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[英]Expression.Lambda variable '' of type 'System.String' referenced from scope '', but it is not defined
[英]Expression.Lambda: Variable 'x' of type '' referenced from scope '', but it is not defined
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.Expressions
API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
每个这些机构都有一套独立的参数,所以只用secondCondition.Parameters
不给firstCondition.Body
的参数。
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。
如果您有兴趣,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());
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