概要

本文主要对Take的优化方法进行源码分析，分析Take在配合Select，Where等常用的Linq扩展方法使用时候，如何实现优化处理。

本文涉及到Select, Where和Take和三个方法的源码分析，其中Select, Where, Take更详尽的源码分析，请参考我之前写的文章。

源码分析

我们之前对Take的源码分析，主要是真对数据序列对象直接调用的Take方法的情况。本文介绍的Take优化方法，主要是针对多个Linq方法配合使用的情况，例如xx.Where.Select.Take或者xx.Select.Take的情况。

Take的优化方法，是定义在Take.SpeedOpt.cs文件中，体现在下面的TakeIterator方法中，Take方法对TakeIterator方法的具体调用方式，请参考C# Linq源码分析之Take方法

private static IEnumerable<TSource> TakeIterator<TSource>(IEnumerable<TSource> source, int count){Debug.Assert(source != null);Debug.Assert(count > 0);returnsource is IPartition<TSource> partition ? partition.Take(count) :source is IList<TSource> sourceList ? new ListPartition<TSource>(sourceList, 0, count - 1) :new EnumerablePartition<TSource>(source, 0, count - 1);}

该优化方法的基本逻辑如下：

1. 如果source序列实现了IPartition接口，则调用IPartition中的Take方法；
2. 如果source序列实现了IList接口，则返回ListPartition对象；
3. 返回EnumerablePartition对象。

案例分析

Select.Take

该场景模拟我们显示中将EF中与数据库关联的对象，转换成Web前端需要的对象，并分页的情况。

将Student对象中的学生姓名和考试成绩取出后，并分页，Student类的定义和初始化请见附录。

 studentList.Select(x => new {x.Name, x.MathResult}).take(3).ToList();

执行流程如上图所示：

1. 进入Select方法后返回一个SelectListIterator对象，该对象存储了source序列数据和selector；
2. 进入Take方法后， 因为SelectListIterator实现了IPartition接口，因此可以调用自己的Take方法，使用source，selector和count（Take方法的参数）实例化SelectListPartitionIterator对象；

  public IPartition<TResult> Take(int count)
{Debug.Assert(count > 0);int maxIndex = _minIndexInclusive + count - 1;return (uint)maxIndex >= (uint)_maxIndexInclusive ? this : new SelectListPartitionIterator<TSource, TResult>(_source, _selector, _minIndexInclusive, maxIndex);
}

3. 进入ToList方法后，根据下面的代码，因为SelectListPartitionIterator对象实现了IPartition接口，而IPartition又继承了IIListProvider接口，所以listProvider.ToList()被执行。

   public static List<TSource> ToList<TSource>(this IEnumerable<TSource> source){
if (source == null){ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);}return source is IIListProvider<TSource> listProvider ? listProvider.ToList() : new List<TSource>(source);
}

listProvider.ToList() 被执行，即SelectListPartitionIterator对象内定义的ToList()方法被执行，该ToList方法可以将
Selecxt的投影操作，Take的取值操作同时执行，代码如下：

 public List<TResult> ToList()
{int count = Count;if (count == 0){return new List<TResult>();}List<TResult> list = new List<TResult>(count);int end = _minIndexInclusive + count;for (int i = _minIndexInclusive; i != end; ++i){list.Add(_selector(_source[i]));}return list;
}

1. 如果Take的取值为0，直接返回空List，不会进行Select操作；
2. 按照Take中指定的Count，取到相应的元素，再进行投影操作；
3. 返回操作结果。

这样，源List序列source，Linq只遍历和一次，就同时完成了投影和过滤两个操作，实现了优化。

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 IIListProvider<TElement> : IEnumerable<TElement>
{TElement[] ToArray();List<TElement> ToList();int GetCount(bool onlyIfCheap);
}

IPartition接口

internal interface IPartition<TElement> : IIListProvider<TElement>
{IPartition<TElement> Skip(int count);IPartition<TElement> Take(int count);TElement? TryGetElementAt(int index, out bool found);TElement? TryGetFirst(out bool found);TElement? TryGetLast(out bool found);
}

SelectListPartitionIterator类

 private sealed class SelectListPartitionIterator<TSource, TResult> : Iterator<TResult>, IPartition<TResult>
{private readonly IList<TSource> _source;private readonly Func<TSource, TResult> _selector;private readonly int _minIndexInclusive;private readonly int _maxIndexInclusive;public SelectListPartitionIterator(IList<TSource> source, Func<TSource, TResult> selector, int minIndexInclusive, int maxIndexInclusive){Debug.Assert(source != null);Debug.Assert(selector != null);Debug.Assert(minIndexInclusive >= 0);Debug.Assert(minIndexInclusive <= maxIndexInclusive);_source = source;_selector = selector;_minIndexInclusive = minIndexInclusive;_maxIndexInclusive = maxIndexInclusive;}public override Iterator<TResult> Clone() =>new SelectListPartitionIterator<TSource, TResult>(_source, _selector, _minIndexInclusive, _maxIndexInclusive);public override bool MoveNext(){// _state - 1 represents the zero-based index into the list.// Having a separate field for the index would be more readable. However, we save it// into _state with a bias to minimize field size of the iterator.int index = _state - 1;if (unchecked((uint)index <= (uint)(_maxIndexInclusive - _minIndexInclusive) && index < _source.Count - _minIndexInclusive)){_current = _selector(_source[_minIndexInclusive + index]);++_state;return true;}Dispose();return false;}public override IEnumerable<TResult2> Select<TResult2>(Func<TResult, TResult2> selector) =>new SelectListPartitionIterator<TSource, TResult2>(_source, CombineSelectors(_selector, selector), _minIndexInclusive, _maxIndexInclusive);public IPartition<TResult> Skip(int count){Debug.Assert(count > 0);int minIndex = _minIndexInclusive + count;return (uint)minIndex > (uint)_maxIndexInclusive ? EmptyPartition<TResult>.Instance : new SelectListPartitionIterator<TSource, TResult>(_source, _selector, minIndex, _maxIndexInclusive);}public IPartition<TResult> Take(int count){Debug.Assert(count > 0);int maxIndex = _minIndexInclusive + count - 1;return (uint)maxIndex >= (uint)_maxIndexInclusive ? this : new SelectListPartitionIterator<TSource, TResult>(_source, _selector, _minIndexInclusive, maxIndex);}public TResult? TryGetElementAt(int index, out bool found){if ((uint)index <= (uint)(_maxIndexInclusive - _minIndexInclusive) && index < _source.Count - _minIndexInclusive){found = true;return _selector(_source[_minIndexInclusive + index]);}found = false;return default;}public TResult? TryGetFirst(out bool found){if (_source.Count > _minIndexInclusive){found = true;return _selector(_source[_minIndexInclusive]);}found = false;return default;}public TResult? TryGetLast(out bool found){int lastIndex = _source.Count - 1;if (lastIndex >= _minIndexInclusive){found = true;return _selector(_source[Math.Min(lastIndex, _maxIndexInclusive)]);}found = false;return default;}private int Count{get{int count = _source.Count;if (count <= _minIndexInclusive){return 0;}return Math.Min(count - 1, _maxIndexInclusive) - _minIndexInclusive + 1;}}public TResult[] ToArray(){int count = Count;if (count == 0){return Array.Empty<TResult>();}TResult[] array = new TResult[count];for (int i = 0, curIdx = _minIndexInclusive; i != array.Length; ++i, ++curIdx){array[i] = _selector(_source[curIdx]);}return array;}public List<TResult> ToList(){int count = Count;if (count == 0){return new List<TResult>();}List<TResult> list = new List<TResult>(count);int end = _minIndexInclusive + count;for (int i = _minIndexInclusive; i != end; ++i){list.Add(_selector(_source[i]));}return list;}public int GetCount(bool onlyIfCheap){// In case someone uses Count() to force evaluation of// the selector, run it provided `onlyIfCheap` is false.int count = Count;if (!onlyIfCheap){int end = _minIndexInclusive + count;for (int i = _minIndexInclusive; i != end; ++i){_selector(_source[i]);}}return count;}
}

SelectListIterator类 Select.cs

 private sealed partial class SelectListIterator<TSource, TResult> : Iterator<TResult>{private readonly List<TSource> _source;private readonly Func<TSource, TResult> _selector;private List<TSource>.Enumerator _enumerator;public SelectListIterator(List<TSource> source, Func<TSource, TResult> selector){Debug.Assert(source != null);Debug.Assert(selector != null);_source = source;_selector = selector;}private int CountForDebugger => _source.Count;public override Iterator<TResult> Clone() => new SelectListIterator<TSource, TResult>(_source, _selector);public override bool MoveNext(){switch (_state){case 1:_enumerator = _source.GetEnumerator();_state = 2;goto case 2;case 2:if (_enumerator.MoveNext()){_current = _selector(_enumerator.Current);return true;}Dispose();break;}return false;}public override IEnumerable<TResult2> Select<TResult2>(Func<TResult, TResult2> selector) =>new SelectListIterator<TSource, TResult2>(_source, CombineSelectors(_selector, selector));}

Select.SpeedOpt.cs

private sealed partial class SelectListIterator<TSource, TResult> : IPartition<TResult>{public TResult[] ToArray(){int count = _source.Count;if (count == 0){return Array.Empty<TResult>();}var results = new TResult[count];for (int i = 0; i < results.Length; i++){results[i] = _selector(_source[i]);}return results;}public List<TResult> ToList(){int count = _source.Count;var results = new List<TResult>(count);for (int i = 0; i < count; i++){results.Add(_selector(_source[i]));}return results;}public int GetCount(bool onlyIfCheap){// In case someone uses Count() to force evaluation of// the selector, run it provided `onlyIfCheap` is false.int count = _source.Count;if (!onlyIfCheap){for (int i = 0; i < count; i++){_selector(_source[i]);}}return count;}public IPartition<TResult> Skip(int count){Debug.Assert(count > 0);return new SelectListPartitionIterator<TSource, TResult>(_source, _selector, count, int.MaxValue);}public IPartition<TResult> Take(int count){Debug.Assert(count > 0);return new SelectListPartitionIterator<TSource, TResult>(_source, _selector, 0, count - 1);}public TResult? TryGetElementAt(int index, out bool found){if (unchecked((uint)index < (uint)_source.Count)){found = true;return _selector(_source[index]);}found = false;return default;}public TResult? TryGetFirst(out bool found){if (_source.Count != 0){found = true;return _selector(_source[0]);}found = false;return default;}public TResult? TryGetLast(out bool found){int len = _source.Count;if (len != 0){found = true;return _selector(_source[len - 1]);}found = false;return default;}}