"Only arguments that can be evaluated on the client are supported for the String.Contains method"

Answer 1

It seems you should be using f.ErrorMessage.Contains(err.ErrorMessage) - linq to sql should then convert this to WHERE ErrorFilter.ErrorMessage LIKE %err.ErrorMessage%. The problem with the way you have it is that the generated SQL would need a dynamic string to match in the where clause, and hence could only be filtered on the client.

Incidently, the var query = from filter in DataContext.ErrorFilters select filter; line is not required and you can just do:

var filters = DataContext.ErrorFilters.Where(f => f.ErrorMessage.Contains(err.ErrorMessage)).ToList();

EDIT:

Ok I see what you're trying to do now, but I'm not sure if this is possible in linq2sql. You could create a stored procedure and add that to your datacontext and do the mapping from the output to a sequence of ErrorFilter objects:

create procedure GetMatchingFilters @message varchar(500)
as
begin
    select *
    from ErrorFilter
    where @message LIKE '%'+ErrorMessage+'%'
end

Then in your datacontext you can do:

DataContext
    .GetMatchingFilters(err.ErrorMessage)
    .Select(result => new ErrorFilter {...})
    .ToList();