dbcontext 和 objectcontext 中的 NULL 处理

我有这个简单的 LINQ 查询

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

Here DesignationID是一个可为空的字段：

In objectcontext查询被翻译为：

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]

虽然相同的查询dbcontext它被翻译为：

 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 的方式与dbcontext?

此行为是可配置的，因此很可能是不同默认值的问题（我不知道为什么默认值不同）。

该控制由提供DbContextConfiguration.UseDatabaseNullSemantics https://msdn.microsoft.com/query/dev14.query?appId=Dev14IDEF1&l=EN-US&k=k(System.Data.Entity.Infrastructure.DbContextConfiguration.UseDatabaseNullSemantics);财产：

获取或设置一个值，该值指示在比较两个操作数（这两个操作数都可能为空）时是否显示数据库空语义。默认值为 false。例如 (operand1 == operand2) 将被翻译为： (operand1 = operand2) 如果 UseDatabaseNullSemantics 为 true，则分别 (((operand1 = operand2) AND (NOT (operand1 IS NULL OR operand2 IS NULL))) OR ((operand1 IS NULL) AND (operand2 IS NULL))) 如果 UseDatabaseNullSemantics 为 false。

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

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