Cannot CompileProjection for a mapping derived types between abstract classes with .Include()

Description

When trying to create a mapping between abstract base classes and including their derived concrete implementations, Mapster throws an exception during CompileProjection(). There appears to be an issue in the expression tree generation.

Environment

Mapster version: 7.4.0
Mapster.EFCore 5.1.1
.NET version: net9.0
EF Core version: 9.0.4

I have tried Mapster 7.4.2-pre02 with net9.0 and net8.0 as well.

Steps to Reproduce

Define the following class hierarchy:

public abstract class PocoA
{
    public int Id { get; set; }
}

public class PocoDerived : PocoA
{
    public int DerivedVal { get; set; }
}

public abstract class DtoA
{
    public int Id { get; set; }
}

public class DtoDerived : DtoA
{
    public int DerivedVal { get; set; }
}

Configure the mapping:

config.NewConfig<PocoA, DtoA>()
    .Include<PocoDerived, DtoDerived>();
  
config.CompileProjection();

Alternatively, edit the WhenIncludeDerivedClasses test in Mapster.Tests so that the Compile() lines are CompileProjection();

Expected Behavior

The mapping should compile successfully, allowing for projection between these types.

Actual Behavior

An exception is thrown during CompileProjection():

Unhandled exception. Mapster.CompileException: Error while compiling
source=Backend.Services.Models.PocoA
destination=Backend.Services.Models.DtoA
type=Projection
 ---> System.InvalidCastException: Unable to cast object of type 'System.Linq.Expressions.UnaryExpression' to type 'System.Linq.Expressions.NewExpression'.
   at Mapster.Adapters.ClassAdapter.CreateInlineExpression(Expression source, CompileArgument arg)
   at Mapster.Adapters.BaseAdapter.CreateInlineExpressionBody(Expression source, CompileArgument arg)
   at Mapster.Adapters.BaseAdapter.CreateExpressionBody(Expression source, Expression destination, CompileArgument arg)
   at Mapster.Adapters.BaseAdapter.CreateAdaptFunc(CompileArgument arg)
   at Mapster.TypeAdapterConfig.CreateMapExpression(CompileArgument arg)
   --- End of inner exception stack trace ---
   at Mapster.TypeAdapterConfig.CreateMapExpression(CompileArgument arg)
   at Mapster.TypeAdapterConfig.CreateMapExpression(TypeTuple tuple, MapType mapType)
   at Mapster.TypeAdapterConfig.CreateProjectionCallExpression(TypeTuple tuple)
   at Mapster.TypeAdapterConfig.CompileProjection()

@Tommsy64

@Tommsy64 .Include to querytable not supported.

Projection is intended to create a query to the database.
And at the moment of creating a query, we cannot know what derived type will be obtained from the database (if this is even possible).

Even if this can be done, the scope of application will be limited to Linq to Object cases (after loading from the database into memory).

It is possible to know the type from within the queryable, this works:

class Address
{
  public int Id { get; set; }
  public string AddressLine { get; set; }
}

class DetailedAddress : Address
{
  public string HouseNumber { get; set; }
}

class AddressDto
{
  public int Id { get; set; }
  public string AddressLine { get; set; }
}

class DetailedAddressDto : AddressDto
{
  public string HouseNumber { get; set; }
}

// ....

var mappedDtos = await source.Select(x =>
    x.GetType() == typeof(DetailedAddress)
        ? new DetailedAddressDto { Id = x.Id, AddressLine = x.AddressLine, HouseNumber = ((DetailedAddress)x).HouseNumber }
        : new Address{ Id = x.Id, AddressLine = x.AddressLine }
).ToListAsync();

I don't know since when, but EFCore supports translating the .GetType() call.

I was able to get this working like so:

private static readonly LambdaExpression SimpleAddressProjection = TypeAdapterConfig
    .CreateMapExpression(new TypeTuple(typeof(Address), typeof(AddressDto)), MapType.Projection);
private static readonly LambdaExpression DetailedAddressProjection = TypeAdapterConfig
    .CreateMapExpression(new TypeTuple(typeof(DetailedAddress), typeof(DetailedAddressDto)), MapType.Projection);

public IQueryable<AddressDto> ProjectTo(IQueryable<Address> source)
{
    var param = Expression.Parameter(typeof(Address));
    var lambda = Expression.Lambda(Expression.Condition(
        Expression.TypeIs(param, typeof(DetailedAddress)),
        Expression.TypeAs(Expression.Invoke(DetailedAddressProjection, Expression.TypeAs(param, typeof(DetailedAddress))), typeof(AddressDto)),
        Expression.Invoke(SimpleAddressProjection, param)
    ), param);

    var selectExpr = CreateProjectionCallExpression<Address, AddressDto>(lambda);
    var finalExpr = Expression.Call(selectExpr.Method, source.Expression, selectExpr.Arguments[1]);

    return source.Provider.CreateQuery<AddressDto>(finalExpr);
}

// From Mapster
private MethodCallExpression CreateProjectionCallExpression<TSource, TDestination>(LambdaExpression lambda)
{
    var source = Expression.Parameter(typeof(IQueryable<>).MakeGenericType(typeof(TSource)));
    var methodInfo = (from method in typeof(Queryable).GetMethods()
                      where method.Name == nameof(Queryable.Select)
                      let p = method.GetParameters()[1]
                      where p.ParameterType.GetGenericArguments()[0].GetGenericTypeDefinition() == typeof(Func<,>)
                      select method).First().MakeGenericMethod(typeof(TSource), typeof(TDestination));
    return Expression.Call(methodInfo, source, Expression.Quote(lambda));
}

@zsr2531

@zsr2531 If x.GetType() == typeof(DetailedAddress) it really is converted into a SQL query to the database. In this case adding is really possible.

Can you show the resulting SQL query?

Sure:

select
	a."Discriminator" = 'DetailedAddress',
	a."AddressLine",
        a."HouseNumber"
from
	"Addresses" as a

It seems like they lower x.GetType() == ... to a discriminator column check.

Forgot to mention, but I am also on EFCore version 9.

It seems like they lower x.GetType() == ... to a discriminator column check.

Yes, if it can work, then only like this :)

@zsr2531

@zsr2531 It's a kind of filter.

Expression.TypeIs(param, typeof(DetailedAddress)),
Expression.TypeAs(Expression.Invoke(DetailedAddressProjection, Expression.TypeAs(param, typeof(DetailedAddress))), typeof(AddressDto)),

But .Include works differently.
Using the source type It create instance of destination type defined in the config.

It seems like an idea of how this can be done has appeared :)
But it will be done, not quickly.

fix: support CompileProjection with Include on abstract types (#801)

chore: Merge pull request #977 from leno23/fix/projection-include-abs…

Fixed as of Mapster 10.0.9. Closing this issue.

I did notice that a derived source type with no matching Include<TDerivedSource, TDerivedDestination>() registration projects to default(TDestination), i.e. a null row rather than an error. So adding a derived type and forgetting to register it with Include<>() in the Mapster config will silently yield nulls.

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Cannot CompileProjection for a mapping derived types between abstract classes with .Include() #801

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Cannot CompileProjection for a mapping derived types between abstract classes with .Include() #801

Description

Description

Environment

Steps to Reproduce

Expected Behavior

Actual Behavior

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DocSvartz commented on May 8, 2025

zsr2531 commented on Jul 11, 2025

zsr2531 commented on Jul 11, 2025

DocSvartz commented on Jul 11, 2025

zsr2531 commented on Jul 11, 2025

zsr2531 commented on Jul 11, 2025

DocSvartz commented on Jul 11, 2025

DocSvartz commented on Jul 11, 2025

DocSvartz commented on Jul 11, 2025

Tommsy64 commented on Aug 20, 2026

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