Nullable int在LINQ（C＃）中没有按预期工作

Question

The following works as expected (LINQ to Entities):

var topics = (from t in ctx.Topics where t.SubjectId == subjectId && t.ParentId == null select new { t.Title, t.Id }).ToList();

However, the following returns nothing:

int? parent = null;
var topics = (from t in ctx.Topics where t.SubjectId == subjectId && t.ParentId == parent select new { t.Title, t.Id }).ToList();

Topic.ParentId is a nullable int. It's easy to work around, but this puzzles me. Can anyone shed any light?

Answer 1

You are definately not the first person to observe this... interesting... behavior.

http://connect.microsoft.com/data/feedback/details/607404/entity-framework-and-linq-to-sql-incorrectly-handling-nullable-variables

In short, it's difficult to handle different ways of expressing null.