如何创建用于运行时表达式树排序？ [英] How do I create an expression tree for run time sorting?

问题描述

使用实体框架4，我想根据成员名称的集合来实现动态排序。基本上，用户可以选择字段进行排序和排序的顺序。我看着表达式树的例子并不能拼凑了一起。这里还有一些细节：

Using Entity Framework 4, I'm trying to implement dynamic sorting based on a collection of member names. Basically, the user can select fields to sort and the order of the sorting. I've looked at expression tree examples and can't piece this together. Here are some details:

列名的集合：

public List<string> sortColumns;
sortColumns = new List<string>();

/// Example subset of video fields.  The collection will vary.
sortColumns.Add("Width");
sortColumns.Add("Height");
sortColumns.Add("Duration");
sortColumns.Add("Title");

视频类的定义如下：

The video class is defined as follows:

public class Video
{
    public string Title { get; set; }
    public int Width { get; set; }
    public int Height { get; set; }
    public float Duration { get; set; }
    public string Filename { get; set; }
    public DateTime DateCreated { get; set; }
    .
    .
    .
}
public List<Video> Videos;

我想要做的就是通过sortColumns集合枚举建立在运行时的表达式树。此外，用户可以指定升序或降序排序和表达式树要么处理。

What I would like to do is enumerate through the sortColumns collection to build an expression tree at run time. Also, the user could specify either ascending or descending sorting and the expression tree should handle either.

我试过了动态的LINQ库VS 2008，但它不会出现在2010年VS上班（我可能做错了什么。）

I tried the Dynamic LINQ library for VS 2008, but it doesn't appear to work in VS 2010. (I could be doing something wrong.)

底线是，我需要一个表达式树动态排序基于用户输入的视频采集。任何帮助，将不胜感激。

The bottom line is I need an expression tree to dynamically sort the Videos collection based on user input. Any help would be appreciated.

推荐答案

首先，你需要在排序依据扩展方法@Slace写的此处。所有信贷 Slace 一个真棒一段代码是迄今为止解决方案的最困难的部分！我做了少许修改它与您的具体情况工作：

First you need the OrderBy extension method that @Slace wrote here. All credit to Slace for an awesome piece of code and by far the most difficult part of the solution! I made a slight modification for it to work with your specific situation:

public static class QueryableExtensions
{
    public static IQueryable<T> OrderBy<T>(this IQueryable<T> source, string sortProperty, ListSortDirection sortOrder)
    {
        var type = typeof(T);
        var property = type.GetProperty(sortProperty);
        var parameter = Expression.Parameter(type, "p");
        var propertyAccess = Expression.MakeMemberAccess(parameter, property);
        var orderByExp = Expression.Lambda(propertyAccess, parameter);
        var typeArguments = new Type[] { type, property.PropertyType };
        var methodName = sortOrder == ListSortDirection.Ascending ? "OrderBy" : "OrderByDescending";
        var resultExp = Expression.Call(typeof(Queryable), methodName, typeArguments, source.Expression, Expression.Quote(orderByExp));

        return source.Provider.CreateQuery<T>(resultExp);
    }
}

创建对列表进行排序的方法。字符串>

Create a method to sort the list. A couple of things to note in the method below:

1. 的列表与LT：一对夫妇的事情，在下面的方法注意>转换为的IQueryable<字符串方式> ，因为可枚举运营商不采取表达式树


2. 通过以相反的顺序排序列清单的方法，迭代（假设你想给的第一个项目在列表中的最高排序优先级）

1. The List<string> is converted to an IQueryable<string> since Enumerable operators don't take expression trees.
2. The method iterates through the list of sort columns in reverse order (assuming you want to give the first item in the list the highest sort priority)

private void PrintVideoList(IEnumerable<string> sortColumns, ListSortDirection sortOrder)
{
    var videos = this.GetVideos();
    var sortedVideos = videos.AsQueryable();

    foreach (var sortColumn in sortColumns.Reverse())
    {
        sortedVideos = sortedVideos.OrderBy(sortColumn, sortOrder);
    }

    // Test the results
    foreach (var video in sortedVideos)
    {
        Console.WriteLine(video.Title);
    }
}

您应该然后就可以使用的方法是这样

You should then be able to use the method like this:

// These values are entered by the user
var sortColumns = new List<string> { "Width", "Title", "Height" };
var sortOrder = ListSortDirection.Ascending;

// Print the video list base on the user selection
this.PrintVideoList(sortColumns, sortOrder);

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