在Linq-To-Sql中避免“Nullable对象必须有值。”

Question

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.

Answer 1

你试过了吗：

where filterId == null || t.Filter == filterId

Answer 2

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.