EF OrderBy dinamically

问题描述

我试图通过EF使用orderby，通过字符串传递参数：

I am trying to use a orderby via EF, passing the parameter via string:

public static List< Book> ListBy（字符串字段）

  &NBSP; &NBSP; &NBSP; {

  &NBSP; &NBSP; &NBSP; &NBSP; &NBSP;列表与LT;图书> books = context.Books.OrderByDescending（m => m.GetType（）。GetProperty（field）.GetValue（m，null）。ToList（）;

       返还书籍;

    }

public static List<Book> ListBy(string field)
        {
            List<Book> books = context.Books.OrderByDescending(m => m.GetType().GetProperty(field).GetValue(m, null).ToList();
            return books;
        }

然而，它不起作用。如果它只是一个字段，我会执行：

However, it's not working. If it was just a field, I would do:

       List< Book> books = context.Books .OrderByDescending（m => m.BookId）。ToList（）;

            List<Book> books = context.Books.OrderByDescending(m => m.BookId).ToList();

我该如何改编呢？

Thx

How can I adapt that ?
Thx

推荐答案

> ;>我如何适应？

因为数据库不支持c＃方法，你可以将记录加载到内存中并使用c＃方法。像这样：

Because database does not support c# method, which you could load the records to memory and use c# method. like this:

public static List<Book> ListBy(string field)
         {
             List<Book> books = context.Books.ToList().OrderByDescending(m => m.GetType().GetProperty(field).GetValue(m, null).ToList();
             return books;
         }

或者您也可以使用以下扩展来实现它。

Or you could also use the following extension to achieve it.

public static partial class QueryableExtensions
{
    public static IOrderedQueryable<T> OrderByMember<T>(this IQueryable<T> source, string memberPath)
    {
        return source.OrderByMemberUsing(memberPath, "OrderBy");
    }
    public static IOrderedQueryable<T> OrderByMemberDescending<T>(this IQueryable<T> source, string memberPath)
    {
        return source.OrderByMemberUsing(memberPath, "OrderByDescending");
    }
    public static IOrderedQueryable<T> ThenByMember<T>(this IOrderedQueryable<T> source, string memberPath)
    {
        return source.OrderByMemberUsing(memberPath, "ThenBy");
    }
    public static IOrderedQueryable<T> ThenByMemberDescending<T>(this IOrderedQueryable<T> source, string memberPath)
    {
        return source.OrderByMemberUsing(memberPath, "ThenByDescending");
    }
    private static IOrderedQueryable<T> OrderByMemberUsing<T>(this IQueryable<T> source, string memberPath, string method)
    {
        var parameter = Expression.Parameter(typeof(T), "item");
        var member = memberPath.Split('.')
            .Aggregate((Expression)parameter, Expression.PropertyOrField);
        var keySelector = Expression.Lambda(member, parameter);
        var methodCall = Expression.Call(
            typeof(Queryable), method, new[] { parameter.Type, member.Type },
            source.Expression, Expression.Quote(keySelector));
        return (IOrderedQueryable<T>)source.Provider.CreateQuery(methodCall);
    }
}

有关详细信息，请参阅：

For more information, please refer to:

https://stackoverflow.com/questions/39908403 /怎么用-a-string-to-create-a-ef-order-by-expression / 39916384＃39916384

祝你好运，

Cole Wu

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