DKNet

DKNet.EfCore.DtoGenerator

A compile-time source generator that automatically creates immutable DTO (Data Transfer Object) types from Entity Framework Core entities or any POCO classes, eliminating boilerplate code while maintaining type safety.

What is this project?

DKNet.EfCore.DtoGenerator is a Roslyn Incremental Source Generator that generates DTO classes at compile time. Instead of manually creating and maintaining DTO classes that mirror your entities, you simply apply the [GenerateDto] attribute to an empty partial record or class, and the generator creates all the properties and mapping methods automatically.

The generator intelligently synthesizes public init properties for every public instance readable property on the source entity, while excluding indexers and static properties. It also generates helpful mapping methods (FromEntity, ToEntity, and FromEntities) that leverage Mapster when available or fall back to property-by-property initialization.

Key Features

• Compile-Time Code Generation: No runtime reflection or performance overhead
• Type-Safe DTOs: Compile-time errors prevent mismatches between entities and DTOs
• Automatic Property Mapping: Generates all properties from source entity automatically
• Mapster Integration: Leverages Mapster for efficient mapping when available
• Fallback Mapping: Provides property-by-property initialization when Mapster is not present
• Partial Class Support: Allows customization and extension of generated DTOs
• Property Exclusion: Exclude specific properties using the Exclude parameter
• Incremental Generation: Efficient generation that only regenerates when needed
• Zero Runtime Dependencies: Generated code has no runtime dependencies on the generator

How it contributes to DDD and Onion Architecture

Application Layer DTO Generation

DKNet.EfCore.DtoGenerator primarily serves the Application Layer and Presentation Layer of the Onion Architecture by providing clean, immutable DTOs that decouple external representations from internal domain models:

┌─────────────────────────────────────────────────────────────────┐
│                    🌐 Presentation Layer                        │
│                   (Controllers, API Endpoints)                  │
│                                                                 │
│  Uses: Generated DTOs for request/response models              │
│  📄 CustomerDto, OrderDto, ProductDto                          │
│  ✅ Validation happens on DTOs, not domain entities            │
│  🔒 Domain entities never exposed directly to clients          │
└─────────────────────────┬───────────────────────────────────────┘
                          │
┌─────────────────────────┴───────────────────────────────────────┐
│                   🎯 Application Layer                          │
│              (Use Cases, Application Services)                  │
│                                                                 │
│  🎯 DKNet.EfCore.DtoGenerator - Generates DTOs at compile time │
│  📋 Maps between domain entities and DTOs                      │
│  🔄 Uses Mapster for efficient transformations                 │
│  ✅ Maintains separation between domain and presentation       │
└─────────────────────────┬───────────────────────────────────────┘
                          │
┌─────────────────────────┴───────────────────────────────────────┐
│                    💼 Domain Layer                             │
│           (Entities, Aggregates, Domain Services)              │
│                                                                 │
│  Domain entities remain pure and focused on business logic     │
│  No knowledge of DTOs or external representations              │
└─────────────────────────┬───────────────────────────────────────┘
                          │
┌─────────────────────────┴───────────────────────────────────────┐
│                 🗄️ Infrastructure Layer                        │
│                  (Data Access, Persistence)                    │
│                                                                 │
│  Domain entities persist without DTO concerns                  │
└─────────────────────────────────────────────────────────────────┘

DDD Benefits

1. Anti-Corruption Layer

DTOs serve as an anti-corruption layer, preventing external API concerns from leaking into the domain model:

// Domain Entity - Pure business logic
public class Customer : AggregateRoot
{
    public string Name { get; private set; }
    public Email Email { get; private set; }
    public CustomerStatus Status { get; private set; }
    
    public void ActivateAccount() 
    {
        // Business logic
    }
}

// Generated DTO - External representation
[GenerateDto(typeof(Customer))]
public partial record CustomerDto;
// Auto-generated properties: Name, Email, Status (as string/primitive types)

2. Bounded Context Boundaries

DTOs help maintain clear boundaries between bounded contexts by providing explicit translation points:

// Order Context
[GenerateDto(typeof(Order))]
public partial record OrderDto;

// Customer Context
[GenerateDto(typeof(Customer))]
public partial record CustomerSummaryDto;

// Integration between contexts uses DTOs, not domain entities

3. Aggregate Protection

DTOs prevent clients from directly modifying aggregate internals:

// Domain aggregate with encapsulated behavior
public class Order : AggregateRoot
{
    private List<OrderItem> _items = new();
    public IReadOnlyList<OrderItem> Items => _items.AsReadOnly();
    
    public void AddItem(Product product, int quantity)
    {
        // Business rules enforced here
    }
}

// DTO for reading - no behavior, just data
[GenerateDto(typeof(Order))]
public partial record OrderDto;

Onion Architecture Benefits

1. Layer Decoupling

Generated DTOs enable complete decoupling between presentation and domain layers:

// Application Service - translates between layers
public class CustomerService
{
    private readonly ICustomerRepository _repository;
    
    public async Task<CustomerDto> GetCustomerAsync(Guid id)
    {
        var customer = await _repository.GetByIdAsync(id);
        return CustomerDto.FromEntity(customer); // Generated mapping
    }
    
    public async Task<Guid> CreateCustomerAsync(CreateCustomerDto dto)
    {
        var customer = dto.ToEntity(); // Generated mapping
        await _repository.AddAsync(customer);
        return customer.Id;
    }
}

2. Testability

DTOs with generated mapping methods are easily testable:

[Fact]
public void CustomerDto_ShouldMapFromEntity()
{
    // Arrange
    var customer = new Customer("John Doe", "john@example.com");
    
    // Act
    var dto = CustomerDto.FromEntity(customer);
    
    // Assert
    dto.Name.ShouldBe("John Doe");
    dto.Email.ShouldBe("john@example.com");
}

3. API Versioning Support

Different DTO versions can be generated from the same entity:

// V1 API
[GenerateDto(typeof(Customer))]
public partial record CustomerDtoV1;

// V2 API - exclude sensitive fields
[GenerateDto(typeof(Customer), Exclude = new[] { "InternalNotes", "CreditScore" })]
public partial record CustomerDtoV2;

Installation and Setup

NuGet Package Installation

<ItemGroup>
  <PackageReference Include="DKNet.EfCore.DtoGenerator" Version="1.0.0" 
                    PrivateAssets="all" OutputItemType="Analyzer" />
  <!-- Optional but recommended for efficient mapping -->
  <PackageReference Include="Mapster" Version="7.4.0" />
</ItemGroup>

Project Configuration

Add these properties to your .csproj file:

<PropertyGroup>
  <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
  <CompilerGeneratedFilesOutputPath>$(BaseIntermediateOutputPath)Generated</CompilerGeneratedFilesOutputPath>
  <!-- Force analyzer to reload on every build to avoid caching issues -->
  <EnforceExtendedAnalyzerRules>true</EnforceExtendedAnalyzerRules>
</PropertyGroup>

Global Exclusion Configuration (Optional)

Configure properties to exclude globally across all DTOs:

<!-- Define global exclusions (comma or semicolon separated) -->
<PropertyGroup>
  <DtoGenerator_GlobalExclusions>CreatedBy,UpdatedBy,CreatedAt,UpdatedAt</DtoGenerator_GlobalExclusions>
</PropertyGroup>

<!-- Make the property visible to the source generator -->
<ItemGroup>
  <CompilerVisibleProperty Include="DtoGenerator_GlobalExclusions" />
</ItemGroup>

This is particularly useful for:

• Audit properties (CreatedBy, UpdatedBy, CreatedAt, UpdatedAt)
• Internal tracking fields
• Properties that should never be exposed in DTOs
• Reducing boilerplate in DTO attribute declarations

Copy Generated Files for Inspection (Optional)

For debugging and verification, copy generated DTOs to your project:

<!-- Custom target to copy generated DTOs to project/GeneratedDtos folder -->
<Target Name="CopyGeneratedDtosToOutputFolder" AfterTargets="CoreCompile" 
        Condition="Exists('$(CompilerGeneratedFilesOutputPath)')">
    <ItemGroup>
        <GeneratedDtoFiles Include="$(CompilerGeneratedFilesOutputPath)\**\*Dto.g.cs"/>
    </ItemGroup>
    <MakeDir Directories="$(ProjectDir)GeneratedDtos" Condition="'@(GeneratedDtoFiles)' != ''"/>
    <Copy SourceFiles="@(GeneratedDtoFiles)"
          DestinationFiles="$(ProjectDir)GeneratedDtos\%(Filename)%(Extension)"
          SkipUnchangedFiles="false"
          OverwriteReadOnlyFiles="true"
          Condition="'@(GeneratedDtoFiles)' != ''"/>
    <Message Text="Copied %(Filename)%(Extension) to $(ProjectDir)GeneratedDtos" 
             Importance="high" Condition="'@(GeneratedDtoFiles)' != ''"/>
</Target>

<!-- Exclude generated DTOs from compilation, but keep them visible in Solution Explorer -->
<ItemGroup>
    <Compile Remove="GeneratedDtos\**\*.cs"/>
    <None Include="GeneratedDtos\**\*.cs"/>
</ItemGroup>

Usage Examples

Basic DTO Generation

// Entity
public class Product
{
    public Guid Id { get; set; }
    public string Name { get; set; } = string.Empty;
    public string Description { get; set; } = string.Empty;
    public decimal Price { get; set; }
    public int StockQuantity { get; set; }
    public DateTime CreatedAt { get; set; }
}

// DTO Declaration
[GenerateDto(typeof(Product))]
public partial record ProductDto;

// Usage
var product = await repository.GetByIdAsync(productId);
var dto = ProductDto.FromEntity(product);
return Results.Ok(dto);

Excluding Properties

// Exclude internal/sensitive properties
[GenerateDto(typeof(Product), Exclude = new[] { "StockQuantity", "CreatedAt" })]
public partial record ProductSummaryDto;

// Generated DTO will only include: Id, Name, Description, Price

Global Exclusions

Configure global exclusions via MSBuild properties to exclude common audit or internal properties across all DTOs:

<!-- In your .csproj file -->
<PropertyGroup>
  <DtoGenerator_GlobalExclusions>CreatedBy,UpdatedBy,CreatedAt,UpdatedAt</DtoGenerator_GlobalExclusions>
</PropertyGroup>

<!-- Make the property visible to the source generator -->
<ItemGroup>
  <CompilerVisibleProperty Include="DtoGenerator_GlobalExclusions" />
</ItemGroup>

Global exclusions are applied to all DTOs by default:

// This DTO will automatically exclude CreatedBy, UpdatedBy, CreatedAt, UpdatedAt
[GenerateDto(typeof(Product))]
public partial record ProductDto;

// Local exclusions are combined with global exclusions
[GenerateDto(typeof(Product), Exclude = ["InternalNotes"])]
public partial record ProductSummaryDto; // Excludes global properties + InternalNotes

// Include parameter overrides global exclusions
[GenerateDto(typeof(Product), Include = ["Id", "Name", "CreatedAt"])]
public partial record ProductNameDto; // Only includes these 3 properties, ignoring global exclusions

Benefits of Global Exclusions:

• Centralized configuration for common exclusions
• Reduces boilerplate in DTO declarations
• Consistent exclusion of audit/internal properties
• Easy to maintain across large projects

Custom Properties

[GenerateDto(typeof(Product))]
public partial record ProductDto
{
    // Add computed property
    public string DisplayPrice => $"${Price:N2}";
    
    // Add custom property not in entity
    public bool IsAvailable => StockQuantity > 0;
}

Collection Mapping

// Map multiple entities to DTOs
var products = await repository.GetAllAsync();
var dtos = ProductDto.FromEntities(products);
return Results.Ok(dtos);

// Async with EF Core and Mapster
var dtos = await dbContext.Products
    .ProjectToType<ProductDto>() // Mapster extension
    .ToListAsync();

Nested DTOs

public class Order
{
    public Guid Id { get; set; }
    public string OrderNumber { get; set; } = string.Empty;
    public Customer Customer { get; set; } = null!;
    public List<OrderItem> Items { get; set; } = new();
    public OrderStatus Status { get; set; }
}

// Generate DTOs for related entities
[GenerateDto(typeof(Customer))]
public partial record CustomerDto;

[GenerateDto(typeof(OrderItem))]
public partial record OrderItemDto;

[GenerateDto(typeof(Order))]
public partial record OrderDto;

// Configure Mapster to map nested objects
TypeAdapterConfig<Order, OrderDto>
    .NewConfig()
    .Map(dest => dest.Customer, src => CustomerDto.FromEntity(src.Customer))
    .Map(dest => dest.Items, src => OrderItemDto.FromEntities(src.Items));

API Request/Response DTOs

// Read DTO
[GenerateDto(typeof(Customer))]
public partial record CustomerDto;

// Create DTO - exclude Id and timestamps
[GenerateDto(typeof(Customer), Exclude = new[] { "Id", "CreatedAt", "UpdatedAt" })]
public partial record CreateCustomerDto;

// Update DTO - exclude Id and CreatedAt
[GenerateDto(typeof(Customer), Exclude = new[] { "Id", "CreatedAt" })]
public partial record UpdateCustomerDto;

// API Endpoint
app.MapPost("/customers", async (CreateCustomerDto dto, ICustomerRepository repo) =>
{
    var customer = dto.ToEntity();
    await repo.AddAsync(customer);
    return Results.Created($"/customers/{customer.Id}", CustomerDto.FromEntity(customer));
});

Mapster Integration

Global Configuration

// Configure mappings at startup
public static class MappingConfiguration
{
    public static void Configure()
    {
        TypeAdapterConfig.GlobalSettings.Scan(typeof(Program).Assembly);
        
        // Custom mapping rules
        TypeAdapterConfig<Customer, CustomerDto>
            .NewConfig()
            .Map(dest => dest.FullName, src => $"{src.FirstName} {src.LastName}")
            .Ignore(dest => dest.InternalId);
    }
}

EF Core Query Projection

// Efficient database queries with projection
public async Task<List<ProductDto>> GetProductsAsync()
{
    return await _dbContext.Products
        .Where(p => p.IsActive)
        .ProjectToType<ProductDto>() // Mapster projects directly from DB
        .ToListAsync();
}

Custom Type Adapters

// Register custom adapter for value objects
TypeAdapterConfig<Email, string>
    .NewConfig()
    .MapWith(email => email.Value);

TypeAdapterConfig<string, Email>
    .NewConfig()
    .MapWith(str => new Email(str));

Advanced Patterns

Version-Specific DTOs

namespace MyApp.V1.Dtos
{
    [GenerateDto(typeof(Product))]
    public partial record ProductDto;
}

namespace MyApp.V2.Dtos
{
    [GenerateDto(typeof(Product), Exclude = new[] { "InternalCode" })]
    public partial record ProductDto
    {
        // V2 adds new computed field
        public string Category { get; init; } = string.Empty;
    }
}

Read vs Write DTOs

// Read model - all properties
[GenerateDto(typeof(Order))]
public partial record OrderDto;

// Command model - only writable properties
[GenerateDto(typeof(Order), Exclude = new[] { "Id", "OrderNumber", "CreatedAt", "Status" })]
public partial record CreateOrderCommand;

// Update model - fewer properties
[GenerateDto(typeof(Order), Exclude = new[] { "Id", "OrderNumber", "CreatedAt" })]
public partial record UpdateOrderCommand;

Flattening Nested Objects

public class Order
{
    public Guid Id { get; set; }
    public Address ShippingAddress { get; set; } = null!;
}

[GenerateDto(typeof(Order))]
public partial record OrderDto
{
    // Override to flatten
    public new string ShippingAddress { get; init; } = string.Empty;
}

// Configure flattening in Mapster
TypeAdapterConfig<Order, OrderDto>
    .NewConfig()
    .Map(dest => dest.ShippingAddress, 
         src => $"{src.ShippingAddress.Street}, {src.ShippingAddress.City}");

Performance Considerations

Compile-Time vs Runtime

DtoGenerator works at compile time, not runtime:

• ✅ No runtime reflection overhead
• ✅ No runtime code generation
• ✅ All mappings are statically compiled
• ✅ Full IDE IntelliSense support

Mapping Performance

With Mapster (Recommended):

• Uses compiled expressions for fast mapping
• Supports query projection for efficient database queries
• Caches mapping expressions

Without Mapster (Fallback):

• Simple property-by-property assignment
• No reflection at runtime
• Suitable for simple scenarios

Memory Efficiency

// Efficient collection mapping with Mapster
var dtos = await dbContext.Products
    .ProjectToType<ProductDto>() // Maps in database, not in memory
    .ToListAsync();

// vs. inefficient approach
var products = await dbContext.Products.ToListAsync(); // Load all entities
var dtos = products.Select(p => ProductDto.FromEntity(p)); // Map in memory

Troubleshooting

Generated Files Not Visible

Ensure these properties are set:

<EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
<CompilerGeneratedFilesOutputPath>$(BaseIntermediateOutputPath)Generated</CompilerGeneratedFilesOutputPath>

Check obj/Generated folder for generated files.

Generator Not Running

• Clean and rebuild the solution
• Ensure EnforceExtendedAnalyzerRules is set to true
• Check for analyzer warnings in the Error List

Compilation Errors

If you get duplicate property errors:

1. Check if properties are declared in both entity and DTO partial
2. Use the Exclude parameter to skip duplicates
3. Override properties explicitly if needed

Mapster Not Detected

Ensure Mapster package reference is added:

<PackageReference Include="Mapster" Version="7.4.0" />

Generator checks for Mapster at compile time.

Global Exclusions Not Working

If global exclusions aren’t being applied:

1. Verify the MSBuild property is correctly set in your .csproj:

   <PropertyGroup>
     <DtoGenerator_GlobalExclusions>Property1,Property2</DtoGenerator_GlobalExclusions>
   </PropertyGroup>
   

2. Ensure CompilerVisibleProperty is configured:

   <ItemGroup>
     <CompilerVisibleProperty Include="DtoGenerator_GlobalExclusions" />
   </ItemGroup>
   

3. Clean and rebuild to regenerate all DTOs

4. Check generated files in obj/Generated or your configured output folder

Diagnostics

The generator reports several diagnostic codes:

• DKDTOGEN001: DTO generation warning - entity type issues
  • Entity type not found or not accessible
  • Generic entity types (limited support)
  • Circular references in entity properties
• DKDTOGEN002: No properties found for entity
  • Entity type wasn’t resolved correctly
  • Entity has no public readable properties
• DKDTOGEN003: Properties filtered from DTO (informational)
  • Shows how many properties were included/excluded
• DKDTOGEN004: Include and Exclude are mutually exclusive
  • Cannot use both Include and Exclude on the same DTO
  • Choose one approach or the other
• DKDTOGEN005: Include parameter ignores global exclusions (informational)
  • When using Include, global exclusions are not applied
  • Only specified properties will be included

Check build output for specific diagnostic messages.

Limitations and Edge Cases

Navigation Properties

Navigation and collection properties are included as shallow copies:

• Complex reference types (e.g., navigation properties like Customer, Order) are generated with the = null!; initializer to satisfy C# nullable reference type requirements
• Collections are initialized with = []; for non-nullable collection types
• For deep copying:
  • Configure Mapster to handle nested objects
  • Override properties in partial DTO
  • Create separate DTOs for related entities
  • Exclude navigation properties using the Exclude parameter if they’re not needed

// Entity with navigation property
public class Order
{
    public Guid Id { get; set; }
    public Customer Customer { get; set; } = null!; // Navigation property
    public List<OrderItem> Items { get; set; } = new();
}

// Generated DTO includes navigation property with null! initializer
[GenerateDto(typeof(Order))]
public partial record OrderDto;
// Generated: public Customer Customer { get; init; } = null!;
// Generated: public List<OrderItem> Items { get; init; } = [];

// Alternative: Exclude navigation properties if not needed
[GenerateDto(typeof(Order), Exclude = new[] { "Customer", "Items" })]
public partial record OrderSummaryDto;

Nullable Reference Types

• Non-nullable strings receive the required modifier
• Non-nullable collections are initialized with = []
• Non-nullable complex reference types (navigation properties, custom classes) receive = null!; initializer to satisfy compiler null-state analysis

Generic Entities

Limited support for generic entity types. DTO shells must be non-generic.

Inheritance

Entity inheritance is not automatically handled. Create separate DTOs for each entity type or use Mapster configuration.

Best Practices

1. Use Partial Records

// Preferred
[GenerateDto(typeof(Product))]
public partial record ProductDto;

// Instead of class
[GenerateDto(typeof(Product))]
public partial class ProductDto; // Works but records are more idiomatic for DTOs

2. Create Purpose-Specific DTOs

[GenerateDto(typeof(Customer))]
public partial record CustomerDto; // Full read model

[GenerateDto(typeof(Customer), Exclude = new[] { "Id", "CreatedAt" })]
public partial record CreateCustomerDto; // Create command

[GenerateDto(typeof(Customer), Exclude = new[] { "Id", "Email" })]
public partial record CustomerSummaryDto; // List view

3. Organize DTOs by Feature

/Features
  /Customers
    /Entities
      Customer.cs
    /Dtos
      CustomerDto.cs
      CreateCustomerDto.cs
      UpdateCustomerDto.cs
  /Orders
    /Entities
      Order.cs
    /Dtos
      OrderDto.cs

4. Configure Mapster Globally

// Startup.cs or Program.cs
var config = TypeAdapterConfig.GlobalSettings;
config.Scan(typeof(Program).Assembly);
config.RequireExplicitMapping = false;
config.RequireDestinationMemberSource = false;

5. Use DTOs at API Boundaries Only

// ✅ Good - DTOs at API boundary
public async Task<CustomerDto> GetCustomerAsync(Guid id)
{
    var customer = await _repository.GetByIdAsync(id); // Domain entity internally
    return CustomerDto.FromEntity(customer); // Convert to DTO for API
}

// ❌ Bad - DTOs in domain layer
public async Task ProcessOrderAsync(OrderDto dto) // DTOs shouldn't be in domain services

Integration with DKNet Framework

DtoGenerator works seamlessly with other DKNet components:

With DKNet.EfCore.Repos

public class CustomerService
{
    private readonly IReadRepository<Customer> _repository;
    
    public async Task<PagedList<CustomerDto>> GetCustomersAsync(int page, int pageSize)
    {
        return await _repository.Gets()
            .ProjectToType<CustomerDto>() // DtoGenerator + Mapster
            .ToPagedListAsync(page, pageSize); // DKNet.EfCore.Repos
    }
}

With SlimBus CQRS

public record GetCustomerQuery : IWitResponse<CustomerDto>
{
    public required Guid CustomerId { get; init; }
}

public class GetCustomerHandler : IHandler<GetCustomerQuery, CustomerDto>
{
    private readonly ICustomerRepository _repository;
    
    public async Task<CustomerDto?> OnHandle(GetCustomerQuery request, CancellationToken ct)
    {
        var customer = await _repository.FindAsync(request.CustomerId, ct);
        return customer != null ? CustomerDto.FromEntity(customer) : null;
    }
}

Related Documentation

• DKNet.EfCore.Repos - Repository pattern implementations
• DKNet.EfCore.Repos.Abstractions - Repository abstractions
• Architecture Guide - Understanding DDD and Onion Architecture
• Examples & Recipes - Practical implementation patterns

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💡 Pro Tip: Use DtoGenerator for all API data transfer needs to maintain a clean separation between your domain model and external representations. Combined with Mapster, it provides a powerful, type-safe, and performant solution for object mapping in DDD applications.

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