c# - 如何检查linq to sql中的空值

Question

我正在尝试使用以下 linq to sql 查询来获取结果。但是如果 parentCategoryId 作为 null 传递，它就不起作用

 public static IEnumerable<Category> GetChildren(this Table<Category> source, int? parentCategoryId)
    {
        var categories = from c in source where c.ParenCategoryId == parentCategoryId select c;
        return categories;
    }   

但如果直接使用 null 代替 parentCategoryId，则以下工作

 public static IEnumerable<Category> GetChildren(this Table<Category> source, int? parentCategoryId)
    {
        var categories = from c in source where c.ParenCategoryId == null select c;
        return categories;
    }

Answer 1

您可以使用object.Equals，它也会匹配null值。

public static IEnumerable<Category> GetChildren(this Table<Category> source, int? parentCategoryId)
{
    var categories = from c in source 
                     where object.Equals(c.ParenCategoryId, parentCategoryId) 
                     select c;
    return categories;
} 

Answer 2

You can try the following

public static IEnumerable<Category> GetChildren(this Table<Category> source, int? parentCategoryId)
{
    var categories = from c in source 
                     where (parentCategoryId != null? c.ParenCategoryId == parentCategoryId : c.ParenCategoryId == null)
                     select c;
    return categories;
}