.cs

public class Buff
{
    public float MeleeRangeFactor { get; protected set; } = 1f;
    public float KnockbackFactor { get; protected set; } = 1f;
    public float DamageFactor { get; protected set; } = 1f;

    public static Buff Knockback = new Buff() { KnockbackFactor = 1.5f };
}

abstract class Character {
    private readonly Dictionary<Buff, TimeSpan> Buffs;

    public void Buff(Buff buff, TimeSpan timeSpan)
    {
        if (Buffs.ContainsKey(buff))
        {
            Buffs[buff] += timeSpan;
        }
        else
        {
            Buffs.Add(buff, timeSpan);
        }
    }

    private void AgeBuffs(CanvasTimingInformation dt)
    {
        foreach (var buff in Buffs.Keys.ToList())
        {
            Buffs[buff] -= dt.ElapsedTime;
            if (Buffs[buff] <= TimeSpan.Zero)
            {
                Buffs.Remove(buff);
            }
        }
    }

    public abstract float BaseMeleeRange { get; }
    public abstract float BaseKnockback { get; }
    public abstract int BaseDamage { get; }
    
    public float MeleeRange => Buffs.Keys.Aggregate(BaseMeleeRange, (r, b) => r * b.MeleeRangeFactor);
    public float Knockback => Buffs.Keys.Aggregate(BaseKnockback, (r, b) => r * b.KnockbackFactor);
    public int Damage => (int)Buffs.Keys.Aggregate((float)BaseDamage, (r, b) => r * b.DamageFactor);
}