将对象的linq表达式应用于该对象是项目属性的项目列表(Wrapper)

Question

我需要能够将属性的表达式应用于该属性的包装器,下面是一个示例; 一个数字列表和一个列表包装数字,表达式是[数字是偶数]

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;

namespace ExpressionTest
{
    class Program
    {
        static void Main(string[] args)
        {
            /* Expression */
            Expression<Func<int, bool>> expression = item => item % 2 == 0;


            /* List */
            IList<int> items = new List<int>();
            for (int i = 0; i < 10; i++)
            {
                items.Add(i);
            }
            IEnumerable<int> evenNumbers = items.Where(expression.Compile());
            foreach (int number in evenNumbers)
            {
                Console.WriteLine(number.ToString());
            }


            /* Wrappers List */
            // How to apply expression to the porpery 'Number'?
            IList<Wrapper> wrappers = new List<Wrapper>();
            for (int i = 0; i < 10; i++)
            {
                wrappers.Add(new Wrapper { Number = i });
            }
            IEnumerable<int> evenWrappedNumbers = ????; 
            foreach (Wrapper wrappedNumber in evenWrappedNumbers)
            {
                Console.WriteLine(wrappedNumber.ToString());
            }
        }
    }

    public class Wrapper
    {
        public int Number;
    }
}

Answer 1

这应该做的伎俩:

Func<int, bool> predicate = expression.Compile();
IEnumerable<Wrapper> evenWrappedNumbers = wrappers.Where(w => predicate(w.Number));

或者,如果你想要一个 IEnumerable<int>

Func<int, bool> predicate = expression.Compile();
IEnumerable<int> evenWrappedNumbers = wrappers.Where(w => predicate(w.Number)).Select(w => w.Number);

编辑:自从了解NHibernate正在使用中,这是另一种可能的解决方案.请记住,我不使用NHibernate并且对表达式树的经验有限.但无论如何,这应该允许NHibernate将表达式树解析为SQL.

首先在静态类中定义一个扩展方法,如下所示:

    public static IQueryable<T> Where<T, TProperty>(this IQueryable<T> source,
        Expression<Func<T, TProperty>> propertySelector,
        Expression<Func<TProperty, bool>> predicate)
    {
        MemberExpression member = propertySelector.Body as MemberExpression;

        if (member == null)
            throw new ArgumentException("Must be a property selector", "propertySelector");

        string propertyName = member.Member.Name;

        // The input type
        ParameterExpression propertyParameter = Expression.Parameter(typeof(T));

        // The property on that type
        MemberExpression itemProperty = Expression.Property(propertyParameter, propertyName);

        // Invoke the specified predicate with the property from the input type
        InvocationExpression invokeExpression = Expression.Invoke(predicate, itemProperty);

        // The lambda expression for use with Linq
        Expression<Func<T, bool>> finalExpression = Expression.Lambda<Func<T, bool>>(invokeExpression, propertyParameter);

        return source.Where(finalExpression);
    }

然后可以像这样使用这种扩展方法:

    IEnumerable<Wrapper> evenWrappedNumbers = wrappers.Where(w => w.Number, expression);

正如我所说,我没有NHibernate的经验,但希望它能够解析这个表达式树并将其转换为SQL.