The receiver is my website, the sender is the same but the medium is noisy, a user. He will read an alphanumeric code of length 6 and later input the same code to identify a resource. A good use for a error correcting code, I thought, and rather than do the research I thought I'd just put the question out there. Or I might be going about it the wrong way, since the situation is rather like sending a perfect dictionary along with every transmission.

The requirements on the code are simply:

1. 6 alphanumeric digits, to start with until I run out, anyway.
2. If the user gets it wrong I should still be able to identify the right resource.
3. No resource is preferable to the wrong one.
4. Easy to code or have free libraries for .net

Any suggestions?