Non-repetitive random number

Question

To generate Random numbers from 1- 20 I need to pick selective and it should not be repetitive.

How to do this in C#

Note I need to loop through as like this

Answer 1

From MSDN

"One way to improve randomness is to make the seed value time-dependent."

Another fact

you should "create one Random to generate many random numbers over time." This will enhance the random generation