Comparing a nullable column throws "Unable to cast the type..." exception

Question

My entity NewsItem has a nullable foreign key property: LibraryID of type int?.

My issue is when I query the property and compare it with any value except null, I get exceptions.

Initially my code was:

int? lid = ...
var results = context.NewsItems
    .Where(n => n.LibraryID == lid);

but it gives me no results at all, no matter what lid is.

So, I tried:

var results = context.NewsItems
    .Where(n => n.LibraryID.Equals(lid));

gives exception:

Unable to create a constant value of type 'System.Object'. Only primitive types or enumeration types are supported in this context.

and then I tried:

var results = context.NewsItems
    .Where(n => lid.Equals(n.LibraryID));

and got:

Unable to cast the type 'System.Nullable`1' to type 'System.Object'. LINQ to Entities only supports casting EDM primitive or enumeration types.

and this:

var results = context.NewsItems
    .Where(n => object.Equals(lid, n.LibraryID));

gives same exception as the last one.

Now I was desperate, so I tried to complicate stuff (like other forums suggested, for example here):

var results = context.NewsItems
    .Where(n => (lid == null ? n.LibraryID == null : n.LibraryID == lid));

but still getting same exception.

So... any SIMPLE workarounds?

Answer 1

嗯，第一个片段应该可以工作。我已经多次使用过这样的可空值。我要做的第一件事是进行健全性检查，以确保LibraryID是真实的int?、不是long?或相似的。

除此之外，你还可以尝试这个：

var results = context.NewsItems
    .Where(n => (lid.HasValue ? n.LibraryID == lid.Value : !n.LibraryID.HasValue));

或者避免?:在查询中：

var results = lid.HasValue 
    ? context.NewsItems.Where(n => n.LibraryID == lid.Value)
    : context.NewsItems.Where(n => !n.LibraryID.HasValue);