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本文在C# Linq源码分析之Take#xff08;四#xff09;的基础上继续从源码角度分析Take的优化方法#xff0c;主要分析Where.Select.Take的使用案例。
Where.Select.Take的案例分析
该场景模拟我们显示中将EF中与数据库关联的对象进行过滤#xff0c;然后转换成Web…概要
本文在C# Linq源码分析之Take四的基础上继续从源码角度分析Take的优化方法主要分析Where.Select.Take的使用案例。
Where.Select.Take的案例分析
该场景模拟我们显示中将EF中与数据库关联的对象进行过滤然后转换成Web前端需要的对象并分页的情况。 studentList.Where(x x.MathResult 90).Select(x new {x.Name,x.MathResult}).Take(3).ToList().ForEach(xConsole.WriteLine(x.Name x.MathResult));找到数学90分以上的学生获取学生的姓名和数学成绩每次只取前三个学生。并将学生信息打印。Student类的代码请见附录。
源码流程分析 第一步进入Where方法返回WhereListIterator对象 第二步进入Select方法将Where和Select两个操作合并返回WhereSelectListIterator对象 第三步进入Take方法调用takeIterator方法由于人WhereSelectListIterator并没有实现IPartition接口和IList接口所以无法再进行操作合并只能返回EnumerablePartition对象。
private static IEnumerableTSource takeIteratorTSource(IEnumerableTSource source, int count){Debug.Assert(count 0);returnsource is IPartitionTSource partition ? partition.Take(count) :source is IListTSource sourceList ? new ListPartitionTSource(sourceList, 0, count - 1) :new EnumerablePartitionTSource(source, 0, count - 1);}第四步进入ToList方法 public static ListTSource ToListTSource(this IEnumerableTSource source){if (source null){ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);}return source is IIListProviderTSource listProvider ? listProvider.ToList() : new ListTSource(source);}此时的source是EnumerablePartition对象它实现了IPartition接口而IPartition接口继承了IIListProvider接口所以可以调用自己的ToList方法
public ListTSource ToList()
{var list new ListTSource();using (IEnumeratorTSource en _source.GetEnumerator()){if (SkipBeforeFirst(en) en.MoveNext()){int remaining Limit - 1; // Max number of items left, not counting the current element.int comparand HasLimit ? 0 : int.MinValue; // If we dont have an upper bound, have the comparison always return true.do{remaining--;list.Add(en.Current);}while (remaining comparand en.MoveNext());}}return list;
}定义迭代器en此时的_source是WhereSelectListIterator对象该ToList方法同样支持Skip所以要判断迭代器的起始位置是不是从第一个开始每次迭代首先从WhereSelectListIterator迭代器中返回一个符合过滤条件并完成Selector操作的元素存入list直到list中包含三个元素返回执行结果。
虽然WhereSelectListIterator没有实现IPartition接口不能实现一次迭代完成全部操作但是现有的流程性能并不差因为WhereSelectListIterator迭代器本身已经合并了过滤和投影操作而且并不需要遍历所有元素只要找到3个符合条件的元素即可。
我认为如果代码需要用到Take方法尽量把它放到Linq的最后。这样做的好处是前面的Linq操作并不需要遍历全部的序列元素只要得到Take方法中需要的元素个数即可。
本文中涉及的源码请见附录关于WhereSelectListIterator的合并优化操作更多详细内容请参考C# LINQ源码分析之Select
附录
Student类
public class Student {public string Id { get; set; }public string Name { get; set; }public string Classroom { get; set; }public int MathResult { get; set; }
}IIListProvider接口
internal interface IIListProviderTElement : IEnumerableTElement
{TElement[] ToArray();ListTElement ToList();int GetCount(bool onlyIfCheap);
}IPartition接口
internal interface IPartitionTElement : IIListProviderTElement
{IPartitionTElement Skip(int count);IPartitionTElement Take(int count);TElement? TryGetElementAt(int index, out bool found);TElement? TryGetFirst(out bool found);TElement? TryGetLast(out bool found);
}EnumerablePartition类
private sealed class EnumerablePartitionTSource : IteratorTSource, IPartitionTSource{private readonly IEnumerableTSource _source;private readonly int _minIndexInclusive;private readonly int _maxIndexInclusive; // -1 if we want everything past _minIndexInclusive.// If this is -1, its impossible to set a limit on the count.private IEnumeratorTSource? _enumerator;internal EnumerablePartition(IEnumerableTSource source, int minIndexInclusive, int maxIndexInclusive){Debug.Assert(source ! null);Debug.Assert(!(source is IListTSource), $The caller needs to check for {nameof(IListTSource)}.);Debug.Assert(minIndexInclusive 0);Debug.Assert(maxIndexInclusive -1);// Note that although maxIndexInclusive cant grow, it can still be int.MaxValue.// We support partitioning enumerables with 2B elements. For example, e.Skip(1).Take(int.MaxValue) should work.// But if it is int.MaxValue, then minIndexInclusive must ! 0. Otherwise, our count may overflow.Debug.Assert(maxIndexInclusive -1 || (maxIndexInclusive - minIndexInclusive int.MaxValue), ${nameof(Limit)} will overflow!);Debug.Assert(maxIndexInclusive -1 || minIndexInclusive maxIndexInclusive);_source source;_minIndexInclusive minIndexInclusive;_maxIndexInclusive maxIndexInclusive;}// If this is true (e.g. at least one Take call was made), then we have an upper bound// on how many elements we can have.private bool HasLimit _maxIndexInclusive ! -1;private int Limit unchecked((_maxIndexInclusive 1) - _minIndexInclusive); // This is that upper bound.public override IteratorTSource Clone() new EnumerablePartitionTSource(_source, _minIndexInclusive, _maxIndexInclusive);public override void Dispose(){if (_enumerator ! null){_enumerator.Dispose();_enumerator null;}base.Dispose();}public int GetCount(bool onlyIfCheap){if (onlyIfCheap){return -1;}if (!HasLimit){// If HasLimit is false, we contain everything past _minIndexInclusive.// Therefore, we have to iterate the whole enumerable.//return Math.Max(_source.Count()- _minIndexInclusive, 0);return 0;}using (IEnumeratorTSource en _source.GetEnumerator()){// We only want to iterate up to _maxIndexInclusive 1.// Past that, we know the enumerable will be able to fit this partition,// so the count will just be _maxIndexInclusive 1 - _minIndexInclusive.// Note that it is possible for _maxIndexInclusive to be int.MaxValue here,// so 1 may result in signed integer overflow. We need to handle this.// At the same time, however, we are guaranteed that our max count can fit// in an int because if that is true, then _minIndexInclusive must 0.uint count SkipAndCount((uint)_maxIndexInclusive 1, en);Debug.Assert(count ! (uint)int.MaxValue 1 || _minIndexInclusive 0, Our return value will be incorrect.);return Math.Max((int)count - _minIndexInclusive, 0);}}public override bool MoveNext(){// Cases where GetEnumerator has not been called or Dispose has already// been called need to be handled explicitly, due to the default: clause.int taken _state - 3;if (taken -2){Dispose();return false;}switch (_state){case 1:_enumerator _source.GetEnumerator();_state 2;goto case 2;case 2:Debug.Assert(_enumerator ! null);if (!SkipBeforeFirst(_enumerator)){// Reached the end before we finished skipping.break;}_state 3;goto default;default:Debug.Assert(_enumerator ! null);if ((!HasLimit || taken Limit) _enumerator.MoveNext()){if (HasLimit){// If we are taking an unknown number of elements, its important not to increment _state.// _state - 3 may eventually end up overflowing well hit the Dispose branch even though// we havent finished enumerating._state;}_current _enumerator.Current;return true;}break;}Dispose();return false;}public override IEnumerableTResult SelectTResult(FuncTSource, TResult selector) new SelectIPartitionIteratorTSource, TResult(this, selector);public IPartitionTSource Skip(int count){int minIndex unchecked(_minIndexInclusive count);if (!HasLimit){if (minIndex 0){// If we dont know our max count and minIndex can no longer fit in a positive int,// then we will need to wrap ourselves in another iterator.// This can happen, for example, during e.Skip(int.MaxValue).Skip(int.MaxValue).return new EnumerablePartitionTSource(this, count, -1);}}else if ((uint)minIndex (uint)_maxIndexInclusive){// If minIndex overflows and we have an upper bound, we will go down this branch.// We know our upper bound must be smaller than minIndex, since our upper bound fits in an int.// This branch should not be taken if we dont have a bound.return EmptyPartitionTSource.Instance;}Debug.Assert(minIndex 0, $We should have taken care of all cases when {nameof(minIndex)} overflows.);return new EnumerablePartitionTSource(_source, minIndex, _maxIndexInclusive);}public IPartitionTSource Take(int count){int maxIndex unchecked(_minIndexInclusive count - 1);if (!HasLimit){if (maxIndex 0){// If we dont know our max count and maxIndex can no longer fit in a positive int,// then we will need to wrap ourselves in another iterator.// Note that although maxIndex may be too large, the difference between it and// _minIndexInclusive (which is count - 1) must fit in an int.// Example: e.Skip(50).Take(int.MaxValue).return new EnumerablePartitionTSource(this, 0, count - 1);}}else if (unchecked((uint)maxIndex (uint)_maxIndexInclusive)){// If we dont know our max count, we cant go down this branch.// Its always possible for us to contain more than count items, as the rest// of the enumerable past _minIndexInclusive can be arbitrarily long.return this;}Debug.Assert(maxIndex 0, $We should have taken care of all cases when {nameof(maxIndex)} overflows.);return new EnumerablePartitionTSource(_source, _minIndexInclusive, maxIndex);}public TSource? TryGetElementAt(int index, out bool found){// If the index is negative or our max count, return early.if (index 0 (!HasLimit || index Limit)){using (IEnumeratorTSource en _source.GetEnumerator()){Debug.Assert(_minIndexInclusive index 0, $Adding {nameof(index)} caused {nameof(_minIndexInclusive)} to overflow.);if (SkipBefore(_minIndexInclusive index, en) en.MoveNext()){found true;return en.Current;}}}found false;return default;}public TSource? TryGetFirst(out bool found){using (IEnumeratorTSource en _source.GetEnumerator()){if (SkipBeforeFirst(en) en.MoveNext()){found true;return en.Current;}}found false;return default;}public TSource? TryGetLast(out bool found){using (IEnumeratorTSource en _source.GetEnumerator()){if (SkipBeforeFirst(en) en.MoveNext()){int remaining Limit - 1; // Max number of items left, not counting the current element.int comparand HasLimit ? 0 : int.MinValue; // If we dont have an upper bound, have the comparison always return true.TSource result;do{remaining--;result en.Current;}while (remaining comparand en.MoveNext());found true;return result;}}found false;return default;}public ListTSource ToList(){var list new ListTSource();using (IEnumeratorTSource en _source.GetEnumerator()){if (SkipBeforeFirst(en) en.MoveNext()){int remaining Limit - 1; // Max number of items left, not counting the current element.int comparand HasLimit ? 0 : int.MinValue; // If we dont have an upper bound, have the comparison always return true.do{remaining--;list.Add(en.Current);}while (remaining comparand en.MoveNext());}}return list;}private bool SkipBeforeFirst(IEnumeratorTSource en) SkipBefore(_minIndexInclusive, en);private static bool SkipBefore(int index, IEnumeratorTSource en) SkipAndCount(index, en) index;private static int SkipAndCount(int index, IEnumeratorTSource en){Debug.Assert(index 0);return (int)SkipAndCount((uint)index, en);}private static uint SkipAndCount(uint index, IEnumeratorTSource en){Debug.Assert(en ! null);for (uint i 0; i index; i){if (!en.MoveNext()){return i;}}return index;}public TSource[] ToArray(){throw new NotImplementedException();}}
