I have a method query like this:
public IList<BusinessObject> GetBusinessObject(Guid? filterId)
{
using (var db = new L2SDataContext())
{
var result = from bo in db.BusinessObjects
where (filterId.HasValue)
? bo.Filter == filterId.value
: true
orderby bo.Name
select SqlModelConverters.ConvertBusinessObject(bo);
return result.ToList();
}
}
At runtime, this throws a System.InvalidOperationException: Nullable object must have a value.
Looking at the Debugger, the problem is my Where Clause: Linq To SQL tries to convert the entire thing to SQL, so even if filterId is NULL, it will still try to access filterId.value.
I thought/hoped the C# compiler/CLR would evaluate that where clause as a code block and only send one of the two branches to Linq To SQL, but that's not how it works.
My refactored version works, but is not very elegant:
public IList<BusinessObject> GetBusinessObject(Guid? filterId)
{
using (var db = new L2SDataContext())
{
var temp = from bo in db.BusinessObjects select bo;
if(filterId.HasValue) temp = temp.Where(t => t.Filter == filterId.Value);
var result = from t in temp
orderby t.Name
select SqlModelConverters.ConvertBusinessObject(bo);
return result.ToList();
}
}
I know that Lazy-Evaluation will make sure that only one query is really sent, but having that temp object in there isn't that great really.
Did you try:
where filterId == null || t.Filter == filterId
Your fix is exactly correct. You are effectively trying to build up a query dynamically, based on your function input. It's a good idea to omit the where clause instead of supplying WHERE TRUE
anyway. If I were writing this query, I would go with your fixed version myself.
It's not as pretty as using the language keywords, but it's still the right way to approach the query in my opinion.
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