Method generating set of unique random numbers.

Random.NextUniqueSet.cs

/// <summary>Generates set of unique random numbers from 1 to specified max value.</summary>
public static ISet<int> NextUniqueSet(
    this Random random,
    int count,
    int maxValueInclusive)
{
    // Taken from https://stackoverflow.com/a/2394292/10401390

    if (maxValueInclusive < count)
    {
        throw new ArgumentException($"{nameof(maxValueInclusive)} cannot be lower than {nameof(count)}.", nameof(maxValueInclusive));
    }

    var m = count;
    var n = maxValueInclusive;
    var s = new HashSet<int>();

    for (var j = n - m + 1; j <= n; j++)
    {
        var t = random.Next(1, j);

        if (!s.Add(t))
        {
            s.Add(j);
        }
    }

    return s;
}