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Entity Framework Core: NullReferenceException after DropTable

I am simply trying to drop a table.
First I tried to just remove all the references to the table in the code. When creating a new migration, I get a "NullReferenceException", as shown here.
Then I tried instead to create an empty migration with migrationBuilder.DropTable("MyTable") in the Up() method. That drops the table, but if I try to make a new empty migration I get the same error.
Are these not the ways to drop a table with EF Core? Do I need something else, or doing something in the wrong order?
I am using F#, but I hope that is not the problem.
Build started...
Build succeeded.
System.NullReferenceException: Object reference not set to an instance of an object.
at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Initialize(ColumnOperation columnOperation, IColumn column, RelationalTypeMapping typeMapping, Boolean isNullable, IEnumerable`1 migrationsAnnotations, Boolean inline)
at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Add(IColumn target, DiffContext diffContext, Boolean inline)+MoveNext()
at Pomelo.EntityFrameworkCore.MySql.Migrations.Internal.MySqlMigrationsModelDiffer.PostFilterOperations(IEnumerable`1 migrationOperations)+MoveNext()
at System.Linq.Enumerable.SelectManySingleSelectorIterator`2.MoveNext()
at System.Linq.Enumerable.CastIterator[TResult](IEnumerable source)+MoveNext()
at System.Collections.Generic.List`1.InsertRange(Int32 index, IEnumerable`1 collection)
at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Add(ITable target, DiffContext diffContext)+MoveNext()
at Pomelo.EntityFrameworkCore.MySql.Migrations.Internal.MySqlMigrationsModelDiffer.PostFilterOperations(IEnumerable`1 migrationOperations)+MoveNext()
at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.DiffCollection[T](IEnumerable`1 sources, IEnumerable`1 targets, DiffContext diffContext, Func`4 diff, Func`3 add, Func`3 remove, Func`4[] predicates)+MoveNext()
at System.Linq.Enumerable.ConcatIterator`1.MoveNext()
at Pomelo.EntityFrameworkCore.MySql.Migrations.Internal.MySqlMigrationsModelDiffer.PostFilterOperations(IEnumerable`1 migrationOperations)+MoveNext()
at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Sort(IEnumerable`1 operations, DiffContext diffContext)
at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.GetDifferences(IRelationalModel source, IRelationalModel target)
at Microsoft.EntityFrameworkCore.Migrations.Design.MigrationsScaffolder.ScaffoldMigration(String migrationName, String rootNamespace, String subNamespace, String language)
at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType, String namespace)
at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType, String namespace)
at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.<>c__DisplayClass0_0.<.ctor>b__0()
at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
Object reference not set to an instance of an object.

System.InvalidCastException on Select when using custom ValueConverter

Problem
I'm trying to introduce a strongly typed ID in one of my entities. To do so, I followed the instructions on https://andrewlock.net/using-strongly-typed-entity-ids-to-avoid-primitive-obsession-part-3/
The only difference to the instructions above is, that I'm trying to persist my ID as a string instead of a Guid.
A short summary of the instructions (a full example can be downloaded here: https://drive.google.com/file/d/1_mtYp1c8W6qJoEAY-8NPNjYvsSAif4mz/view?usp=sharing):
Create a struct "OrderId" with a property "Value", which returns the string representation of the ID
Create the class for the Order entity and add a property "ID" of type "OrderId"
Create the DbContext with a DbSet
Create a custom ValueConverter that converts OrderId to string and vice versa
Apply the value converter to the Id column of the Order entity
Add an explicit conversion operator from OrderId to string to the OrderId in order to avoid client-side evaluations
With this setup, it's no problem to add entries to the Orders table. BUT: when I try to read them, the following exception is thrown:
System.InvalidCastException
HResult=0x80004002
Message=Invalid cast from 'System.String' to 'StronglyTypedIds.OrderId'.
Source=System.Private.CoreLib
StackTrace:
at System.Convert.DefaultToType(IConvertible value, Type targetType, IFormatProvider provider) in E:\A\_work\644\s\src\mscorlib\shared\System\Convert.cs:line 309
at Microsoft.EntityFrameworkCore.Storage.ValueConversion.ValueConverter`2.Sanitize[T](Object value) in /_/src/EFCore/Storage/ValueConversion/ValueConverter`.cs:line 52
at Microsoft.EntityFrameworkCore.Storage.ValueConversion.ValueConverter`2.<>c__DisplayClass3_0`2.<SanitizeConverter>b__0(Object v) in /_/src/EFCore/Storage/ValueConversion/ValueConverter`.cs:line 43
at Microsoft.EntityFrameworkCore.Storage.RelationalTypeMapping.CreateParameter(DbCommand command, String name, Object value, Nullable`1 nullable) in /_/src/EFCore.Relational/Storage/RelationalTypeMapping.cs:line 515
at Microsoft.EntityFrameworkCore.Storage.Internal.TypeMappedRelationalParameter.AddDbParameter(DbCommand command, Object value) in /_/src/EFCore.Relational/Storage/Internal/TypeMappedRelationalParameter.cs:line 66
at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalParameterBase.AddDbParameter(DbCommand command, IReadOnlyDictionary`2 parameterValues) in /_/src/EFCore.Relational/Storage/Internal/RelationalParameterBase.cs:line 45
at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.CreateCommand(IRelationalConnection connection, IReadOnlyDictionary`2 parameterValues) in /_/src/EFCore.Relational/Storage/Internal/RelationalCommand.cs:line 375
at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.Execute(IRelationalConnection connection, DbCommandMethod executeMethod, IReadOnlyDictionary`2 parameterValues) in /_/src/EFCore.Relational/Storage/Internal/RelationalCommand.cs:line 149
at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.ExecuteReader(IRelationalConnection connection, IReadOnlyDictionary`2 parameterValues) in /_/src/EFCore.Relational/Storage/Internal/RelationalCommand.cs:line 119
at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.Enumerator.BufferlessMoveNext(DbContext _, Boolean buffer) in /_/src/EFCore.Relational/Query/Internal/QueryingEnumerable.cs:line 111
at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.Enumerator.MoveNext() in /_/src/EFCore.Relational/Query/Internal/QueryingEnumerable.cs:line 93
at System.Linq.Enumerable.TryGetFirst[TSource](IEnumerable`1 source, Boolean& found) in E:\A\_work\286\s\corefx\src\System.Linq\src\System\Linq\First.cs:line 63
at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.ResultEnumerable`1.GetEnumerator() in /_/src/EFCore/Query/Internal/LinqOperatorProvider.cs:line 294
at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.<_TrackEntities>d__17`2.MoveNext() in /_/src/EFCore/Query/Internal/LinqOperatorProvider.cs:line 185
at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.MoveNext() in /_/src/EFCore/Query/Internal/LinqOperatorProvider.cs:line 143
at System.Linq.Enumerable.TryGetFirst[TSource](IEnumerable`1 source, Boolean& found) in E:\A\_work\286\s\corefx\src\System.Linq\src\System\Linq\First.cs:line 63
at System.Linq.Enumerable.First[TSource](IEnumerable`1 source) in E:\A\_work\286\s\corefx\src\System.Linq\src\System\Linq\First.cs:line 14
at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass15_1`1.<CompileQueryCore>b__0(QueryContext qc) in /_/src/EFCore/Query/Internal/QueryCompiler.cs:line 132
at System.Linq.Queryable.FirstOrDefault[TSource](IQueryable`1 source, Expression`1 predicate) in E:\A\_work\286\s\corefx\src\System.Linq.Queryable\src\System\Linq\Queryable.cs:line 741
at StronglyTypedIds.Program.Main() in <path_to_project>\StronglyTypedIds\StronglyTypedIds\Program.cs:line 29
The weird thing is, that it seems to depend on how I build my Where-query:
var order = dbContext.Orders.Where(o => (string)o.Id == "1").ToList(); // works
const string idAsStringConstant = "1";
order = dbContext.Orders.Where(o => (string)o.Id == idAsStringConstant).ToList(); // works
var idAsStringVariable = "1";
order = dbContext.Orders.Where(o => (string)o.Id == idAsStringVariable).ToList(); // doesn't work
order = dbContext.Orders.Where(o => (string)o.Id == OrderId.Parse("1").StringValue).ToList(); // doesn't work
Steps to reproduce
Here is a small VS2019 solution with an example: https://drive.google.com/file/d/1_mtYp1c8W6qJoEAY-8NPNjYvsSAif4mz/view?usp=sharing
Further technical details
EF Core version: 2.2.6
Database Provider: Microsoft.EntityFrameworkCore.SqlServer and Microsoft.EntityFrameworkCore.Sqlite
Operating system: Windows 10 1809
IDE: (e.g. Visual Studio 2019 16.1.6)

Ok, it's a bug in Entity Framework. If I install the preview of version 3.0 of .NET Core and Entity Framework Core it works as expected. Further there seems to be a fix for version 2.2 which just wasn't released yet (see https://github.com/aspnet/EntityFrameworkCore/issues/17610).

NHibernate firebird error - Index was out of range

What can cause the following error in c# when using NHibernate and Firebird database?
2015-08-17 08:27:04,962 [21] [(null)] ERROR Smartsign.Server.Core.Server Unhandled exception: System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
at System.ThrowHelper.ThrowArgumentOutOfRangeException()
at System.Collections.Generic.List`1.get_Item(Int32 index)
at FirebirdSql.Data.FirebirdClient.FbParameterCollection.get_Item(Int32 index) in c:\Users\Jiri\Documents\devel\NETProvider\working\NETProvider\src\FirebirdSql.Data.FirebirdClient\FirebirdClient\FbParameterCollection.cs:line 58
at FirebirdSql.Data.FirebirdClient.FbParameterCollection.GetParameter(Int32 index) in c:\Users\Jiri\Documents\devel\NETProvider\working\NETProvider\src\FirebirdSql.Data.FirebirdClient\FirebirdClient\FbParameterCollection.cs:line 315
at System.Data.Common.DbParameterCollection.System.Collections.IList.get_Item(Int32 index)
at NHibernate.Type.Int32Type.Set(IDbCommand rs, Object value, Int32 index)
at NHibernate.Type.NullableType.NullSafeSet(IDbCommand cmd, Object value, Int32 index)
at NHibernate.Type.NullableType.NullSafeSet(IDbCommand st, Object value, Int32 index, ISessionImplementor session)
at NHibernate.Persister.Entity.AbstractEntityPersister.Dehydrate(Object id, Object[] fields, Object rowId, Boolean[] includeProperty, Boolean[][] includeColumns, Int32 table, IDbCommand statement, ISessionImplementor session, Int32 index)
at NHibernate.Persister.Entity.AbstractEntityPersister.Insert(Object id, Object[] fields, Boolean[] notNull, Int32 j, SqlCommandInfo sql, Object obj, ISessionImplementor session)
at NHibernate.Persister.Entity.AbstractEntityPersister.Insert(Object id, Object[] fields, Object obj, ISessionImplementor session)
at NHibernate.Action.EntityInsertAction.Execute()
at NHibernate.Engine.ActionQueue.Execute(IExecutable executable)
at NHibernate.Engine.ActionQueue.ExecuteActions(IList list)
at NHibernate.Engine.ActionQueue.ExecuteActions()
at NHibernate.Event.Default.AbstractFlushingEventListener.PerformExecutions(IEventSource session)
at NHibernate.Event.Default.DefaultFlushEventListener.OnFlush(FlushEvent event)
at NHibernate.Impl.SessionImpl.Flush()
at NHibernate.Transaction.AdoTransaction.Commit()

Just for completeness. The exception:
Index was out of range...
with NHibernate WRITE operation mostly means: there is doubled column mapping.
Usually combination of the <property> and <many-to-one>. Also check:
NHibernate mapping index out of range

Dapper throws "Invalid type owner for DynamicMethod."

So I'm trying to use Dapper.net and I'm liking it. What I'm not liking is when I try to batch-insert entities and I get the following error thrown:
Invalid type owner for DynamicMethod.
at System.Reflection.Emit.DynamicMethod.Init(String name,
MethodAttributes attributes, CallingConventions callingConvention,
Type returnType, Type[] signature, Type owner, Module m, Boolean
skipVisibility, Boolean transparentMethod, StackCrawlMark& stackMark)
at System.Reflection.Emit.DynamicMethod..ctor(String name, Type
returnType, Type[] parameterTypes, Type owner, Boolean skipVisibility)
at Dapper.SqlMapper.CreateParamInfoGenerator(Identity identity,
Boolean checkForDuplicates, Boolean removeUnused, IList1 literals) in
D:\Dev\dapper-dot-net\Dapper NET40\SqlMapper.cs:line 3033 at
Dapper.SqlMapper.GetCacheInfo(Identity identity, Object
exampleParameters, Boolean addToCache) in D:\Dev\dapper-dot-net\Dapper
NET40\SqlMapper.cs:line 2138 at
Dapper.SqlMapper.<QueryImpl>d__611.MoveNext() in
D:\Dev\dapper-dot-net\Dapper NET40\SqlMapper.cs:line 1578 at
System.Collections.Generic.List1..ctor(IEnumerable1 collection)
at System.Linq.Enumerable.ToList[TSource](IEnumerable1 source) at
Dapper.SqlMapper.Query[T](IDbConnection cnn, String sql, Object param,
IDbTransaction transaction, Boolean buffered, Nullable1
commandTimeout, Nullable1 commandType) in
D:\Dev\dapper-dot-net\Dapper NET40\SqlMapper.cs:line 1479 at
Dapper.SqlMapper.Query(IDbConnection cnn, String sql, Object param,
IDbTransaction transaction, Boolean buffered, Nullable1
commandTimeout, Nullable1 commandType) in
D:\Dev\dapper-dot-net\Dapper NET40\SqlMapper.cs:line 1418 at
NinjaEvaluation.Data.Database.DapperWrapper.<>c__DisplayClass41.b__3(SqlConnection
sqlConnection, SqlTransaction transaction) in
c:\Projects\InHouse\ninjaevaluation\NinjaEvaluation\NinjaEvaluation.Data\Database\DapperWrapper.cs:line
52 at NinjaEvaluation.Data.Database.DapperWrapper.Invoke(Action`2
action) in
c:\Projects\InHouse\ninjaevaluation\NinjaEvaluation\NinjaEvaluation.Data\Database\DapperWrapper.cs:line
68
This happens in a completely normal situation when I run my query like this:
string sql = #" INSERT INTO XXX
(XXXId, AnotherId, ThirdId, Value, Comment)
VALUES
(#XXXId, #AnotherId, #ThirdId, #Value, #Comment)";
var parameters = command
.MyModels
.Select(model => new
{
XXXId= model.XXXId,
AnotherId= model.AnotherId,
ThirdId= model.ThirdId,
Value = model.Value,
Comment = model.Comment
})
.ToArray();
...
sqlConnection.Query(sql, parameters, commandType: commandType, transaction: transaction)
I found the following SO-thread started by someone having the same problem BUT the issue there seems to have been the .NET version (3.5) but I'm running .NET 4.5 and I can't figure out what the problem is.
Any suggestions?

I ran into this error when using an interface instead of the class:
Query<MyObject> worked, while Query<IMyObject> did not

It fails because this scenario using Query[<T>] isn't expecting an array / sequence of parameters. The Execute call-path does expect this, and unrolls the data automatically, executing the SQL once per item - but this isn't the case for Query[<T>], so it tries to create the dynamic method bound to the array (in your case), which isn't allowed. The code should probably detect this much earlier, and just say "nope, that isn't allowed".
You probably want to change your .ToArray() to .Single().
This will be clearer after the next build; the following passes:
public void SO30435185_InvalidTypeOwner()
{
try {
// not shown for brevity: something very similar to your code
Assert.Fail();
} catch(InvalidOperationException ex)
{
ex.Message.IsEqualTo("An enumerable sequence of parameters (arrays, lists, etc) is not allowed in this context");
}
}

LINQ-NHibernate - Selecting only a few fields (including a Collection) for a complex object

I'm using Fluent NHibernate in one of my projects (and ASP.NET MVC application), with LINQ to query to data (using the LINQ to NHibernate libraries).
The objet names are changed to protect the innocent.
Let's say I have the following classes Foo, Bar, Baz, and their appropriate tables in the database (MySQL).
Foo has a many-to-many relationship with both Bar (table "FooBar") and Baz (table "FooBaz"), defined in the Fluent mappings. As such, the class interface is defined as follows:
public class Foo {
public virtual int id { get; set; }
public virtual string name { get; set; }
public virtual string email { get; set; }
public virtual IList<Bar> bars { get; set; }
public virtual IList<Baz> bazes { get; set; }
}
This is a pretty standard class. We can see that a Foo object will have a list of bars and bazes.
The problem comes when trying to do a LINQ query.
If I do a simple query like this, it works fine (the where clause is unimportant) :
var foos = from foo in session.Linq<Foo>()
where email.equals("foo#bar.com")
select foo;
IList<Foo> listFoos = foos.ToList();
This will return a list of Foos, with all the fields populated (id, name, email, bars, bazes). log4net shows that NHibernate performs separate queries for the collections.
The problem arises when I want to load only some fields. For example, I might want to load only the bars in the query, but not the bazes.
This query compiles, but produces an error at runtime:
var foos = from foo in session.Linq<Foo>()
where email.equals("foo#bar.com")
select new Foo()
{
id = foo.id,
name = foo.name,
email = foo.email,
bars = foo.bars
};
IList<Foo> listFoos = foos.ToList();
The error I get is something along the lines of an array index out of bounds exception. The stack trace shows some methods names relating the collection handling on LINQ-NHibernate's side, but nothing else. The query reported by log4net shows no sign of a query on bars, which means than an error was caught before it had the time to perform the query to select the bars.
Did anyone else had this kind of problem before? How did you solve it? I don't want to have to select all the objects everytime I open a web page in my application!
Thank you!
Edit: here is the stacktrace, as requested.
System.IndexOutOfRangeException: L'index se trouve en dehors des limites du tableau. (read: "Index is out of bounds for the array.")
[IndexOutOfRangeException: L'index se trouve en dehors des limites du tableau.]
NHibernate.Transform.TypeSafeConstructorMemberInitResultTransformer.InvokeMemberInitExpression(MemberInitExpression expression, Object[] args, Int32& argumentCount) +404
NHibernate.Transform.TypeSafeConstructorMemberInitResultTransformer.TransformTuple(Object[] tuple, String[] aliases) +150
[QueryException: could not instantiate: Foo]
NHibernate.Transform.TypeSafeConstructorMemberInitResultTransformer.TransformTuple(Object[] tuple, String[] aliases) +265
NHibernate.Loader.Criteria.CriteriaLoader.GetResultColumnOrRow(Object[] row, IResultTransformer resultTransformer, IDataReader rs, ISessionImplementor session) +171
NHibernate.Loader.Loader.GetRowFromResultSet(IDataReader resultSet, ISessionImplementor session, QueryParameters queryParameters, LockMode[] lockModeArray, EntityKey optionalObjectKey, IList hydratedObjects, EntityKey[] keys, Boolean returnProxies) +330
NHibernate.Loader.Loader.DoQuery(ISessionImplementor session, QueryParameters queryParameters, Boolean returnProxies) +704
NHibernate.Loader.Loader.DoQueryAndInitializeNonLazyCollections(ISessionImplementor session, QueryParameters queryParameters, Boolean returnProxies) +70
NHibernate.Loader.Loader.DoList(ISessionImplementor session, QueryParameters queryParameters) +111
NHibernate.Loader.Loader.ListIgnoreQueryCache(ISessionImplementor session, QueryParameters queryParameters) +18
NHibernate.Loader.Loader.List(ISessionImplementor session, QueryParameters queryParameters, ISet`1 querySpaces, IType[] resultTypes) +79
NHibernate.Impl.SessionImpl.List(CriteriaImpl criteria, IList results) +407
NHibernate.Impl.CriteriaImpl.List(IList results) +41
NHibernate.Impl.CriteriaImpl.List() +35
NHibernate.Linq.<GetEnumerator>d__0.MoveNext() +71
System.Collections.Generic.List`1..ctor(IEnumerable`1 collection) +7665172
System.Linq.Enumerable.ToList(IEnumerable`1 source) +61
FooRepository.List(Int32 count) in C:\...\FooRepository.cs:38
FooController.List() in C:\...\FooController.cs:30
lambda_method(ExecutionScope , ControllerBase , Object[] ) +39
System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters) +17
System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +178
System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +24
System.Web.Mvc.<>c__DisplayClassa.<InvokeActionMethodWithFilters>b__7() +52
System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) +254
System.Web.Mvc.<>c__DisplayClassc.<InvokeActionMethodWithFilters>b__9() +19
System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodWithFilters(ControllerContext controllerContext, IList`1 filters, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +192
System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +399
System.Web.Mvc.Controller.ExecuteCore() +126
System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) +27
System.Web.Mvc.ControllerBase.System.Web.Mvc.IController.Execute(RequestContext requestContext) +7
System.Web.Mvc.MvcHandler.ProcessRequest(HttpContextBase httpContext) +151
System.Web.Mvc.MvcHandler.ProcessRequest(HttpContext httpContext) +57
System.Web.Mvc.MvcHandler.System.Web.IHttpHandler.ProcessRequest(HttpContext httpContext) +7
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +181
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75

A couple of suggestions...
First, NHibernate defaults to lazy loading, so there may not even be a need to do the more "efficient" query you're attempting.
Second, the following code does not quite do what your example code does - it creates a list of anonymous types called miniFoos that contain a subset of the fields in Foo. But the net effect is similar to what you seem to be attempting. It also uses what I believe is called LINQ method syntax.
I tested this in on an entity in my own application, which also has an IList property, and it does work (though I should note that I'm currently using Eager loading). I just cut and pasted the working code and substituted your example's names.
using( var session = sessionFactory.OpenSession() )
{
session.BeginTransaction();
var foos =
session.CreateCriteria(typeof(Foo))
.List<Foo>();
var miniFoos =
foos.Select(f => new { f.email, f.bars })
.Where(f => f.email.Equals("foo#bar.com)
.ToList();
session.Close();
}

I tried answering this already, but am not happy with it.
The more I think about your question, the more it seems like you are trying to do your own version of Lazy Loading, which NHibernate handles pretty well on it's own, in my experience.
I know (from a comment) that you switched to Eager Loading because you were not keeping a session open. I think it would be more useful for you in the long run if you focused on that angle.
If I've misunderstood, perhaps you could edit the question to explain why you think you need to handle it yourself?

I could manage to project not the whole collection but its fields
...
Select(a => new Forum {
Id = a.Id,
Name = a.Name,
CategoryName = a.Categories.Select(b => b.Name).First()
})
CategoryName is an computed filed in the Forum Entity
which does not have any mapping with DB
In result I got a small projection of the huge Forum Entity and a possibility to sort on the field.
Hope that it will help somebody.

i think you can solve your problem by defining the result transformer and the result set names, if in some way you can access the Criteria that being generated you can avoid anonymous object.
the problem is that the property names and column names are not the same, and when using projection nhibernate/linq to nhibernate does not check what the column name and the property name. so if u'll define them by yourself in the result properties (i think its called result set) in the criteria before you execute the query it should work.
it's pretty simple, give it a try