表达式的结果始终为真

Question

我在 linq 中进行了左连接，并且 UserId 的值为 NULLABLE（不在表中）但在连接中，Visual Studio 显示此警告。

我认为这是一个误报，因为当 userid 为 null IsSelected 为 false 时代码运行良好。

UserPrivilege 类

//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated from a template.
//
//     Manual changes to this file may cause unexpected behavior in your application.
//     Manual changes to this file will be overwritten if the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------
using System;
using System.Collections.Generic;

public partial class UserPrivileges
{
    public int Id { get; set; }
    public int UserId { get; set; }
    public int PrivilegeId { get; set; }

    public virtual Privileges Privileges { get; set; }
    public virtual UserProfile UserProfile { get; set; }
}

linq 查询：

using (var dataContext = new MyEntities())
{
List<UserPrivilegesModel> result = (from privileges in dataContext.Privileges
                                    join userPrivileges in dataContext.UserPrivileges on privileges.Id equals userPrivileges.PrivilegeId into tmpUserPrivileges
                                    from userPrivileges in tmpUserPrivileges.DefaultIfEmpty()
                                        //where userPrivileges.UserId == userId
                                    select new UserPrivilegesModel { IsSelected = (userPrivileges.UserId != null), Description = privileges.Description, PrivilegeId = privileges.Id }).ToList();
return result;
}

在 Linq 用户 ID == null 为 TRUE

SQL 脚本：

GO
/****** Object:  Table [dbo].[Privileges]    Script Date: 07/12/2016 10:35:21 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[Privileges](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [Name] [nvarchar](20) NOT NULL,
    [Description] [nvarchar](100) NOT NULL,
 CONSTRAINT [PK_Privileges] PRIMARY KEY CLUSTERED 
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

GO
/****** Object:  Table [dbo].[UserPrivileges]    Script Date: 07/12/2016 10:35:21 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[UserPrivileges](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [UserId] [int] NOT NULL,
    [PrivilegeId] [int] NOT NULL,
 CONSTRAINT [PK_UserPrivileges_1] PRIMARY KEY CLUSTERED 
(
    [UserId] ASC,
    [PrivilegeId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

GO
/****** Object:  Table [dbo].[UserProfile]    Script Date: 07/12/2016 10:35:21 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[UserProfile](
    [UserId] [int] IDENTITY(1,1) NOT NULL,
    [UserName] [nvarchar](56) NOT NULL,
    [Enabled] [smallint] NULL,
 CONSTRAINT [PK__UserProf__1788CC4CB96C92F3] PRIMARY KEY CLUSTERED 
(
    [UserId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
 CONSTRAINT [UQ__UserProf__C9F28456208EB813] UNIQUE NONCLUSTERED 
(
    [UserName] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

GO
ALTER TABLE [dbo].[UserPrivileges]  WITH CHECK ADD  CONSTRAINT [FK_UserPrivileges_Privileges] FOREIGN KEY([PrivilegeId])
REFERENCES [dbo].[Privileges] ([Id])
GO
ALTER TABLE [dbo].[UserPrivileges] CHECK CONSTRAINT [FK_UserPrivileges_Privileges]
GO
ALTER TABLE [dbo].[UserPrivileges]  WITH CHECK ADD  CONSTRAINT [FK_UserPrivileges_UserProfile] FOREIGN KEY([UserId])
REFERENCES [dbo].[UserProfile] ([UserId])
GO
ALTER TABLE [dbo].[UserPrivileges] CHECK CONSTRAINT [FK_UserPrivileges_UserProfile]
GO

SQL 关系

Answer 1

您是否应该检查 userPrivlages == null 而不是 userPrivlages.UserId == null？另外，第二个 from 和 join 是否应该使用相同的名称？

尝试纠正这些问题。

Answer 2

database 中的类型可以为空，但code 中的类型不能。 UserId 是 int 类型，它永远不能是 null。 （因此出现错误。）所以大概在某个地方的某个类上你有这样的东西：

public int UserId { get; set; }

要使类型可以为空，您需要这样：

public int? UserId { get; set; }

或者，如果您愿意，可以这样：

public Nullable<int> UserId { get; set; }

这可能会对此处未显示的有关如何使用该属性的其他代码产生影响，因为您正在更改该属性的类型。但这将允许该属性携带一个空值。

Answer 3

userPrivalegs.UserId 不可为空，因此 (userPrivalegs.UserId != null) 将始终为真。