创建表达式并不难，但是棘手的部分是当您不知道属性的类型（因此选择器表达式结果的类型）时如何将其绑定到相应的OrderBy(Descending)/
ThenBy(Descendig)方法。

这是封装在自定义扩展方法中的所有内容：

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);
    }