Die.cs

public sealed class Die
{
    private readonly int m_numberOfSides;
    private readonly IRandomNumberGenerator m_rng;

    public Die(int numberOfSides, IRandomNumberGenerator rng) {
        if (0 > numberOfSides) {
            throw new ArgumentOutOfRangeException(actualValue: numberOfSides, message: "value must be greater than zero", paramName: nameof(numberOfSides));
        }

        m_numberOfSides = --numberOfSides;
        m_rng = rng;
    }

    public int Next() => m_rng.NextInt32(0, m_numberOfSides);
}
public sealed class Dice
{
    private readonly Die[] m_dice;

    public Dice(params Die[] dice) {
        m_dice = dice;
    }

    public long Throw() {
        var result = 0L;

        foreach (var die in m_dice) {
            result += die.Next();
        }

        return result;
    }
}