StringBuffer.cs

public class StringBuffer
{
    private Queue<string> _buffer = new Queue<string>();

    private int _offset;

    public int Length;

    public void Append(string content)
    {
        _buffer.Enqueue(content);
        Length += content.Length;
    }

    public char this[int index]
    {
        get
        {
            var i = index - _offset;

            foreach (var element in _buffer)
            {
                if (i >= element.Length)
                {
                    i -= element.Length;
                    continue;
                }

                return element[i];
            }

            throw new IndexOutOfRangeException();
        }
    }

    public void Clear()
    {
        _buffer.Clear();
        Length = 0;
        _offset = 0;
    }

    public void Trim(int index)
    {
        index = index - _offset - 1;

        while (index > 0)
        {
            var str = _buffer.Peek();

            index -= str.Length;

            if (index > 0)
            {
                _offset += str.Length;
                _buffer.Dequeue();
            }
        }
    }
}