Question

我看到了联系topic但是......

我试图实现规范模式。如果我使用System.Linq.Expressions API明确创建Or或And Expression，我将收到错误

从范围引用的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

已编辑

我尝试使用Specification pattern with Linq to Nhibernate，因此在我的工作代码中看起来像是：

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());

所以我想使用像这样的代码x =＆gt; x.Id＆gt; 4.

被修改

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，因此仅使用secondCondition.Parameters不会给firstCondition.Body一个参数。

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

Answer 2

如果您感兴趣，这是您必须使用的表达式树：

var param = Expression.Parameter(typeof(Employee), "x");
var firstCondition = Expression.Lambda<Func<Employee, bool>>(
    Expression.Equal(
        Expression.Property(param, "Id"),
        Expression.Constant(2)
    ),
    param
);
var secondCondition = Expression.Lambda<Func<Employee, bool>>(
    Expression.GreaterThan(
        Expression.Property(param, "Id"),
        Expression.Constant(4)
    ),
    param
);

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

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

2 个答案: