创建一个排序函数的泛型列表 [英] Creating a sort function for a generic list

问题描述

我对由对象字段排序泛型列表的方法：

I have a method for sorting generic lists by the object fields:

public static IQueryable<T> SortTable<T>(IQueryable<T> q, string sortfield, bool ascending)
{
    var p = Expression.Parameter(typeof(T), "p");

    if (typeof(T).GetProperty(sortfield).PropertyType == typeof(int?))
    {
        var x = Expression.Lambda<Func<T, int?>>(Expression.Property(p, sortfield), p);
        if (ascending)
            q = q.OrderBy(x);
        else
            q = q.OrderByDescending(x);
    }
    else if (typeof(T).GetProperty(sortfield).PropertyType == typeof(int))
    {
        var x = Expression.Lambda<Func<T, int>>(Expression.Property(p, sortfield), p);
        if (ascending)
            q = q.OrderBy(x);
        else
            q = q.OrderByDescending(x);
    }
    else if (typeof(T).GetProperty(sortfield).PropertyType == typeof(DateTime))
    {
        var x = Expression.Lambda<Func<T, DateTime>>(Expression.Property(p, sortfield), p);
        if (ascending)
            q = q.OrderBy(x);
        else
            q = q.OrderByDescending(x);
    }
    // many more for every type
    return q;
}

有什么办法我可以折叠的IFS到一个通用的声明？
的主要问题是，对于部分
Expression.Lambda< Func键< T，INT>>
我不知道怎么样它一般写的。

Is there any way I can collapse those ifs to a single generic statement? The main problem is that for the part Expression.Lambda<Func<T, int>> I am not sure how to write it generically.

推荐答案

如果您展开 Queryable.OrderBy 来它的定义，那么你就不必使用 Expression.Lambda 通用过载：

If you expand Queryable.OrderBy to its definition then you don't have to use the generic overload of Expression.Lambda:

public static IQueryable<T> SortTable<T>(
    IQueryable<T> q, string sortfield, bool ascending)
{
    var p = Expression.Parameter(typeof(T), "p");
    var x = Expression.Lambda(Expression.Property(p, sortfield), p);

    return q.Provider.CreateQuery<T>(
               Expression.Call(typeof(Queryable),
                               ascending ? "OrderBy" : "OrderByDescending",
                               new Type[] { q.ElementType, x.Body.Type },
                               q.Expression,
                               x));
}

这篇关于创建一个排序函数的泛型列表的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！