Mapster

Dosaic.Plugins.Mapping.Mapster is a Dosaic plugin that wires Mapster into the Dosaic plugin pipeline. It automatically scans all application assemblies at startup, discovers types annotated with [MapFrom<TSource>], and registers the corresponding Mapster mapping rules in TypeAdapterConfig.GlobalSettings — no manual mapping registration required.

Installation

dotnet add package Dosaic.Plugins.Mapping.Mapster

Or add a package reference directly to your .csproj:

<PackageReference Include="Dosaic.Plugins.Mapping.Mapster" Version="" />

Configuration

No configuration is required. MapsterPlugin is discovered and activated automatically by the Dosaic source generator (Dosaic.Hosting.Generator). At startup it calls IImplementationResolver.FindAssemblies() to obtain every assembly in the application and passes them to the internal MapsterInitializer, which registers all [MapFrom<T>]-based rules into Mapster's global configuration.

There are no appsettings.yml / appsettings.json keys for this plugin.

Usage

Basic property rename

Use [MapFrom<TSource>(nameof(TSource.Property))] on the destination property to map it from a differently-named property on the source type.

public class DbModel
{
    public string Id { get; set; }
    public string LongName { get; set; }
}

public class ModelDto
{
    public string Id { get; set; }

    [MapFrom<DbModel>(nameof(DbModel.LongName))]
    public string Name { get; set; }
}

// Mapping
var dbModel = new DbModel { Id = "1", LongName = "Hello World" };
var dto = dbModel.Adapt<ModelDto>();
// dto.Name == "Hello World"

Nested navigation path

Pass multiple property names to traverse nested objects. Each element in the array is a step in the navigation chain evaluated against the current traversal position.

public class Order
{
    public int Id { get; set; }
    public Address ShippingAddress { get; set; }
}

public class Address
{
    public string Street { get; set; }
    public string City { get; set; }
}

public class OrderDto
{
    public int Id { get; set; }

    // Maps from order.ShippingAddress.City
    [MapFrom<Order>(nameof(Order.ShippingAddress), nameof(Address.City))]
    public string City { get; set; }
}

var order = new Order { Id = 42, ShippingAddress = new Address { Street = "Main St", City = "Berlin" } };
var dto = order.Adapt<OrderDto>();
// dto.City == "Berlin"

Collection projection — scalar values

When the navigation path passes through a collection, each element is projected individually. The target property type must be a compatible IEnumerable<T>.

public class BlogPost
{
    public int Id { get; set; }
    public List<Tag> Tags { get; set; }
}

public class Tag
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class BlogPostDto
{
    public int Id { get; set; }

    // Extracts Tag.Name from every element in BlogPost.Tags
    [MapFrom<BlogPost>(nameof(BlogPost.Tags), nameof(Tag.Name))]
    public IEnumerable<string> TagNames { get; set; }
}

var post = new BlogPost
{
    Id = 1,
    Tags = [new Tag { Id = 10, Name = "dotnet" }, new Tag { Id = 11, Name = "mapster" }]
};
var dto = post.Adapt<BlogPostDto>();
// dto.TagNames == ["dotnet", "mapster"]

Collection projection — object mapping

When the navigation ends at a collection of objects, each element is adapted to the target element type using the globally registered Mapster rules.

public class SourceModel
{
    public List<NestedSource> Items { get; set; }
}

public class NestedSource
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class NestedTarget
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class TargetModel
{
    // Each NestedSource element is adapted to NestedTarget
    [MapFrom<SourceModel>(nameof(SourceModel.Items))]
    public IEnumerable<NestedTarget> Items { get; set; }
}

var source = new SourceModel
{
    Items = [new NestedSource { Id = 1, Name = "A" }, new NestedSource { Id = 2, Name = "B" }]
};
var target = source.Adapt<TargetModel>();
// target.Items contains two NestedTarget instances

Null-safe collection mapping

If the source collection is null, the mapped destination property is also null — no NullReferenceException is thrown.

public class SourceModel
{
    public List<NestedSource> Items { get; set; } // null
}

public class TargetModel
{
    [MapFrom<SourceModel>(nameof(SourceModel.Items))]
    public IEnumerable<NestedTarget> Items { get; set; }
}

var source = new SourceModel { Items = null };
var target = source.Adapt<TargetModel>();
// target.Items == null  (no exception)

Multiple [MapFrom] attributes on a single property

The attribute allows AllowMultiple = true, so a single destination property can have multiple source mappings applied (e.g., for different source types).

public class TargetModel
{
    [MapFrom<SourceA>(nameof(SourceA.Title))]
    [MapFrom<SourceB>(nameof(SourceB.Headline))]
    public string Name { get; set; }
}

EF Core LINQ projection (ProjectToType<T>)

Because the mappings are registered as Mapster expression rules, they are fully translatable to SQL when used with EF Core's ProjectToType<T>() extension.

// Retrieve only the columns you need — translated to SQL
var dtos = await dbContext.Orders
    .Where(o => o.CustomerId == customerId)
    .ProjectToType<OrderDto>()
    .ToListAsync();

Features

• Zero configuration — no YAML/JSON settings required; the plugin activates automatically via the Dosaic source generator.

• Attribute-driven mapping — declare all mapping rules inline on destination types using [MapFrom<TSource>(navigationPath)], keeping mapping logic co-located with the DTO.

• Deep navigation paths — navigate arbitrarily deep object graphs by providing a sequence of property names; each name is resolved against the current traversal type.

• Collection projection to scalars — extract a scalar property from every element of a source collection into an IEnumerable<TScalar>.

• Collection projection to objects — map a source collection to a destination collection of a different element type, with each element adapted via Mapster.

• Null-safe collections — null source collections produce null destination properties without throwing.

• EF Core LINQ projection — registered expression rules are LINQ-translatable, enabling efficient ProjectToType<T>() queries that are converted to SQL.

• Global rule registration — rules land in TypeAdapterConfig.GlobalSettings, making them available everywhere Adapt<T>() or ProjectToType<T>() is called with no additional setup.

• Assembly scanning — leverages IImplementationResolver.FindAssemblies() so every assembly in the application is scanned, including plugin assemblies.