dbcontext和objectcontext中的NULL处理 [英] NULL handling in dbcontext and objectcontext

问题描述

我有这个简单的LINQ查询

I have this simple LINQ query

from e in Employees 
where e.DesignationID !=558
select e

这里指定ID 是一个可空字段：

在 objectcontext 中将查询转换为：

SELECT 
[Extent1].[EmployeeID] AS [EmployeeID], 
[Extent1].[EmployeeCode] AS [EmployeeCode], 
[Extent1].[EmployeeName] AS [EmployeeName],
[Extent1].[DesignationID] AS [DesignationID] 
FROM [dbo].[setupEmployees] AS [Extent1]
WHERE 558 <> [Extent1].[DesignationID]

虽然在 dbcontext 它被翻译成：

While the same query in dbcontext it is translated to:

 SELECT 
    [Extent1].[EmployeeID] AS [EmployeeID], 
    [Extent1].[EmployeeCode] AS [EmployeeCode], 
    [Extent1].[EmployeeName] AS [EmployeeName],
    [Extent1].[DesignationID] AS [DesignationID] 
    FROM [dbo].[setupEmployees] AS [Extent1]
 WHERE  NOT ((558 = [Extent1].[DesignationID]) AND ([Extent1].[DesignationID] IS NOT NULL))

为什么 objectcontext 处理NULL不同于 dbcontext ？

Why does objectcontext handle NULL differently than dbcontext?

推荐答案

这个行为是可配置的，所以很有可能是一个不同的默认（我不知道为什么默认是不同的）。

This behavior is configurable, so most likely it's a matter of a different default (I don't know why the default is different).

该控件由 DbContextConfiguration.UseDatabaseNullSemantics 属性： p>

The control is provided by the DbContextConfiguration.UseDatabaseNullSemantics property:

获取或设置一个值，指示在比较两个可能为空的操作数时是否显示数据库空语义。 默认值为false 。 （（operand1 = operand2）AND（NOT（operand1 IS NULL OR operand2 IS NULL）））OR（（operand1 IS is（operand1 = operand2））将被转换为（operand1 = operand2），如果UseDatabaseNullSemantics为true NULL）AND（operand2 IS NULL）））如果UseDatabaseNullSemantics为false。

Gets or sets a value indicating whether database null semantics are exhibited when comparing two operands, both of which are potentially nullable. The default value is false. For example (operand1 == operand2) will be translated as: (operand1 = operand2) if UseDatabaseNullSemantics is true, respectively (((operand1 = operand2) AND (NOT (operand1 IS NULL OR operand2 IS NULL))) OR ((operand1 IS NULL) AND (operand2 IS NULL))) if UseDatabaseNullSemantics is false.

要获得与第一个示例相同的翻译，您应该将其设置为 true （例如在您的数据库上下文构造函数中）：

To get the same translation as in your first example, you should set it to true (for instance inside your db context constructor):

public YourDbContext()
{
    // ...
    this.Configuration.UseDatabaseNullSemantics = true;
}

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