Здравствуйте!
Возможно, что-то не то делаю, тогда поправьте.
Просто запускаю тест из статьи Linq-провайдер для BLToolkit (предварительно вставив свежий BLToolkit.4.dll и переделав проект в .NET Framework 4):

И спотыкается на BLToolkit.Data.Linq.Builder.SelectContext.BuildQuery<T>(...):

        public virtual void BuildQuery<T>(Query<T> query, ParameterExpression queryParameter)
        {
            var expr   = BuildExpression(null, 0);
            var mapper = Expression.Lambda<Func<QueryContext,IDataContext,IDataReader,Expression,object[],T>>(
                Builder.BuildBlock(expr), new []
                {
                    ExpressionBuilder.ContextParam,
                    ExpressionBuilder.DataContextParam,
                    ExpressionBuilder.DataReaderParam,
                    ExpressionBuilder.ExpressionParam,
                    ExpressionBuilder.ParametersParam,
                });

            query.SetQuery(mapper.Compile());
        }

System.ArgumentException was unhandled

  Message=Expression of type 'Linq.Demo.DataModel.Northwind+Product' cannot be used for return type 'Linq.Demo.DataModel.Northwind+DiscontinuedProduct'
  Source=System.Core
  StackTrace:
       at System.Linq.Expressions.Expression.ValidateLambdaArgs(Type delegateType, Expression& body, ReadOnlyCollection`1 parameters)
       at System.Linq.Expressions.Expression.Lambda[TDelegate](Expression body, String name, Boolean tailCall, IEnumerable`1 parameters)
       at System.Linq.Expressions.Expression.Lambda[TDelegate](Expression body, Boolean tailCall, IEnumerable`1 parameters)
       at BLToolkit.Data.Linq.Builder.SelectContext.BuildQuery[T](Query`1 query, ParameterExpression queryParameter) in P:\C#\Libs\BLToolkit\github.com\current\Source\Data\Linq\Builder\SelectContext.cs:line 64
       at BLToolkit.Data.Linq.Builder.ExpressionBuilder.Build[T]() in P:\C#\Libs\BLToolkit\github.com\current\Source\Data\Linq\Builder\ExpressionBuilder.cs:line 165
       at BLToolkit.Data.Linq.Query`1.GetQuery(IDataContextInfo dataContextInfo, Expression expr) in P:\C#\Libs\BLToolkit\github.com\current\Source\Data\Linq\Query.cs:line 134
       at BLToolkit.Data.Linq.Table`1.GetQuery(Expression expression, Boolean cache) in P:\C#\Libs\BLToolkit\github.com\current\Source\Data\Linq\TableT.cs:line 106
       at BLToolkit.Data.Linq.Table`1.Execute(IDataContextInfo dataContextInfo, Expression expression) in P:\C#\Libs\BLToolkit\github.com\current\Source\Data\Linq\TableT.cs:line 98
       at BLToolkit.Data.Linq.Table`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator() in P:\C#\Libs\BLToolkit\github.com\current\Source\Data\Linq\TableT.cs:line 193
       at Linq.Demo.Program.InheritanceTest1() in P:\C#\Libs\BLToolkit\temp_test\LinqWithBLToolkit\Program.cs:line 284
       at Linq.Demo.Program.Main() in P:\C#\Libs\BLToolkit\temp_test\LinqWithBLToolkit\Program.cs:line 31
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()