implicit

class Fraction(int numerator, int denominator)
{
    public int Numerator { get; } = numerator;
    public int Denominator { get; } = denominator;
    // ...
    public static implicit operator double(Fraction f)
    {
        return f.Numerator / (double) f.Denominator;
    }
    public static implicit operator Fraction(int i)
    {
        return new Fraction(i, 1);
    }
}