure<{Feature}Response>(new Error(
                    "Get{Feature}.Null",
                    "The {feature} with the specified ID was not found"));
            }
            
            var response = {feature}.Adapt<{Feature}Response>();
            
            // Optional: Enrich response with additional data
            
            return response;
        }
    }
}

Key Patterns:

• Returns NotFound for missing entities
• Validation ensures query parameters are valid
• Response mapping allows projection/transformation
• Separate method for complex retrieval logic

Query Pattern (Get Collection)

Use when: Retrieving multiple entities with filtering

Structure:

namespace {ProjectName}.Features.{FeatureName};

// 1. Carter Endpoint Module
public class Get{Feature}sEndpoint : ICarterModule {
    public void AddRoutes(IEndpointRouteBuilder app) {
        app.MapGet("{route}", async (int param1, string param2, ISender sender) => {
            var query = new Get{Feature}s.{Feature}Query { 
                Param1 = param1, 
                Param2 = param2 
            };
            var result = await sender.Send(query);
            
            if (result.IsFailure) {
                return Results.NotFound(result.Error);
            }
            
            return Results.Ok(result.Value);
        })
        .Produces<IEnumerable<{Feature}Response>>()
        .RequireAuthorization("{PolicyName}")
        .WithMetadata(new RouteMetadata { Tags = new[] { "{TagName}" } });
    }
}

// 2. Query Definition
public static class Get{Feature}s {
    public class {Feature}Query : IRequest<Result<IEnumerable<{Feature}Response>>> {
        public int Param1 { get; set; } = 0;
        public string Param2 { get; set; } = string.Empty;
        // Filter/pagination properties
    }
    
    // 3. Validation Rules
    public class Validator : AbstractValidator<{Feature}Query> {
        public Validator() {
            RuleFor(x => x.Param1)
                .GreaterThan(0)
                .WithMessage("Param1 cannot be zero.");
            
            // Date format validation example
            RuleFor(x => x.DateParam)
                .Matches(@"\d{4}-\d{2}-\d{2}")
                .WithMessage("DateParam must be in the format yyyy-MM-dd.");
        }
    }
    
    // 4. Query Handler
    internal sealed class Handler : IRequestHandler<{Feature}Query, Result<IEnumerable<{Feature}Response>>> {
        private readonly ILogger<Handler> _logger;
        private readonly IValidator<{Feature}Query> _validator;
        private readonly I{Feature}Data _data;
        
        public Handler(
            ILogger<Handler> logger,
            IValidator<{Feature}Query> validator,
            I{Feature}Data data
        ) {
            _logger = logger;
            _validator = validator;
            _data = data;
        }
        
        public async Task<Result<IEnumerable<{Feature}Response>>> Handle({Feature}Query request, CancellationToken cancellationToken) {
            // Validate
            var validationResult = _validator.Validate(request);
            if (!validationResult.IsValid) {
                return Result.Failure<IEnumerable<{Feature}Response>>(new Error("Get{Feature}s.Validation", validationResult.ToString()));
            }
            
            // Retrieve collection
            var {feature}s = await _data.List{Feature}s(/* filter params */);
            
            if ({feature}s is null) {
                return Result.Failure<IEnumerable<{Feature}Response>>(new Error(
                    "Get{Feature}s.Null",
                    "Failed to get {feature}s"));
            }
            
            var response = {feature}s.Adapt<List<{Feature}Response>>();
            
            return response;
        }
    }
}

Key Patterns:

• Query parameters come from route or query string
• Returns collections (IEnumerable)
• Validation includes format checks (dates, etc.)
• Consider pagination for large datasets

Command Pattern (Update)

Use when: Modifying existing entities

Structure:

namespace {ProjectName}.Features.{FeatureName};

// 1. Carter Endpoint Module
public class Update{Feature}Endpoint : ICarterModule {
    public void AddRoutes(IEndpointRouteBuilder app) {
        app.MapPut("{route}", async ({Feature}UpdateRequest request, ISender sender) => {
            var command = new Update{Feature}.Command { {Feature} = request };
            var result = await sender.Send(command);
            
            if (result.IsFailure) {
                return Results.BadRequest(result.Error);
            }
            
            return Results.Ok(result.Value);
        })
        .Produces<int>()
        .RequireAuthorization("{PolicyName}")
        .WithMetadata(new RouteMetadata { Tags = new[] { "{TagName}" } });
    }
}

// 2. Command Definition
public static partial class Update{Feature} {
    public class Command : IRequest<Result<int>> {
        public {Feature}UpdateRequest? {Feature} { get; set; }
    }
    
    // 3. Validation Rules
    public class Validator : AbstractValidator<Command> {
        public Validator() {
            RuleFor(x => x.{Feature})
                .NotNull()
                .WithMessage("No {feature} data provided.");
            
            RuleFor(x => x.{Feature}!.ID)
                .GreaterThan(0)
                .WithMessage("ID cannot be zero.");
            
            // Additional field validations
        }
    }
    
    // 4. Command Handler
    internal sealed partial class {Feature}Handler : IRequestHandler<Command, Result<int>> {
        private readonly ILogger<{Feature}Handler> _logger;
        private readonly IValidator<Command> _validator;
        private readonly I{Feature}Data _data;
        
        public {Feature}Handler(
            IValidator<Command> validator,
            ILogger<{Feature}Handler> logger,
            I{Feature}Data data
        ) {
            _validator = validator;
            _logger = logger;
            _data = data;
        }
        
        public async Task<Result<int>> Handle(Command request, CancellationToken cancellationToken) {
            // Validate
            var validationResult = _validator.Validate(request);
            if (!validationResult.IsValid) {
                return Result.Failure<int>(new Error("Update{Feature}.Validation", validationResult.ToString()));
            }
            
            try {
                // Verify entity exists
                var existing{Feature} = await _data.Get{Feature}(request.{Feature}!.ID);
                if (existing{Feature} is null) {
                    return Result.Failure<int>(new Error("Update{Feature}", "The {feature} with the specified ID was not found"));
                }
                
                // Perform update
                await _data.Update{Feature}(/* updated values */);
                
                return existing{Feature}.ID;
            } catch (Exception ex) {
                return Result.Failure<int>(new Error("Update{Feature}.Exception", ex.Message));
            }
        }
    }
}

Key Patterns:

• Verify entity exists before update
• Update request includes entity ID
• Returns updated entity ID
• BadRequest for validation failures
• Consider optimistic concurrency with version tokens

DTOs and Contracts

Request DTOs (Commands)

namespace {ProjectName}.Features.{FeatureName};

public class {Feature}AddRequest {
    public required string PropertyName { get; init; }
    public int NumericProperty { get; init; }
    // Only properties needed for creation
    // Exclude: ID, CreatedDate, UpdatedDate
}

public class {Feature}UpdateRequest {
    public required int ID { get; init; }  // Identity required
    public required string PropertyName { get; init; }
    public int NumericProperty { get; init; }
    // Properties that can be updated
}

Best Practices:

• Use init accessors for immutability
• Mark non-nullable properties as required (C# 11+)
• Exclude computed/system properties
• Consider separate DTOs for different update scenarios

Response DTOs (Queries)

namespace {ProjectName}.Features.{FeatureName};

public class {Feature}Response {
    public int ID { get; init; }
    public string PropertyName { get; init; } = string.Empty;
    public int NumericProperty { get; init; }
    public DateTime CreatedDate { get; init; }
    // Include all properties consumers need
    // Can include computed properties
}

Best Practices:

• Inherit from domain model if structures align
• Add view-specific computed properties
• Use init-only properties
• Default string properties to string.Empty

Validation Strategies

FluentValidation Patterns

Required Field Validation:

RuleFor(x => x.Property)
    .NotNull()
    .WithMessage("Property is required.");

RuleFor(x => x.StringProperty)
    .NotEmpty()
    .WithMessage("StringProperty cannot be empty.");

Range Validation:

RuleFor(x => x.Age)
    .GreaterThan(0)
    .WithMessage("Age must be greater than zero.")
    .LessThanOrEqualTo(150)
    .WithMessage("Age must be realistic.");

RuleFor(x => x.Percentage)
    .InclusiveBetween(0, 100)
    .WithMessage("Percentage must be between 0 and 100.");

Format Validation:

RuleFor(x => x.Email)
    .EmailAddress()
    .WithMessage("Invalid email format.");

RuleFor(x => x.DateString)
    .Matches(@"\d{4}-\d{2}-\d{2}")
    .WithMessage("Date must be in yyyy-MM-dd format.");

RuleFor(x => x.PhoneNumber)
    .Matches(@"^\+?[1-9]\d{1,14}$")
    .WithMessage("Invalid phone number format.");

Nested Object Validation:

RuleFor(x => x.Address)
    .NotNull()
    .WithMessage("Address is required.")
    .SetValidator(new AddressValidator());  // Use separate validator

Conditional Validation:

RuleFor(x => x.CompanyName)
    .NotEmpty()
    .When(x => x.CustomerType == CustomerType.Business)
    .WithMessage("Company name is required for business customers.");

Cross-Field Validation:

RuleFor(x => x.EndDate)
    .GreaterThan(x => x.StartDate)
    .WithMessage("End date must be after start date.");

Custom Validation:

RuleFor(x => x.Username)
    .Must(BeUniqueUsername)
    .WithMessage("Username already exists.");

private bool BeUniqueUsername(string username) {
    // Custom validation logic
    return !_userRepository.UsernameExists(username);
}

Error Handling Patterns

Result Pattern Implementation

The templates use a Result pattern for explicit error handling:

// Success case
return Result.Success(value);
return value;  // Implicit conversion

// Failure case
return Result.Failure<T>(new Error("Category.Operation", "Description"));

// Checking results
if (result.IsFailure) {
    return Results.BadRequest(result.Error);
}

return Results.Ok(result.Value);

Error Categories

Organize errors by category for better diagnostics:

• Validation: "{Feature}.Validation" - Input validation failures
• NotFound: "{Feature}.Null" or "{Feature}.NotFound" - Entity not found
• Exception: "{Feature}.Exception" - Unexpected errors
• Business: "{Feature}.BusinessRule" - Business rule violations

HTTP Status Code Mapping

• 200 OK: Successful query/command
• 400 Bad Request: Validation failure, business rule violation
• 404 Not Found: Entity not found
• 401 Unauthorized: Authentication failure
• 403 Forbidden: Authorization failure
• 500 Internal Server Error: Unhandled exceptions (should be rare)

Dependency Injection Patterns

Required Registrations

For each feature, register:

// MediatR handlers (usually auto-registered)
services.AddMediatR(cfg => cfg.RegisterServicesFromAssembly(assembly));

// FluentValidation validators
services.AddValidatorsFromAssembly(assembly);

// Carter modules
services.AddCarter();

// Data access services
services.AddScoped<I{Feature}Data, {Feature}Data>();

// Logging is automatically available via ILogger<T>

Handler Constructor Pattern

public {Feature}Handler(
    IValidator<Command> validator,     // FluentValidation validator
    ILogger<{Feature}Handler> logger,  // Structured logging
    I{Feature}Data data                // Data access
    // Add other dependencies as needed
) {
    _validator = validator;
    _logger = logger;
    _data = data;
}

Best Practices:

• Constructor injection only
• Inject interfaces, not concrete types
• Use scoped lifetime for data services
• Use singleton for stateless services

Carter Module Configuration

Endpoint Registration

public class {Feature}Endpoint : ICarterModule {
    public void AddRoutes(IEndpointRouteBuilder app) {
        app.MapPost("{route}", handler)
            .Produces<TResponse>()              // Documents response type
            .ProducesProblem(400)               // Documents error responses
            .RequireAuthorization("{Policy}")    // Authorization policy
            .WithMetadata(new RouteMetadata {   // OpenAPI metadata
                Tags = new[] { "{TagName}" }
            })
            .WithName("{OperationId}")          // OpenAPI operation ID
            .WithDescription("{Description}");   // OpenAPI description
    }
}

Routing Conventions

RESTful routes:

• GET /api/{features} - Get collection
• GET /api/{features}/{id} - Get single item
• POST /api/{features} - Create new item
• PUT /api/{features} - Update item (ID in body)
• PUT /api/{features}/{id} - Update item (ID in route)
• DELETE /api/{features}/{id} - Delete item

Nested resources:

• GET /api/{parent}/{parentId}/{features} - Get child collection
• GET /api/{parent}/{parentId}/{features}/{id} - Get child item

Testing Strategies

Unit Testing Handlers

Test handlers in isolation:

[Fact]
public async Task Handle_ValidCommand_ReturnsSuccess() {
    // Arrange
    var validator = new Add{Feature}.Validator();
    var logger = new Mock<ILogger<Add{Feature}.{Feature}Handler>>();
    var data = new Mock<I{Feature}Data>();
    
    data.Setup(x => x.Add{Feature}(It.IsAny<{Feature}Model>()))
        .ReturnsAsync(new {Feature}Model { ID = 1 });
    
    var handler = new Add{Feature}.{Feature}Handler(validator, logger.Object, data.Object);
    var command = new Add{Feature}.Command { /* ... */ };
    
    // Act
    var result = await handler.Handle(command, CancellationToken.None);
    
    // Assert
    Assert.True(result.IsSuccess);
    Assert.Equal(1, result.Value);
}

[Fact]
public async Task Handle_InvalidCommand_ReturnsValidationFailure() {
    // Test validation failures
}

[Fact]
public async Task Handle_DataLayerException_ReturnsFailure() {
    // Test exception handling
}

Integration Testing Endpoints

Test full request/response cycle:

[Fact]
public async Task Post_{Feature}_ValidRequest_Returns200() {
    // Arrange
    var client = _factory.CreateClient();
    var request = new {Feature}AddRequest { /* ... */ };
    
    // Act
    var response = await client.PostAsJsonAsync("/api/{features}", request);
    
    // Assert
    response.StatusCode.Should().Be(HttpStatusCode.OK);
    var id = await response.Content.ReadFromJsonAsync<int>();
    id.Should().BeGreaterThan(0);
}

Best Practices Summary

Do's ✓

1. Organize by feature, not technical layer
2. Validate in handlers, not endpoints
3. Use Result for explicit error handling
4. Separate commands and queries clearly
5. Make DTOs immutable with init-only properties
6. Use meaningful error messages with context
7. Log exceptions with structured logging
8. Document with OpenAPI metadata
9. Test handlers independently from endpoints
10. Follow RESTful conventions for routing
11. Use CancellationToken for all async operations
12. Apply authorization policies at endpoint level

Don'ts ✗

1. Don't bypass validation "for convenience"
2. Don't return domain models directly from endpoints
3. Don't catch and swallow exceptions without logging
4. Don't mix query and command logic in handlers
5. Don't use primitive obsession - use value objects
6. Don't skip null checks on nullable properties
7. Don't hardcode strings - use constants or enums
8. Don't forget CancellationToken parameter
9. Don't create anemic DTOs - include business logic where appropriate
10. Don't ignore validation errors - always check IsValid

Advanced Patterns

Pagination Support

public class {Feature}Query : IRequest<Result<PagedResult<{Feature}Response>>> {
    public int PageNumber { get; set; } = 1;
    public int PageSize { get; set; } = 20;
    // Filter properties
}

public class PagedResult<T> {
    public IEnumerable<T> Items { get; init; } = Enumerable.Empty<T>();
    public int TotalCount { get; init; }
    public int PageNumber { get; init; }
    public int PageSize { get; init; }
    public int TotalPages => (int)Math.Ceiling(TotalCount / (double)PageSize);
    public bool HasPrevious => PageNumber > 1;
    public bool HasNext => PageNumber < TotalPages;
}

Sorting and Filtering

public class {Feature}Query : IRequest<Result<IEnumerable<{Feature}Response>>> {
    public string? SortBy { get; set; }
    public bool SortDescending { get; set; }
    public string? SearchTerm { get; set; }
    public DateTime? FromDate { get; set; }
    public DateTime? ToDate { get; set; }
}

Soft Delete Pattern

public class Delete{Feature}.Command : IRequest<Result<bool>> {
    public int ID { get; set; }
}

// Handler marks as deleted rather than removing
await _data.SoftDelete{Feature}(request.ID);

Audit Trail

public abstract class AuditableEntity {
    public DateTime CreatedDate { get; init; }
    public string CreatedBy { get; init; } = string.Empty;
    public DateTime? ModifiedDate { get; set; }
    public string? ModifiedBy { get; set; }
}

// Apply in handler
entity.CreatedBy = _currentUserService.Username;
entity.CreatedDate = DateTime.UtcNow;

Enrichment Pattern

For queries that need additional data:

private async Task<{Feature}Response> Enrich{Feature}Response({Feature}Model {feature}) {
    var response = {feature}.Adapt<{Feature}Response>();
    
    // Add related data
    response.RelatedItems = await _data.GetRelatedItems({feature}.ID);
    
    // Add computed properties
    response.DisplayName = $"{feature.FirstName} {feature.LastName}";
    
    return response;
}

Common Pitfalls and Solutions

Problem: Validation in Multiple Places

Solution: Centralize all validation in FluentValidation validators within handlers

Problem: Anemic Domain Models

Solution: Add behavior methods to domain models, not just properties

Problem: Tight Coupling to Data Layer

Solution: Use repository interfaces, inject via constructor

Problem: Missing Null Checks

Solution: Use nullable reference types, validate with FluentValidation

Problem: Poor Error Messages

Solution: Provide context in error messages - what failed and why

Problem: Testing Difficulties

Solution: Keep handlers focused, inject dependencies, use interfaces

Problem: Performance Issues with Large Collections

Solution: Implement pagination, filtering, and projection at data layer

Problem: Inconsistent HTTP Status Codes

Solution: Follow the error handling patterns consistently

Configuration Checklist

When implementing a new feature, verify:

• Feature organized in dedicated namespace/folder
• Request DTOs defined (Add, Update as needed)
• Response DTO defined
• Carter endpoint module created with correct HTTP verb
• Command/Query class defined with IRequest<Result>
• Validator class defined with validation rules
• Handler class implements IRequestHandler
• Handler validates input using validator
• Handler implements error handling with Result
• Handler returns appropriate result types
• Endpoint maps failures to HTTP status codes
• Authorization policy applied
• OpenAPI metadata configured
• Data interface and implementation created
• Dependencies registered in DI container
• Unit tests written for handler
• Integration tests written for endpoint

Reference: Full Feature Example

See the provided templates (AddXxxx.cs, GetXxxx.cs, GetXxxxs.cs, UpdateXxxx.cs) for complete, working examples of each pattern.

Summary

This skill provides a structured approach to building maintainable, testable .NET APIs using:

• CQRS for clear separation of reads and writes
• MediatR for loose coupling and testability
• FluentValidation for declarative validation rules
• Carter for clean endpoint organization
• Result pattern for explicit error handling

Follow these patterns consistently to create a cohesive, predictable API surface that's easy to understand, maintain, and extend.