问题描述
请考虑以下课程:
this.choosenEmployee
每个类的属性都具有[Key]属性。是否可以通过它们各自的[Key]属性将这些类的IEnumerable动态分组?
解决方法
我会为每种类型添加扩展方法,例如
选项1:
static class Extensions
{
public static IEnumerable<IGrouping<Tuple<string,int>,Potato>>
GroupByPrimaryKey(this IEnumerable<Potato> e)
{
return e.GroupBy(p => Tuple.Create(p.Farm,p.Size));
}
public static IEnumerable<IGrouping<Tuple<bool,bool>,Frog>>
GroupByPrimaryKey(this IEnumerable<Frog> e)
{
return e.GroupBy(p => Tuple.Create(p.IsAlive,p.IsVirulent));
}
}
如果类型很多,则可以使用t4生成代码。
用法:.GroupByPrimaryKey()
。
选项2:
一个更简单的变体:
static class Extensions
{
public static Tuple<string,int> GetPrimaryKey(this Potato p)
{
return Tuple.Create(p.Farm,p.Size);
}
public static Tuple<bool,bool> GetPrimaryKey(this Frog p)
{
return Tuple.Create(p.IsAlive,p.IsVirulent);
}
}
用法:.GroupBy(p => p.GetPrimaryKey())
。
选项3:
有反射的解决方案是可能的,但是会很慢。草图(远未投入生产!)
class CombinedKey : IEquatable<CombinedKey>
{
object[] _keys;
CombinedKey(object[] keys)
{
_keys = keys;
}
public bool Equals(CombinedKey other)
{
return _keys.SequenceEqual(other._keys);
}
public override bool Equals(object obj)
{
return obj is CombinedKey && Equals((CombinedKey)obj);
}
public override int GetHashCode()
{
return 0;
}
public static CombinedKey GetKey<T>(T instance)
{
return new CombinedKey(GetKeyAttributes(typeof(T)).Select(p => p.GetValue(instance,null)).ToArray());
}
private static PropertyInfo[] GetKeyAttributes(Type type)
{
// you definitely want to cache this
return type.GetProperties()
.Where(p => Attribute.GetCustomAttribute(p,typeof(KeyAttribute)) != null)
.ToArray();
}
}
用法:GroupBy(p => CombinedKey.GetKey(p))
这里的挑战是,您需要构建一个匿名类型才能拥有一个GroupBy
表达式,该表达式可以转换为SQL或任何其他LINQ提供程序。
我不确定您可以使用反射来做到这一点(不是没有一些真正复杂的代码来在运行时创建匿名类型)。但是,如果您愿意提供匿名类型的示例作为种子,则可以创建分组表达式。
public static Expression<Func<TSource,TAnon>> GetAnonymous<TSource,TAnon>(TSource dummy,TAnon example)
{
var ctor = typeof(TAnon).GetConstructors().First();
var paramExpr = Expression.Parameter(typeof(TSource));
return Expression.Lambda<Func<TSource,TAnon>>
(
Expression.New
(
ctor,ctor.GetParameters().Select
(
(x,i) => Expression.Convert
(
Expression.Property(paramExpr,x.Name),// fetch same named property
x.ParameterType
)
)
),paramExpr);
}
这是您将如何使用它(注意:传递给该方法的伪匿名类型是为了使该匿名类型成为编译时类型,该方法不在乎您传入的值是什么为此。):
static void Main()
{
var groupByExpression = GetAnonymous(new Frog(),new {IsAlive = true,IsVirulent = true});
Console.WriteLine(groupByExpression);
var frogs = new []{ new Frog{ IsAlive = true,IsVirulent = false},new Frog{ IsAlive = false,IsVirulent = true},new Frog{ IsAlive = true,IsVirulent = true}};
var grouped = frogs.AsQueryable().GroupBy(groupByExpression);
foreach (var group in grouped)
{
Console.WriteLine(group.Key);
}
}
哪个会产生:
Param_0 => new <>f__AnonymousType0`2(Convert(Param_0.IsAlive,Boolean),Convert(Param_0.IsVirulent,Boolean))
{ IsAlive = True,IsVirulent = False }
{ IsAlive = False,IsVirulent = True }
{ IsAlive = True,IsVirulent = True }
,
有人发布了有效答案,并出于某种原因以后将其删除。在这里:
组合键类:
$dir1 = "/path/1/";
$dir2 = "/path/2/";
$commaSeparatedDirectories = implode(',',[$dir1,$dir2]);
$count = count(glob("{{$commaSeparatedDirectories}}*.gz",GLOB_BRACE));
呼叫者函数及其实际用法:
class CombinedKey<T> : IEquatable<CombinedKey<T>>
{
readonly object[] _keys;
public bool Equals(CombinedKey<T> other)
{
return _keys.SequenceEqual(other._keys);
}
public override bool Equals(object obj)
{
return obj is CombinedKey<T> key && Equals(key);
}
public override int GetHashCode()
{
int hash = _keys.Length;
foreach (object o in _keys)
{
if (o != null)
{
hash = hash * 13 + o.GetHashCode();
}
}
return hash;
}
readonly Lazy<Func<T,object[]>> lambdaFunc = new Lazy<Func<T,object[]>>(() =>
{
Type type = typeof(T);
var paramExpr = Expression.Parameter(type);
var arrayExpr = Expression.NewArrayInit(
typeof(object),type.GetProperties()
.Where(p => (Attribute.GetCustomAttribute(p,typeof(KeyAttribute)) != null))
.Select(p => Expression.Convert(Expression.Property(paramExpr,p),typeof(object)))
.ToArray()
);
return Expression.Lambda<Func<T,object[]>>(arrayExpr,paramExpr).Compile();
},System.Threading.LazyThreadSafetyMode.PublicationOnly);
public CombinedKey(T instance)
{
_keys = lambdaFunc.Value(instance);
}
}
是的,它相对较慢,因此,如果有问题,那么KlausGütter的解决方案就是可行的方法。