List<br>JIT 할당 전략

defaultCapacity   new List      4。

_items   List      ，List         。

   size  2 （ newSize = oldSize*2）

https://github.com/dotnet

https://github.com/dotnet/corefx/tree/master/src

https://github.com/dotnet/corefx/tree/master/src/System.Collections/src/System/Collections/Generic

    ，  


        static readonly T[]  _emptyArray = new T[0];        
            
        // Constructs a List. The list is initially empty and has a capacity
        // of zero. Upon adding the first element to the list the capacity is
        // increased to 16, and then increased in multiples of two as required.
        public List() {
            _items = _emptyArray;
        }
    
        // Constructs a List with a given initial capacity. The list is
        // initially empty, but will have room for the given number of elements
        // before any reallocations are required.
        // 
        public List(int capacity) {
            if (capacity < 0) ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.capacity, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
            Contract.EndContractBlock();
 
            if (capacity == 0)
                _items = _emptyArray;
            else
                _items = new T[capacity];
        }


        // Adds the given object to the end of this list. The size of the list is
        // increased by one. If required, the capacity of the list is doubled
        // before adding the new element.
        //
        public void Add(T item) {
            if (_size == _items.Length) EnsureCapacity(_size + 1);
            _items[_size++] = item;
            _version++;
        }
 


        // Ensures that the capacity of this list is at least the given minimum
        // value. If the currect capacity of the list is less than min, the
        // capacity is increased to twice the current capacity or to min,
        // whichever is larger.
        private void EnsureCapacity(int min) {
            if (_items.Length < min) {
                int newCapacity = _items.Length == 0? _defaultCapacity : _items.Length * 2;
                // Allow the list to grow to maximum possible capacity (~2G elements) before encountering overflow.
                // Note that this check works even when _items.Length overflowed thanks to the (uint) cast
                if ((uint)newCapacity > Array.MaxArrayLength) newCapacity = Array.MaxArrayLength;
                if (newCapacity < min) newCapacity = min;
                Capacity = newCapacity;
            }
        }


       // Inserts the elements of the given collection at a given index. If
        // required, the capacity of the list is increased to twice the previous
        // capacity or the new size, whichever is larger.  Ranges may be added
        // to the end of the list by setting index to the List's size.
        //
        public void InsertRange(int index, IEnumerable collection) {
            if (collection==null) {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection);
            }
            
            if ((uint)index > (uint)_size) {
                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_Index);
            }
            Contract.EndContractBlock();
 
            ICollection c = collection as ICollection;
            if( c != null ) {    // if collection is ICollection
                int count = c.Count;
                if (count > 0) {
                    EnsureCapacity(_size + count);
                    if (index < _size) {
                        Array.Copy(_items, index, _items, index + count, _size - index);
                    }
                    
                    // If we're inserting a List into itself, we want to be able to deal with that.
                    if (this == c) {
                        // Copy first part of _items to insert location
                        Array.Copy(_items, 0, _items, index, index);
                        // Copy last part of _items back to inserted location
                        Array.Copy(_items, index+count, _items, index*2, _size-index);
                    }
                    else {
                        T[] itemsToInsert = new T[count];
                        c.CopyTo(itemsToInsert, 0);
                        itemsToInsert.CopyTo(_items, index);                    
                    }
                    _size += count;
                }                
            }
            else {
                using(IEnumerator en = collection.GetEnumerator()) {
                    while(en.MoveNext()) {
                        Insert(index++, en.Current);                                    
                    }                
                }
            }
            _version++;            
        }

// ==++==
// 
//   Copyright (c) Microsoft Corporation.  All rights reserved.
// 
// ==--==
/*============================================================
**
** Class:  List
** 
** Microsoft
**
** Purpose: Implements a generic, dynamically sized list as an 
**          array.
**
** 
===========================================================*/
namespace System.Collections.Generic {
 
    using System;
    using System.Runtime;
    using System.Runtime.Versioning;
    using System.Diagnostics;
    using System.Diagnostics.Contracts;
    using System.Collections.ObjectModel;
    using System.Security.Permissions;
 
    // Implements a variable-size List that uses an array of objects to store the
    // elements. A List has a capacity, which is the allocated length
    // of the internal array. As elements are added to a List, the capacity
    // of the List is automatically increased as required by reallocating the
    // internal array.
    // 
    [DebuggerTypeProxy(typeof(Mscorlib_CollectionDebugView<>))]
    [DebuggerDisplay("Count = {Count}")]
    [Serializable]
    public class List : IList, System.Collections.IList, IReadOnlyList
    {
        private const int _defaultCapacity = 4;
 
        private T[] _items;
        [ContractPublicPropertyName("Count")]
        private int _size;
        private int _version;
        [NonSerialized]
        private Object _syncRoot;
        
        static readonly T[]  _emptyArray = new T[0];        
            
        // Constructs a List. The list is initially empty and has a capacity
        // of zero. Upon adding the first element to the list the capacity is
        // increased to 16, and then increased in multiples of two as required.
        public List() {
            _items = _emptyArray;
        }
    
        // Constructs a List with a given initial capacity. The list is
        // initially empty, but will have room for the given number of elements
        // before any reallocations are required.
        // 
        public List(int capacity) {
            if (capacity < 0) ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.capacity, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
            Contract.EndContractBlock();
 
            if (capacity == 0)
                _items = _emptyArray;
            else
                _items = new T[capacity];
        }
    
        // Constructs a List, copying the contents of the given collection. The
        // size and capacity of the new list will both be equal to the size of the
        // given collection.
        // 
        public List(IEnumerable collection) {
            if (collection==null)
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection);
            Contract.EndContractBlock();
 
            ICollection c = collection as ICollection;
            if( c != null) {
                int count = c.Count;
                if (count == 0)
                {
                    _items = _emptyArray;
                }
                else {
                    _items = new T[count];
                    c.CopyTo(_items, 0);
                    _size = count;
                }
            }    
            else {                
                _size = 0;
                _items = _emptyArray;
                // This enumerable could be empty.  Let Add allocate a new array, if needed.
                // Note it will also go to _defaultCapacity first, not 1, then 2, etc.
                
                using(IEnumerator en = collection.GetEnumerator()) {
                    while(en.MoveNext()) {
                        Add(en.Current);                                    
                    }
                }
            }
        }
        
        // Gets and sets the capacity of this list.  The capacity is the size of
        // the internal array used to hold items.  When set, the internal 
        // array of the list is reallocated to the given capacity.
        // 
        public int Capacity {
            get {
                Contract.Ensures(Contract.Result() >= 0);
                return _items.Length;
            }
            set {
                if (value < _size) {
                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.value, ExceptionResource.ArgumentOutOfRange_SmallCapacity);
                }
                Contract.EndContractBlock();
 
                if (value != _items.Length) {
                    if (value > 0) {
                        T[] newItems = new T[value];
                        if (_size > 0) {
                            Array.Copy(_items, 0, newItems, 0, _size);
                        }
                        _items = newItems;
                    }
                    else {
                        _items = _emptyArray;
                    }
                }
            }
        }
            
        // Read-only property describing how many elements are in the List.
        public int Count {
            get {
                Contract.Ensures(Contract.Result() >= 0);
                return _size; 
            }
        }
 
        bool System.Collections.IList.IsFixedSize {
            get { return false; }
        }
 
            
        // Is this List read-only?
        bool ICollection.IsReadOnly {
            get { return false; }
        }
 
        bool System.Collections.IList.IsReadOnly {
            get { return false; }
        }
 
        // Is this List synchronized (thread-safe)?
        bool System.Collections.ICollection.IsSynchronized {
            get { return false; }
        }
    
        // Synchronization root for this object.
        Object System.Collections.ICollection.SyncRoot {
            get { 
                if( _syncRoot == null) {
                    System.Threading.Interlocked.CompareExchange