C#:Dictionary<key,key>的Clear到底有多多块

有时我们希望复用一个Dictionary的时候，可以Clear（）也可以，直接new一个新的对象，

两者到底是哪个划算呢？

我们看程序：

  private void Experiment()
        {

            System.Diagnostics.Stopwatch stopwatch = new Stopwatch();
            stopwatch.Start();

            //Dictionary<int, int> aa = new Dictionary<int, int>();

            for (var loop = 0; loop < 30000; loop++)
            {
                Dictionary<int, int> aa = new Dictionary<int, int>();
              
                
                for (var i = 0; i < 10000; i++)
                { aa.Add(i,i); }

                //aa.Clear();
                

            }

            stopwatch.Stop();

            Console.WriteLine("All timeL:"+stopwatch.Elapsed.TotalSeconds);
        }

数据结果如下：

使用Clear（）时间为6秒。而使用new object的方法，时间为20秒，我觉得只所以这样

是因为NEW object的方案中，需要执行很多次Resize的操作。

这些东西看基类代码如下：

Add操作如下：

  public void Add(TKey key, TValue value)
        {
            this.Insert(key, value, true);
        }

其调用Insert操作，代码如下:

  private void Insert(TKey key, TValue value, bool add)
        {
            int freeList;
            if (key == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);
            }
            if (this.buckets == null)
            {
                this.Initialize(0);
            }
            int num = this.comparer.GetHashCode(key) & 0x7fffffff;
            int index = num % this.buckets.Length;
            for (int i = this.buckets[index]; i >= 0; i = this.entries[i].next)
            {
                if ((this.entries[i].hashCode == num) && this.comparer.Equals(this.entries[i].key, key))
                {
                    if (add)
                    {
                        ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_AddingDuplicate);
                    }
                    this.entries[i].value = value;
                    this.version++;
                    return;
                }
            }
            if (this.freeCount > 0)
            {
                freeList = this.freeList;
                this.freeList = this.entries[freeList].next;
                this.freeCount--;
            }
            else//散列已满
            {
                if (this.count == this.entries.Length)
                {
                    this.Resize();//为散列重新开辟一块儿内存空间
                    index = num % this.buckets.Length;
                }
                freeList = this.count;
                this.count++;
            }
            this.entries[freeList].hashCode = num;
            this.entries[freeList].next = this.buckets[index];
            this.entries[freeList].key = key;
            this.entries[freeList].value = value;
            this.buckets[index] = freeList;
            this.version++;
        }

Resize（）如下：

 private void Resize()
        {
            int prime = HashHelpers.GetPrime(this.count * 2);
            int[] numArray = new int[prime];//开辟新的数组
            for (int i = 0; i < numArray.Length; i++)
            {
                numArray[i] = -1;
            }
            Entry<TKey, TValue>[] destinationArray = new Entry<TKey, TValue>[prime];//开辟新的数组
            Array.Copy(this.entries, 0, destinationArray, 0, this.count);
            for (int j = 0; j < this.count; j++)
            {
                int index = destinationArray[j].hashCode % prime;
                destinationArray[j].next = numArray[index];
                numArray[index] = j;
            }
            this.buckets = numArray;
            this.entries = destinationArray;
        }

下个总结吧：凡是重复用可能为resize(）的结构，比如dictionary、Arraylist，使用clear（）的方法比较好。因为resize(）很浪费时间