実際のバグは、式の文脈で関数を呼び出そうとしています。式はデータベース操作を表す式の意味がありません。
私はあなたが実際に以下を試したと思います:
// Database operation, no soap client funny business here.
var list = (
from u in _db.Sessions
where u.DeletedOn == null
orderby x.CreatedOn descending
select new
{
SessionId = u.UploadSessionId,
FileName = u.FileName,
CreatedBy = u.User.UserName,
Partner = u.Company.Name,
CreatedOn = u.CreatedOn,
}).ToList();
// From now on you are not in a database anymore, it's pure linq-to-objects. You can call the service.
ServiceSoapClient service = new ServiceSoapClient();
var dataSource = (from u in list select new {
SessionId = u.SessionId,
FileName = u.FileName,
CreatedBy = u.CreatedBy,
Partner = u.Partner,
CreatedOn = u.CreatedOn,
Visible = service.IsSessionProcessing(u.SessionId)
}).ToList();