首先比较空值会产生奇怪的查询

Question

我试图列出一些类别(父子关系)，我有以下命令仅列出父类别

context.Categories.Where(c => c.ParentId == null)

但是EF生成的sql查询没有返回任何

EF 生成的sql查询

SELECT 
CAST(NULL AS int) AS [C1], 
CAST(NULL AS varchar(1)) AS [C2], 
CAST(NULL AS bit) AS [C3], 
CAST(NULL AS int) AS [C4]
FROM  ( SELECT 1 AS X ) AS [SingleRowTable1]
WHERE 1 = 0

分类模型

public class Category
{
  public int Id { get; set; }
  public string Name { get; set; }
  public bool IsActive { get; set; }
  public virtual IList<Category> SubCategories { get; set; }
  internal int? ParentId { get; set; }
  public virtual Category Parent { get; set; }
  public override bool Equals(object obj)
  {
    var categoryToCompare = obj as Category;
    if (categoryToCompare == null) return false;
    return categoryToCompare.Id == Id;
  }
  public override int GetHashCode()
  {
    return Id.GetHashCode();
  }
}

映射

public class CategoryConfiguration : EntityTypeConfiguration<Category>
{
  public CategoryConfiguration()
  {
    ToTable("tbl_category");
    HasKey(c => c.Id);
    Property(c => c.Id).HasColumnName("cd_category");
    Property(c => c.Name).HasColumnName("ds_category");
    Property(c => c.IsActive).HasColumnName("fl_active");
    Property(c => c.ParentId).HasColumnName("cd_base_category").IsOptional();
    HasMany(c => c.SubCategories).WithRequired(c => c.Parent).HasForeignKey(c => c.ParentId);
  }
}

Answer 1

我把这个作为一个答案，因为肯定其中一个会起作用:

你的ParentId属性需要是public virtual -试着先改变它。

然后，试试context.Categories.Where(c => !c.ParentId.HasValue)

然后，尝试context.Categories.Where(c => c.Parent == null)

我有一些应用程序，上面的工作很好。

然后，试试context.Categories.Where(c => object.Equals(c.Parent, null))

最后一个是微软某个人的hack，但显然应该可以工作。