1:

0123456789

1:

0129456783

1:

0129486753

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5:
6:

HashSet<int> numbers = new HashSet<int> ();
Random r = new Random ();
while (numbers.Count < 6) {
	numbers.Add (r.Next (10));
}
Console.WriteLine (string.Join (", ", numbers.OrderBy (x => x)));

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10:

int n = 10; int k = 6;
int[] numbers = Enumerable.Range (0, n).ToArray ();
Random r = new Random ();
for (int i = 0; i < k; i++) {
	int index = r.Next (i, n);
	int tmp = numbers [index];
	numbers [index] = numbers [i];
	numbers [i] = tmp;
}
Console.WriteLine (string.Join(", ", numbers.Take(k)));