在上面介绍过栈（Stack）的存储结构，接下来介绍另一种存储结构字典（Dictionary）。 字典（Dictionary）里面的每一个元素都是一个键值对(由二个元素组成：键和值) 键必须是唯一的,而值不需要唯一的,键和值都可以是任何类型。字典(Dictionary)是常用于查找和排序的列表。
接下来看一下Dictionary的部分方法和类的底层实现代码：
1.Add：将指定的键和值添加到字典中。
public void Add(TKey key, TValue value) { Insert(key, value, true); }private void Insert(TKey key, TValue value, bool add) { if( key == null ) { ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key); } if (buckets == null) Initialize(0); int hashCode = comparer.GetHashCode(key) & 0x7FFFFFFF; int targetBucket = hashCode % buckets.Length; #if FEATURE_RANDOMIZED_STRING_HASHING int collisionCount = 0; #endif for (int i = buckets[targetBucket]; i >= 0; i = entries[i].next) { if (entries[i].hashCode == hashCode && comparer.Equals(entries[i].key, key)) { if (add) { ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_AddingDuplicate); } entries[i].value = value; version++; return; } #if FEATURE_RANDOMIZED_STRING_HASHING collisionCount++; #endif } int index; if (freeCount > 0) { index = freeList; freeList = entries[index].next; freeCount--; } else { if (count == entries.Length) { Resize(); targetBucket = hashCode % buckets.Length; } index = count; count++; } entries[index].hashCode = hashCode; entries[index].next = buckets[targetBucket]; entries[index].key = key; entries[index].value = value; buckets[targetBucket] = index; version++; #if FEATURE_RANDOMIZED_STRING_HASHING if(collisionCount > HashHelpers.HashCollisionThreshold && HashHelpers.IsWellKnownEqualityComparer(comparer)) { comparer = (IEqualityComparer<TKey>) HashHelpers.GetRandomizedEqualityComparer(comparer); Resize(entries.Length, true); } #endif }