具有动态列组的 LINQ GroupBy

Question

我有一张这样的桌子：

 variety category  quantity
----------------------------------------------
  rg      pm         10
  gs      pm          5
  rg      com         8

我想根据这些bool参数制作一个GroupBy：

• IncludeVariety
• IncludeCategory

例如：

IncludeVariety = true;
IncludeCategory = true;

会返回这个：

 variety category  quantity
----------------------------------------------
  rg      pm         10
  gs      pm          5
  rg      com         8

还有这个：

IncludeVariety = true;
IncludeCategory = false;

会返回这个：

  variety category  quantity
----------------------------------------------
    rg      -         18
    gs      -          5

还有这个：

IncludeVariety = false;
IncludeCategory = true;

会返回这个：

  variety category  quantity
----------------------------------------------
     -      pm         15
     -      com         8

你懂的……

问题：如何使用 LINQ 实现这一目标？

重要提示：我已将问题简化为 两个布尔变量（IncludeVariety 和 IncludeCategory），但实际上我会有更多列 （说五个）

我不知道如何动态生成查询（.GroupBy 和 .Select）：

 rows.GroupBy(r => new { r.Variety, r.Category })
 .Select(g => new 
  {
        Variety = g.Key.Variety,
        Category = g.Key.Category,
        Quantity = g.Sum(a => a.Quantity),
  });

 rows.GroupBy(r => new { r.Category })
 .Select(g => new 
  {
        Variety = new {},
        Category = g.Key.Category,
        Quantity = g.Sum(a => a.Quantity),
  });

 rows.GroupBy(r => new { r.Variety })
 .Select(g => new 
  {
        Variety = g.Key.Variety,
        Category = new {},
        Quantity = g.Sum(a => a.Quantity),
  });

我过去做过的类似的事情是连接Where，比如：

  var query = ...

  if (foo) {
       query = query.Where(...)
  }

  if (bar) {
       query = query.Where(...)
  }

  var result = query.Select(...)

我可以在这里做类似的事情吗？

Answer 1

var results=items
  .Select(i=>
    new {
      variety=includevariety?t.variety:null,
      category=includecategory?t.category:null,
      ...
    })
  .GroupBy(g=>
    new { variety, category, ... }, g=>g.quantity)
  .Select(i=>new {
    variety=i.Key.variety,
    category=i.Key.category,
    ...
    quantity=i.Sum()
  });

缩短：

var results=items
  .GroupBy(g=>
    new {
      variety=includevariety?t.variety:null,
      category=includecategory?t.category:null,
      ... 
    }, g=>g.quantity)
  .Select(i=>new {
    variety=i.Key.variety,
    category=i.Key.category,
    ...
    quantity=i.Sum()
  });

Answer 2

如果您需要它是真正动态的，请使用 Scott Gu 的 Dynamic LINQ 库。

您只需要弄清楚要在结果中包含哪些列并按它们分组。

public static IQueryable GroupByColumns(this IQueryable source,
    bool includeVariety = false,
    bool includeCategory = false)
{
    var columns = new List<string>();
    if (includeVariety) columns.Add("Variety");
    if (includeCategory) columns.Add("Category");
    return source.GroupBy($"new({String.Join(",", columns)})", "it");
}

然后你可以将它们分组。

var query = rows.GroupByColumns(includeVariety: true, includeCategory: true);

Answer 3

在任何情况下，仍然需要在没有动态 LINQ 和类型安全的情况下按动态列进行分组。您可以向此 IQueryable 扩展方法提供一个匿名对象，其中包含您可能想要分组的所有属性（具有匹配的类型！）以及您想要用于此组调用的属性名称列表。在第一个重载中，我使用匿名对象来获取它的构造函数和属性。我使用它们通过表达式动态地构建组。在第二个重载中，我将匿名对象仅用于 TKey 的类型推断，不幸的是，在 C# 中没有办法解决，因为它具有有限的类型别名能力。
仅适用于像这样的可为空的属性，可能很容易为非空值扩展，但现在不能打扰

public static IQueryable<IGrouping<TKey, TElement>> GroupByProps<TElement, TKey>(this IQueryable<TElement> self, TKey model, params string[] propNames)
{
    var modelType = model.GetType();
    var props = modelType.GetProperties();
    var modelCtor = modelType.GetConstructor(props.Select(t => t.PropertyType).ToArray());

    return self.GroupByProps(model, modelCtor, props, propNames);
}

public static IQueryable<IGrouping<TKey, TElement>> GroupByProps<TElement, TKey>(this IQueryable<TElement> self, TKey model, ConstructorInfo modelCtor, PropertyInfo[] props, params string[] propNames)
{
    var parameter = Expression.Parameter(typeof(TElement), "r");
    var propExpressions = props
        .Select(p =>
        {
            Expression value;

            if (propNames.Contains(p.Name))
                value = Expression.PropertyOrField(parameter, p.Name);
            else
                value = Expression.Convert(Expression.Constant(null, typeof(object)), p.PropertyType);

            return value;
        })
        .ToArray();

    var n = Expression.New(
        modelCtor,
        propExpressions,
        props
    );

    var expr = Expression.Lambda<Func<TElement, TKey>>(n, parameter);
    return self.GroupBy(expr);
}

我实现了两个重载，以防您想缓存构造函数和属性以避免每次调用时的反射。像这样使用：

//Class with properties that you want to group by
class Record
{
    public string Test { get; set; }
    public int? Hallo { get; set; }
    public DateTime? Prop { get; set; }

    public string PropertyWhichYouNeverWantToGroupBy { get; set; }
}

//usage

IQueryable<Record> queryable = ...; //the queryable

var grouped = queryable.GroupByProps(new
{
    Test = (string)null,        //put all properties that you might want to group by here
    Hallo = (int?)null,
    Prop = (DateTime?)null
}, nameof(Record.Test), nameof(Record.Prop));   //This will group by Test and Prop but not by Hallo

//Or to cache constructor and props
var anonymous = new
{
    Test = (string)null,        //put all properties that you might want to group by here
    Hallo = (int?)null,
    Prop = (DateTime?)null
};
var type = anonymous.GetType();
var constructor = type.GetConstructor(new[]
{
    typeof(string),             //Put all property types of your anonymous object here 
    typeof(int?),
    typeof(DateTime?)
});
var props = type.GetProperties();
//You need to keep constructor and props and maybe anonymous 
//Then call without reflection overhead
queryable.GroupByProps(anonymous, constructor, props, nameof(Record.Test), nameof(Record.Prop));

您将作为 TKey 接收的匿名对象将仅填充您用于分组的键（即在此示例中为“测试”和“道具”），其他的将为空。