对于StringBuilder自动分配空间的疑惑

Question

有很多朋友提出，StringBuilder中Capacity的值是成倍扩充的，但是我今天使用StringBuilder的时候发现了在某些情况下其Capacity不是成倍扩充的，望高手能给出解释，谢谢！下面是具体代码及执行结果
System.Text.StringBuilder sb = new System.Text.StringBuilder();
Console.WriteLine("Capacity:" + sb.Capacity);
Console.WriteLine("Length:" + sb.Length);
sb.Append('1',16);
Console.WriteLine("Capacity:" + sb.Capacity);
Console.WriteLine("Length:" + sb.Length);
sb.Append('2',32);
Console.WriteLine("Capacity:" + sb.Capacity);
Console.WriteLine("Length:" + sb.Length);
sb.Append('3',64);
Console.WriteLine("Capacity:" + sb.Capacity);
Console.WriteLine("Length:" + sb.Length);
try
{
sb.Remove(0,sb.Length);
Console.WriteLine("Capacity:" + sb.Capacity);
Console.WriteLine("Length:" + sb.Length);
sb.Capacity = 1;
sb.Append('a',2);
Console.WriteLine("Capacity:" + sb.Capacity);
Console.WriteLine("Length:" + sb.Length);
sb.Append('b',4);
Console.WriteLine("Capacity:" + sb.Capacity);
Console.WriteLine("Length:" + sb.Length);
sb.Append('c',6);
Console.WriteLine("Capacity:" + sb.Capacity);
Console.WriteLine("Length:" + sb.Length);
}
catch(ArgumentOutOfRangeException e)
{
Console.WriteLine(e.Message);
return;
}
catch(Exception ex)
{
Console.WriteLine(ex.Message);
return;
}
Console.ReadLine();
其执行结果是：
Capacity:16     Length:0    
Capacity:16     Length:16    
Capacity:49     Length:48    
Capacity:113     Length:112
Capacity:113     Length:0    
Capacity:3      Length:2    
Capacity:7     Length:6
Capacity:14     Length:12

Answer 1

测试代码有问题？？
这是stringbuilder添加元素所进行的操作private string GetNewString(string currentString, int requiredLength)
{
      requiredLength++;
      if (requiredLength < 0)
      {
            throw new OutOfMemoryException();
      }
      if (requiredLength > this.m_MaxCapacity)
      {
            throw new ArgumentOutOfRangeException(Environment.GetResourceString("ArgumentOutOfRange_NegativeCapacity"), "requiredLength");
      }
      int num1 = currentString.Capacity * 2;//注意这里
      if (num1 < requiredLength)//还有这里
      {
            num1 = requiredLength;
      }
      if (num1 > this.m_MaxCapacity)
      {
            num1 = this.m_MaxCapacity;
      }
      if (num1 <= 0)
      {
            throw new ArgumentOutOfRangeException(Environment.GetResourceString("ArgumentOutOfRange_NegativeCapacity"));
      }
      return string.GetStringForStringBuilder(currentString, num1);
}

Answer 2

但是Capacity的大小为什么有113这些呢

Answer 3

你最好用反编译软件看看StringBuilder的实现，上面那些代码就是我用工具反编译的的，
因为代码比较多一些，我不好往上全贴，简单的说就是stringbuilder最后一位需要加一个'\0'来代表字符串结束。
internal unsafe void AppendInPlace(char value, int repeatCount, int currentLength)
{
      int num1 = currentLength + repeatCount;
      fixed (char* chRef1 = &this.m_firstChar)
      {
            int num2 = currentLength;
            while (num2 < num1)
            {
                  chRef1[num2] = value;
                  num2++;
            }
            chRef1[num2] = '\0';//这里      }
      this.m_stringLength = num1;
}