c# - linq中的动态排序

Question

请考虑这种情况:

我有一个包含大约 50 个字段的类列表。我想要一个组合框，用户可以根据要排序的字段列表进行选择。例如，如果用户选择“F1”列表，则根据“F1”进行排序。

我不想对每个字段都使用 if-else 进行排序。我看到了这个主题:

Sorting a gridview when databinding a collection or list of objects

但我不能使用它的答案。我如何使用 Expression Tree 来达到这个目的？

谢谢

编辑 1):

根据亲爱的@Thom Smith 的回答，我写了这段代码:

 using (NorthwindModel1.NorthwindEntities2 ent = new NorthwindModel1.NorthwindEntities2())
    {
        var query = from o in ent.Orders
                    where o.OrderID < 10257
                    select o;

        query.OrderBy("CustomerID", SortDirection.Ascending);

        GridView1.DataSource = query;
        GridView1.DataBind();
    }

但是没有排序。如果我这样写代码:

GridView1.DataSource = query.OrderBy(o=>o.CustomerID);

它正在排序。问题出在哪里？

Answer 1

这是我为此使用的方法:

private IQueryable<T> OrderQuery<T>(IQueryable<T> query, OrderParameter orderBy)
{
    string orderMethodName = orderBy.Direction == SortDirection.Ascending ? "OrderBy" : "OrderByDescending";
    Type t = typeof(T);

    var param = Expression.Parameter(t, "shipment");
    var property = t.GetProperty(orderBy.Attribute);

    /* We can't just call OrderBy[Descending] with an Expression
     * parameter because the second type argument to OrderBy is not
     * known at compile-time.
     */
    return query.Provider.CreateQuery<T>(
        Expression.Call(
            typeof(Queryable),
            orderMethodName,
            new Type[] { t, property.PropertyType },
            query.Expression,
            Expression.Quote(
                Expression.Lambda(
                    Expression.Property(param, property),
                    param))
        ));
}

OrderParameter 只是一个具有属性和方向的结构。

编辑:补充说明。

此方法来 self 的 DynamicOrderList 类，它是 OrderParameter 对象的列表。如果你只需要按一个字段排序，那么你可以稍微简化一下:

private IQueryable<T> OrderByDynamic<T>(this IQueryable<T> query, string attribute, SortDirection direction)
{
    try
    {
        string orderMethodName = direction == SortDirection.Ascending ? "OrderBy" : "OrderByDescending";
        Type t = typeof(T);

        var param = Expression.Parameter(t);
        var property = t.GetProperty(attribute);

        return query.Provider.CreateQuery<T>(
            Expression.Call(
                typeof(Queryable),
                orderMethodName,
                new Type[] { t, property.PropertyType },
                query.Expression,
                Expression.Quote(
                    Expression.Lambda(
                        Expression.Property(param, property),
                        param))
            ));
    }
    catch (Exception) // Probably invalid input, you can catch specifics if you want
    {
        return query; // Return unsorted query
    }
}

然后像这样使用它:

myQuery = myQuery.OrderByDynamic("name", SortDirection.Ascending);

编辑 2:

public IQueryable<T> OrderBy<T>(this IQueryable<T> query, string attribute, SortDirection direction)
{
    return ApplyOrdering(query, attribute, direction, "OrderBy");
}

public IQueryable<T> ThenBy<T>(this IQueryable<T> query, string attribute, SortDirection direction)
{
    return ApplyOrdering(query, attribute, direction, "ThenBy");
}

private IQueryable<T> ApplyOrdering<T>(IQueryable<T> query, string attribute, SortDirection direction, string orderMethodName)
{
    try
    {
        if (direction == SortDirection.Descending) orderMethodName += "Descending";

        Type t = typeof(T);

        var param = Expression.Parameter(t);
        var property = t.GetProperty(attribute);

        return query.Provider.CreateQuery<T>(
            Expression.Call(
                typeof(Queryable),
                orderMethodName,
                new Type[] { t, property.PropertyType },
                query.Expression,
                Expression.Quote(
                    Expression.Lambda(
                        Expression.Property(param, property),
                        param))
            ));
    }
    catch (Exception) // Probably invalid input, you can catch specifics if you want
    {
        return query; // Return unsorted query
    }
}

和:

myQuery=myQuery.OrderBy("name", SortDirection.Ascending).ThenBy("date", SortDirection.Descending);