Question

我正在编写一个列表排序扩展方法。我的输入是列表和带有属性名称和排序方向的字符串。这个字符串可以有多个属性，如下所示： “命名ASC，日期DESC”等。

我已经实现了字符串解析并使用了反射来从字符串中获取属性本身，但我现在所困扰的是如何动态链接orderby方法。

像这样的东西： _list.orderBy(x=>x.prop1).thenBy(x=>x.prop2)等。

有没有办法动态构建它？

Answer 1

使用反射从字符串属性名称获取到PropertyInfo。然后，您可以使用PropertyInfo构建表达式树，以动态构建所有orderbys。获得表达式树后，将其编译为委托，（比如Func，IEnumerable＆gt;）将_list参数传递给此委托，它将为您提供另一个可枚举的有序结果。

要获取Enumerable上泛型方法的反射信息，请查看此帖子的答案： Get a generic method without using GetMethods

public static class Helper
{
    public static IEnumerable<T> BuildOrderBys<T>(
        this IEnumerable<T> source,
        params SortDescription[] properties)
    {
        if (properties == null || properties.Length == 0) return source;

        var typeOfT = typeof (T);

        Type t = typeOfT;

        IOrderedEnumerable<T> result = null;
        var thenBy = false;

        foreach (var item in properties
            .Select(prop => new {PropertyInfo = t.GetProperty(prop.PropertyName), prop.Direction}))
        {
            var oExpr = Expression.Parameter(typeOfT, "o");
            var propertyInfo = item.PropertyInfo;
            var propertyType = propertyInfo.PropertyType;
            var isAscending = item.Direction == ListSortDirection.Ascending;

            if (thenBy)
            {
                var prevExpr = Expression.Parameter(typeof (IOrderedEnumerable<T>), "prevExpr");
                var expr1 = Expression.Lambda<Func<IOrderedEnumerable<T>, IOrderedEnumerable<T>>>(
                    Expression.Call(
                        (isAscending ? thenByMethod : thenByDescendingMethod).MakeGenericMethod(typeOfT, propertyType),
                        prevExpr,
                        Expression.Lambda(
                            typeof (Func<,>).MakeGenericType(typeOfT, propertyType),
                            Expression.MakeMemberAccess(oExpr, propertyInfo),
                            oExpr)
                        ),
                    prevExpr)
                    .Compile();

                result = expr1(result);
            }
            else
            {
                var prevExpr = Expression.Parameter(typeof (IEnumerable<T>), "prevExpr");
                var expr1 = Expression.Lambda<Func<IEnumerable<T>, IOrderedEnumerable<T>>>(
                    Expression.Call(
                        (isAscending ? orderByMethod : orderByDescendingMethod).MakeGenericMethod(typeOfT, propertyType),
                        prevExpr,
                        Expression.Lambda(
                            typeof (Func<,>).MakeGenericType(typeOfT, propertyType),
                            Expression.MakeMemberAccess(oExpr, propertyInfo),
                            oExpr)
                        ),
                    prevExpr)
                    .Compile();

                result = expr1(source);
                thenBy = true;
            }
        }
        return result;
    }

    private static MethodInfo orderByMethod =
        MethodOf(() => Enumerable.OrderBy(default(IEnumerable<object>), default(Func<object, object>)))
            .GetGenericMethodDefinition();

    private static MethodInfo orderByDescendingMethod =
        MethodOf(() => Enumerable.OrderByDescending(default(IEnumerable<object>), default(Func<object, object>)))
            .GetGenericMethodDefinition();

    private static MethodInfo thenByMethod =
        MethodOf(() => Enumerable.ThenBy(default(IOrderedEnumerable<object>), default(Func<object, object>)))
            .GetGenericMethodDefinition();

    private static MethodInfo thenByDescendingMethod =
        MethodOf(() => Enumerable.ThenByDescending(default(IOrderedEnumerable<object>), default(Func<object, object>)))
            .GetGenericMethodDefinition();

    public static MethodInfo MethodOf<T>(Expression<Func<T>> method)
    {
        MethodCallExpression mce = (MethodCallExpression) method.Body;
        MethodInfo mi = mce.Method;
        return mi;
    }
}

public static class Sample
{
    private static void Main()
    {
      var data = new List<Customer>
        {
          new Customer {ID = 3, Name = "a"},
          new Customer {ID = 3, Name = "c"},
          new Customer {ID = 4},
          new Customer {ID = 3, Name = "b"},
          new Customer {ID = 2}
        };

      var result = data.BuildOrderBys(
        new SortDescription("ID", ListSortDirection.Ascending),
        new SortDescription("Name", ListSortDirection.Ascending)
        ).Dump();
    }
}

public class Customer
{
    public int ID { get; set; }
    public string Name { get; set; }
}

LinqPad中显示的样本结果

enter image description here

Answer 2

我不确定你是如何通过反射添加顺序的（太懒了检查）但这里是伪代码的基本思想：

var query = list.OrderBy(properties.First());
bool first = true;
foreach(var property in properties.Skip(1))
{
    query = query.ThenBy(property);
}

Answer 3

您可以使用以下内容：

var query = _list.OrderBy(x=>x.prop1);
if (shouldOrderByProp2Too)
  query = query.ThenBy(x=>x.prop2);
if (shouldOrderByProp3Too)
  query = query.ThenBy(x=>x.prop3);
// ...
// then use query the way you had your code

征求意见：在这种情况下，我会对列表中的对象进行实现IComparable，然后对其进行排序/或使用动态检查这一点的Compareres - 我能想到的另一件事就是用反射和动态来做这件事 - 调用...如果不是真的必要的话，不是你想要做的事情;）

List <t> orderby </t>的动态链接

3 个答案: