链接 Lambda 表达式

Question

我正在尝试创建代码来过滤可重用和可读的实体框架查询。我下面的代码有效，除非我想加入两个过滤器，如注释掉的部分中所尝试的那样。有没有人能够就如何完成这项工作提供指导和指导？

下面是vs2012在该行时显示的编译时错误信息

//IList<Department> departments3 = context.Set<Department>().Where(isHumanResourcesDepartment || isAccountsDepartment).ToList(); is uncommented

运算符“||”不能应用于类型的操作数 'System.Func ' 和 'System.Func '

    private static void TestCodeReadability2()
    {

        EmployeeDepartmentsConnection context = new EmployeeDepartmentsConnection();

        IList<Department> departments = context.Set<Department>().Where(isAccountsDepartment).ToList();
        IList<Department> departments2 = context.Set<Department>().Where(isHumanResourcesDepartment).ToList();

        //IList<Department> departments3 = context.Set<Department>().Where(isHumanResourcesDepartment || isAccountsDepartment).ToList();

    }


    private static Func<Department, bool> isAccountsDepartment = d=>d.Name.ToLower().Contains("accounts");
    private static Func<Department, bool> isHumanResourcesDepartment = d=>d.Name.ToLower().Contains("human resources");

Answer 1

只是补充一下，通过使用 Func，您会强制 EF 从数据库中提取所有数据，然后在本地进行过滤，而不是在数据库上进行过滤。如果要过滤数据库，请将 Func 包装在 Expression 中，方法是将静态方法声明更改为

Expression<Func<Department, bool>>

Answer 2

为了让 EF 能够将这些查询转换为 SQL 并在数据库方面执行工作，您需要传入 Expression 对象，而不是委托。创建表达式而不是委托非常简单：

private static Expression<Func<Department, bool>> isAccountsDepartment = 
    d => d.Name.ToLower().Contains("accounts");
private static Expression<Func<Department, bool>> isHumanResourcesDepartment = 
    d => d.Name.ToLower().Contains("human resources");

接下来，要获取其中任一为真的项目集，您可以使用Union 获取两个查询中任一查询中的所有项目。为了确保这项工作是在数据库而不是在内存中完成的，我们需要从您的内部查询中删除 ToList 调用，以防止它们被加载到内存中：

private static void TestCodeReadability2()
{
    EmployeeDepartmentsConnection context = new EmployeeDepartmentsConnection();

    var accountsDepartments = context.Set<Department>().Where(isAccountsDepartment);
    var hrDepartments= context.Set<Department>().Where(isHumanResourcesDepartment);

    var combined = accountsDepartments.Union(HRDepartments);
}