Question

Specified cast is not valid.

OrderItemState is enum.

IEnumerable<OrderItemState> states = ...;
IEnumerable<byte> stateIds = Enumerable.Cast<byte>(states);

List<OrderEntry> entries =
  (from m in dc.OrderItemMotions
   where stateIds.Contains(m.OrderItemStateId)
   select ...).ToList();

Why?

Part of StackTrace:

at System.Linq.Enumerable.d__b11.MoveNext()
at System.Linq.Enumerable.<OfTypeIterator>d__aa1.MoveNext()
at System.Linq.Enumerable.WhereSelectEnumerableIterator2.MoveNext()
at System.Collections.Generic.List1..ctor(IEnumerable1 collection)
at System.Linq.Enumerable.ToList[TSource](IEnumerable1 source)
at System.Data.Linq.SqlClient.QueryConverter.VisitContains(Expression sequence, Expression value)
at System.Data.Linq.SqlClient.QueryConverter.VisitSequenceOperatorCall(MethodCallExpression mc)
at System.Data.Linq.SqlClient.QueryConverter.VisitInner(Expression node)
at System.Data.Linq.SqlClient.QueryConverter.Visit(Expression node)
at System.Data.Linq.SqlClient.QueryConverter.VisitWhere(Expression sequence, LambdaExpression predicate)
at System.Data.Linq.SqlClient.QueryConverter.VisitSequenceOperatorCall(MethodCallExpression mc)
at System.Data.Linq.SqlClient.QueryConverter.VisitInner(Expression node)
at System.Data.Linq.SqlClient.QueryConverter.VisitWhere(Expression sequence, LambdaExpression predicate)
at System.Data.Linq.SqlClient.QueryConverter.VisitSequenceOperatorCall(MethodCallExpression mc)
at System.Data.Linq.SqlClient.QueryConverter.VisitInner(Expression node)
at System.Data.Linq.SqlClient.QueryConverter.VisitGroupBy(Expression sequence, LambdaExpression keyLambda, LambdaExpression elemLambda, LambdaExpression resultSelector)
at System.Data.Linq.SqlClient.QueryConverter.VisitSequenceOperatorCall(MethodCallExpression mc)
at System.Data.Linq.SqlClient.QueryConverter.VisitInner(Expression node)
at System.Data.Linq.SqlClient.QueryConverter.VisitJoin(Expression outerSequence, Expression innerSequence, LambdaExpression outerKeySelector, LambdaExpression innerKeySelector, LambdaExpression resultSelector)
at System.Data.Linq.SqlClient.QueryConverter.VisitSequenceOperatorCall(MethodCallExpression mc)
at System.Data.Linq.SqlClient.QueryConverter.VisitInner(Expression node)
at System.Data.Linq.SqlClient.QueryConverter.ConvertOuter(Expression node)
at System.Data.Linq.SqlClient.SqlProvider.BuildQuery(Expression query, SqlNodeAnnotations annotations)
at System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Execute(Expression query)
at System.Data.Linq.DataQuery1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
at System.Collections.Generic.List1..ctor(IEnumerable1 collection)
at System.Linq.Enumerable.ToList[TSource](IEnumerable1 source)

Answer 1

You can’t cast an enum to a byte without data loss so the cast will fail (enum is an int under the hood).

But you can define an enum to use a byte under-the-hood if you only want values in the range 0-255 using:

public enum Values : byte { val1, val2};

See here: http://msdn.microsoft.com/en-us/library/sbbt4032.aspx