c# - EF Code First 比较空值会生成奇怪的查询

Question

我正在尝试列出一些类别(父子关系)并且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查询没有返回任何内容

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

类别模型

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

我将此作为答案发布，因为肯定其中之一会起作用:

您的 ParentId 属性需要是 public virtual - 首先尝试更改它。

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

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

我的应用程序可以正常运行上述内容。

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

最后一个是 bit of a hack from someone at MS ，但显然应该有效。