Why do enum permissions often have 0, 1, 2, 4 values?

Question

Why are people always using enum values like 0, 1, 2, 4, 8 and not 0, 1, 2, 3, 4?

Has this something to do with bit operations, etc.?

Answer 1

Because these values represent unique bit locations in binary:

1 == binary 00000001
2 == binary 00000010
4 == binary 00000100

etc., so

1 | 2 == binary 00000011

EDIT:

3 == binary 00000011

3 in binary is represented by a value of 1 in both the ones place and the twos place. It is actually the same as the value 1 | 2. So when you are trying to use the binary places as flags to represent some state, 3 isn't usually meaningful (unless there is a logical value that actually is the combination of the two)

For further clarification, you might want to extend your example enum as follows:

[Flags]
public Enum Permissions
{
  None = 0,   // Binary 0000000
  Read = 1,   // Binary 0000001
  Write = 2,  // Binary 0000010
  Delete = 4, // Binary 0000100
  All = 7,    // Binary 0000111
}

Therefore in I have Permissions.All, I also implicitly have Permissions.Read, Permissions.Write, and Permissions.Delete