.NET[C#]LINQ中如何按多个属性排序？

.NET[C#]LINQ中如何按多个属性排序？

问题描述

比如有如下实体：

public class Movies {
    public int Id{get;set;}
   public int CategoryID {get;set;}
   public string Name {get;set;}
}

需要按分类ID CategoryID 和名称 Name 排序，类似：

var movies = _db.Movies.OrderBy( m => { m.CategoryID, m.Name })

如何实现呢？

方案一

使用lambda：

var movies = _db.Movies.OrderBy(c => c.CategoryID).ThenBy(n => n.Name)

方案二

使用非lambda:

var movies = from row in _db.Movies 
             orderby row.CategoryID, row.Name
             select row;

如果需要指定排序方向，则：

var movies = from row in _db.Movies 
             orderby row.CategoryID descending, row.Name
             select row;

方案三

var movies = _db.Movies.OrderBy( m => new { m.CategoryID, m.Name })

方案四

编写一个静态扩展方法，如下：

public static class IQueryableExtension
{
    public static bool IsOrdered<T>(this IQueryable<T> queryable) {
        if(queryable == null) {
            throw new ArgumentNullException("queryable");
        }

        return queryable.Expression.Type == typeof(IOrderedQueryable<T>);
    }

    public static IQueryable<T> SmartOrderBy<T, TKey>(this IQueryable<T> queryable, Expression<Func<T, TKey>> keySelector) {
        if(queryable.IsOrdered()) {
            var orderedQuery = queryable as IOrderedQueryable<T>;
            return orderedQuery.ThenBy(keySelector);
        } else {
            return queryable.OrderBy(keySelector);
        }
    }

    public static IQueryable<T> SmartOrderByDescending<T, TKey>(this IQueryable<T> queryable, Expression<Func<T, TKey>> keySelector) {
        if(queryable.IsOrdered()) {
            var orderedQuery = queryable as IOrderedQueryable<T>;
            return orderedQuery.ThenByDescending(keySelector);
        } else {
            return queryable.OrderByDescending(keySelector);
        }
    }
}

调用方法：

queryable.SmartOrderBy(i => i.Property1).SmartOrderByDescending(i => i.Property2);