I was speeding up my app by using compiled queries for queries which were getting hit over and over.

I tried to implement it like this:

Function Select(ByVal fk_id As Integer) As List(SomeEntity)
    Using db As New DataContext()
        db.ObjectTrackingEnabled = False
        Return CompiledSelect(db, fk_id)
    End Using
End Function

Shared CompiledSelect As Func(Of DataContext, Integer, List(Of SomeEntity)) = _
    CompiledQuery.Compile(Function(db As DataContext, fk_id As Integer) _
         (From u In db.SomeEntities _
          Where u.SomeLinkedEntity.ID = fk_id _
          Select u).ToList())

This did not work and I got this error message:

Type : System.ArgumentNullException, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
Message : Value cannot be null.

Parameter name: value

However, when I changed my compiled query to return IQueryable instead of List like so:

Function Select(ByVal fk_id As Integer) As List(SomeEntity)
    Using db As New DataContext()
        db.ObjectTrackingEnabled = False
        Return CompiledSelect(db, fk_id).ToList()
    End Using
End Function

Shared CompiledSelect As Func(Of DataContext, Integer, IQueryable(Of SomeEntity)) = _
    CompiledQuery.Compile(Function(db As DataContext, fk_id As Integer) _
         From u In db.SomeEntities _
         Where u.SomeLinkedEntity.ID = fk_id _
         Select u)

It worked fine. Can anyone shed any light as to why this is?

BTW, compiled queries rock! They sped up my app by a factor of 2.