Question

我正在尝试创建可过滤可重用和可读的实体框架查询的代码。我在下面的代码工作，除非我想加入两个过滤器，如在注释掉的部分中尝试的那样。有人能够就如何使这项工作给出指导和指导吗？

以下是vs2012在行

时显示的编译时错误消息

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

运营商'||'不能应用于类型的操作数 'System.Func＆lt; EfGenericRepositoryPoc.DataModel.Department，bool＆gt;'和 'System.Func＆lt; EfGenericRepositoryPoc.DataModel.Department，bool＆gt;'

    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

只需添加，通过使用Func，您强制EF从数据库中提取所有数据，然后在本地过滤，而不是过滤数据库。如果要对数据库进行过滤，请通过将静态方法声明更改为

将Func包装在Expression中

Expression<Func<Department, bool>>

Answer 2

为了使EF能够将这些查询转换为SQL并在数据库端执行需要传递Expression个对象而不是委托的事物。创建表达式而不是委托是非常简单的：

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，要获取其中任一项为真的项集，您可以使用Union来获取两个查询中的任何一项。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);
}

链接Lambda表达式

2 个答案: