[Dykam]

For others interested in the source of StringBuilder: https://github.com/mono/mono/blob/master/mcs/class/corlib/Sy...

Note that the actual magic consists of internal methods in String.cs:

- FormatHelper https://github.com/mono/mono/blob/master/mcs/class/corlib/Sy...

- InternalSetChar https://github.com/mono/mono/blob/master/mcs/class/corlib/Sy...

- CharCopy https://github.com/mono/mono/blob/master/mcs/class/corlib/Sy...