StringBuilder的內(nèi)部數(shù)據(jù)結(jié)構(gòu)如下：

StringBuilder.png

添加字符串：
StringBuilder Append(String value)
當 value 的大小可以放入當前 sb淘菩，則直接字符串拷貝。

         unsafe {
             fixed (char* valuePtr = value)
             fixed (char* destPtr = &chunkChars[chunkLength])
                 string.wstrcpy(destPtr, valuePtr, valueLen);
         }

否則寻行，需要擴容。
擴容代碼：

        int newBlockLength = Math.Max(minBlockCharCount, Math.Min(Length, MaxChunkSize));

        // Copy the current block to the new block, and initialize this to point at the new buffer. 
        m_ChunkPrevious = new StringBuilder(this);
        m_ChunkOffset += m_ChunkLength;
        m_ChunkLength = 0;

        // Check for integer overflow (logical buffer size > int.MaxInt)
        if (m_ChunkOffset + newBlockLength < newBlockLength)
        {
            m_ChunkChars = null;
            throw new OutOfMemoryException();
        }
        m_ChunkChars = new char[newBlockLength];

先將當前 chunk 填滿醋奠，然后將當前 chunk 的內(nèi)容填入一個新建的 sb莫换，并作為前置，然后重置當前sb响蕴，再拷貝剩余字符串內(nèi)容谆焊。
每發(fā)生一次擴容時，都會多一次前置操作浦夷，從而形成了單鏈表辖试，并且由m_ChunkOffset標識出每個sb的首字符在總字符串的位置。

輸出
public override String ToString()

了解了如何添加军拟，再理解如何輸出就不難了剃执。

        string ret = string.FastAllocateString(Length);
        StringBuilder chunk = this;
        unsafe {
            fixed (char* destinationPtr = ret)
            {
                do
                {
                    if (chunk.m_ChunkLength > 0)
                    {
                        // Copy these into local variables so that they are stable even in the presence of ----s (hackers might do this)
                        char[] sourceArray = chunk.m_ChunkChars;
                        int chunkOffset = chunk.m_ChunkOffset;
                        int chunkLength = chunk.m_ChunkLength;

                        // Check that we will not overrun our boundaries. 
                        if ((uint)(chunkLength + chunkOffset) <= ret.Length && (uint)chunkLength <= (uint)sourceArray.Length)
                        {
                            fixed (char* sourcePtr = sourceArray)
                                string.wstrcpy(destinationPtr + chunkOffset, sourcePtr, chunkLength);
                        }
                        else
                        {
                            throw new ArgumentOutOfRangeException("chunkLength", Environment.GetResourceString("ArgumentOutOfRange_Index"));
                        }
                    }
                    chunk = chunk.m_ChunkPrevious;
                } while (chunk != null);
            }
        }

從后向前，按照offset指示的起始位置懈息，將內(nèi)容拷貝至總的字符串肾档。

可以看出，由于是一次性分配了所需要的所有內(nèi)存辫继，所以在【很多次循環(huán)】場景下怒见，比String.Concat效率要高出很多。