>>From the above link:
>>Although this is useful, it does not allow tables to be created from a sequence of scalar types, from queries that project anonymous types, or from queries that perform table joins
>
>Right.
>
>I tried implementing the MS documented solution of creating custom CopyToTable method, but this seems to lead to other problems. First warning message I posted. I found I need to set ComVisible(false) to fix that message.
>
>But now I am getting the other warning
>
>Warning 4 "C:\Dev\Siriusware.Middleware\Siriusware.Middleware\bin\Debug\Siriusware.Middleware.dll" does not contain any types that can be registered for COM Interop. Siriusware.Middleware
>
>So, I am not sure if I need to pursue that method or try to figure something else. My end result is to produce a string and the only reason I need a datatable is because the method that generates that string accepts datatable.

Since your original query uses a join (and CopyToDataTable() doesn't support it) there doesn't seem much point in pursuing that approach :-}

Seems to me your options are:
(a) Convert the query results into the required DataTable using code or
(b) Writing a version of the method that accepts the IEnumerable.

If the code in the method is complex then maybe write an overload that accepts the IEnumerable, converts it to a DataTable and passes that to the existing method?