在.NET
2.0中，它在String内部使用该类。String仅在System名称空间之外是不可变的，因此StringBuilder可以做到这一点。

在.NET 4.0中String已更改为使用char[]。

在2.0中StringBuilder看起来像这样

public sealed class StringBuilder : ISerializable
{
    // Fields
    private const string CapacityField = "Capacity";
    internal const int DefaultCapacity = 0x10;
    internal IntPtr m_currentThread;
    internal int m_MaxCapacity;
    internal volatile string m_StringValue; // HERE ----------------------
    private const string MaxCapacityField = "m_MaxCapacity";
    private const string StringValueField = "m_StringValue";
    private const string ThreadIDField = "m_currentThread";

但是在4.0中，它看起来像这样：

public sealed class StringBuilder : ISerializable
{
    // Fields
    private const string CapacityField = "Capacity";
    internal const int DefaultCapacity = 0x10;
    internal char[] m_ChunkChars; // HERE --------------------------------
    internal int m_ChunkLength;
    internal int m_ChunkOffset;
    internal StringBuilder m_ChunkPrevious;
    internal int m_MaxCapacity;
    private const string MaxCapacityField = "m_MaxCapacity";
    internal const int MaxChunkSize = 0x1f40;
    private const string StringValueField = "m_StringValue";
    private const string ThreadIDField = "m_currentThread";

因此，显然将其从使用a更改string为使用a char[]。

编辑：更新了答案以反映.NET 4（我才刚刚发现）中的更改。