Gutmann method uses 35 passes over the region to be deleted, 8 of them are Pseudo-random. Considered by some to be highly effective for preventing file recovery.
Requires .NET Framework 4.0
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Systems & Safety Establishment
https://sarab.me
Source Code available in c# with comments.
Reference: http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html