How to optimize string allocation?

Here is the code...

private char[] buffer = new char[32];

internal string ExtractString(int len)
{
    if (len > buffer.Length)
    {
        // the buffer is too small, make it bigger
        Array.Resize(ref buffer, buffer.Length * 2);
    }

    for (int i = 0; i < len; i++)
        buffer[i] = (char)_data[_currentIndex++];

    return new string(buffer, 0, len);
}

The profiler shows that the hot spot of this method is "new string(buffer, 0, len)". Anyway to optimize this?

I am thinking to keep the result string in Dictionary and then searching buffer[] in Dictionary to not allocate the string again if it does exist in the Dictionary. Would this help improve the performance? but I am not sure how to search buffer[] in Dictionary with vary length, any example?

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atanai