Expression.Lambda:变量'''类型''引用范围'',但它没有定义

Question

我看到了连接主题但是......

我试图实现规范模式.如果我使用System.Linq.ExpressionsAPI 显式创建Or或And Expression ,我将收到错误

从作用域引用的InvalidOperationExpression变量'x'.

例如,这是我的代码

public class Employee
{
    public int Id { get; set; }
}

Expression<Func<Employee, bool>> firstCondition = x => x.Id.Equals(2);
Expression<Func<Employee, bool>> secondCondition = x => x.Id > 4;


Expression predicateBody = Expression.OrElse(firstCondition.Body, secondCondition.Body);
Expression<Func<Employee, bool>> expr = 
    Expression.Lambda<Func<Employee, bool>>(predicateBody, secondCondition.Parameters);
Console.WriteLine(session.Where(expr).Count()); - //I got error here

EDITED

我尝试使用Linq到Nhibernate的规范模式,所以在我的工作代码中它看起来像:

ISpecification<Employee> specification = new AnonymousSpecification<Employee>(x => x.Id.Equals(2)).Or(new AnonymousSpecification<Employee>(x => x.Id > 4));
var results = session.Where(specification.is_satisfied_by());

所以我想使用像x => x.Id> 4这样的代码.

编辑

所以我的解决方案是

 InvocationExpression invokedExpr = Expression.Invoke(secondCondition, firstCondition.Parameters);
var expr = Expression.Lambda<Func<Employee, bool>>(Expression.OrElse(firstCondition.Body, invokedExpr), firstCondition.Parameters);
Console.WriteLine(session.Where(expr).Count());

谢谢@Jon Skeet

Answer 1

每个主体都有一组单独的参数,因此使用just secondCondition.Parameters不会给出firstCondition.Body参数.

幸运的是,您根本不需要自己编写所有这些内容.只需使用PredicateBuilderJoe Albahari - 这一切都是为你完成的.