创建排序功能的通用列表

Question

我对由对象字段排序泛型列表的方法：

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; 
} 

有什么方法可以让我崩溃的IFS到一个通用的声明？ 主要问题是，对于部分 Expression.Lambda<Func<T, int>> 我不知道如何一般地写它。

Answer 1

如果展开Queryable.OrderBy它的定义，那么你不必使用Expression.Lambda通用超载：

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)); 
} 

Answer 2

不会在工作？

public static IQueryable<T> SortTable<T>(IQueryable<T> q, string sortfield, bool ascending) 
    { 
     var type = typeof(T).GetProperty(sortfield).PropertyType; 
     var p = Expression.Parameter(typeof(T), "p"); 
     var x = Expression.Lambda<Func<T, type> >(Expression.Property(p, sortfield), p); 
     if (ascending) 
      q = q.OrderBy(x); 
     else 
      q = q.OrderByDescending(x); 
     return q; 
    }