LINQ 表达式的运行时创建

假设我有这样的表达：

int setsize = 20;
Expression<Func<Foo, bool>> predicate = x => x.Seed % setsize == 1
                                          || x.Seed % setsize == 4;

这基本上将一组元素“分区”为 20 个分区，并从每个组中检索第一个和第四个元素。

该表达式被传递到MongoDB http://www.mongodb.org/这是哪个driver https://www.nuget.org/packages/mongocsharpdriver/完全能够转换为 MongoDB“查询”。但是，谓词也可以用在对象列表（LINQ2Objects）等上。我希望这个表达式可以重用（DRY http://en.wikipedia.org/wiki/Don%27t_repeat_yourself）。但是，我希望能够通过IEnumerable<int> http://msdn.microsoft.com/en-us/library/9eekhta0.aspx指定要检索的项目（因此 1 和 4 不会“硬编码”到其中）：

public Expression<Func<Foo, bool>> GetPredicate(IEnumerable<int> items) {
    //Build expression here and return it
}

With LINQPad http://www.linqpad.net/使用此代码：

int setsize = 20;
Expression<Func<Foo, bool>> predicate = x => x.Seed % setsize == 1 || x.Seed % setsize == 4;
predicate.Dump();

} 

class Foo
{
    public int Seed { get; set; }

我可以检查这个表达式：

现在，我希望能够构建该表达式的精确再现，但要传递可变数量的整数（因此我可以传递而不是 1 和 4，例如，[1, 5, 9, 11] or [8] or [1, 2, 3, 4, 5, 6, ..., 16]).

我尝试过使用二元表达式 http://msdn.microsoft.com/en-us/library/system.linq.expressions.binaryexpression.aspx等等，但无法正确构造此消息。主要问题是我的大部分attempt http://pastebin.com/qDwXGGit将谓词传递给 MongoDB 时，s 将失败。“硬编码”版本工作正常但不知怎的，我所有传递动态表达式的尝试都未能被 C# 驱动程序转换为 MongoDB 查询：

{
    "$or" : [{
        "Seed" : { "$mod" : [20, 1] }
    }, {
        "Seed" : { "$mod" : [20, 4] }
    }]
}

基本上，我想在运行时动态构建表达式，使其精确复制编译器为“硬编码”版本生成的内容。

任何帮助将不胜感激。

EDIT

按照评论中的要求 https://stackoverflow.com/questions/16583565/runtime-creation-of-linq-expression#comment23832188_16583565 (and 发布到 帕斯特宾 http://pastebin.com/qDwXGGit），下面是我的尝试之一。我将其发布在问题中以供将来参考，pastebin 是否应该将其删除或停止他们的服务或...

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

class Program
{
    static void Main(string[] args)
    {
        MongoRepository<Foo> repo = new MongoRepository<Foo>();
        var reporesult = repo.All().Where(IsInSet(new[] { 1, 4 }, 20)).ToArray();
    }

    private static Expression<Func<Foo, bool>> IsInSet(IEnumerable<int> seeds, int setsize)
    {
        if (seeds == null)
            throw new ArgumentNullException("s");

        if (!seeds.Any())
            throw new ArgumentException("No sets specified");

        return seeds.Select<int, Expression<Func<Foo, bool>>>(seed => x => x.Seed % setsize == seed).JoinByOr();
    }
}

public class Foo : Entity
{
    public int Seed { get; set; }
}

public static class Extensions
{
    public static Expression<Func<T, bool>> JoinByOr<T>(this IEnumerable<Expression<Func<T, bool>>> filters)
    {
        var firstFilter = filters.First();
        var body = firstFilter.Body;
        var param = firstFilter.Parameters.ToArray();
        foreach (var nextFilter in filters.Skip(1))
        {
            var nextBody = Expression.Invoke(nextFilter, param);
            body = Expression.Or(body, nextBody);
        }
        return Expression.Lambda<Func<T, bool>>(body, param);
    }
}

这导致：Unsupported where clause: <InvocationExpression>.

尝试这个：

public Expression<Func<Foo, bool>> GetExpression<T>(
    int setSize, int[] elements,
    Expression<Func<Foo, T>> property)
{
    var seedProperty = GetPropertyInfo(property);
    var parameter = Expression.Parameter(typeof(Foo));
    Expression body = null;

    foreach(var element in elements)
    {
        var condition = GetCondition(parameter, seedProperty, setSize, element);
        if(body == null)
            body = condition;
        else
            body = Expression.OrElse(body, condition);
    }

    if(body == null)
        body = Expression.Constant(false);        

    return Expression.Lambda<Func<Foo, bool>>(body, parameter);    
}

public Expression GetCondition(
    ParameterExpression parameter, PropertyInfo seedProperty,
    int setSize, int element)
{
    return Expression.Equal(
        Expression.Modulo(Expression.Property(parameter, seedProperty),
                          Expression.Constant(setSize)),
        Expression.Constant(element));
}

public static PropertyInfo GetPropertyInfo(LambdaExpression propertyExpression)
{
    if (propertyExpression == null)
        throw new ArgumentNullException("propertyExpression");

    var body = propertyExpression.Body as MemberExpression;
    if (body == null)
    {
        throw new ArgumentException(
            string.Format(
                "'propertyExpression' should be a member expression, "
                + "but it is a {0}", propertyExpression.Body.GetType()));
    }

    var propertyInfo = body.Member as PropertyInfo;
    if (propertyInfo == null)
    {
        throw new ArgumentException(
            string.Format(
                "The member used in the expression should be a property, "
                + "but it is a {0}", body.Member.GetType()));
    }

    return propertyInfo;
}

你可以这样称呼它：

GetExpression(setSize, elements, x => x.Seed);

如果你希望它是通用的Foo另外，你需要像这样改变它：

public static Expression<Func<TEntity, bool>> GetExpression<TEntity, TProperty>(
    int setSize, int[] elements,
    Expression<Func<TEntity, TProperty>> property)
{
    var propertyInfo = GetPropertyInfo(property);
    var parameter = Expression.Parameter(typeof(TEntity));
    Expression body = null;

    foreach(var element in elements)
    {
        var condition = GetCondition(parameter, propertyInfo , setSize, element);
        if(body == null)
            body = condition;
        else
            body = Expression.OrElse(body, condition);
    }

    if(body == null)
        body = Expression.Constant(false);

    return Expression.Lambda<Func<TEntity, bool>>(body, parameter);    
}

现在，调用将如下所示：

GetExpression(setSize, elements, (Foo x) => x.Seed);

在这种情况下，指定类型很重要x明确地，否则类型推断将不起作用，您必须指定两者Foo以及属性的类型作为通用参数GetExpression.