Assert.Equal("1", FizzBuzz(1));

public String FizzBuzz(int n) {
    return 1.ToString();
}

Assert.Equal("2", FizzBuzz(2));

public String FizzBuzz(int n) {
    return n.ToString();
}

Assert.Equal("Fizz", FizzBuzz(3));

public String FizzBuzz(int n) {
    if (n == 3)
        return "Fizz";
    return n.ToString();
}

Assert.Equal("Fizz", FizzBuzz(6));

public String FizzBuzz(int n) {
    if (n % 3 == 0)
        return "Fizz";
    return n.ToString();
}

Assert.Equal("Buzz", FizzBuzz(5));

public String FizzBuzz(int n) {
    if (n % 3 == 0)
        return "Fizz";
    if (n == 5)
        return "Buzz"
    return n.ToString();
}

Assert.Equal("Buzz", FizzBuzz(10));

public String FizzBuzz(int n) {
    if (n % 3 == 0)
        return "Fizz";
    if (n % 5 == 0)
        return "Buzz"
    return n.ToString();
}

private bool isDivisibleBy(int divisor, into input) {
    return (input % divisor == 0);
}

public String FizzBuzz(int n) {
    if (isDivisibleBy(3, n))
        return "Fizz";
    if (isDivisibleBy(5,n))
        return "Buzz"
    return n.ToString();
}

Assert.Equal("FizzBuzz", FizzBuzz(15));

public String FizzBuzz(int n) {
    if (isDivisibleBy(3, n) && isDivisibleBy(5, n))
        return "FizzBuzz";
    if (isDivisibleBy(3, n))
        return "Fizz";
    if (isDivisibleBy(5,n))
        return "Buzz"
    return n.ToString();
}

public String FizzBuzz(int n) {

    var isDivisibleBy3 = isDivisibleBy(3, n);
    var isDivisibleBy5 = isDivisibleBy(5, n);

    if ( isDivisibleBy3 && isDivisibleBy5 )
        return "FizzBuzz";
    if ( isDivisibleBy3 )
        return "Fizz";
    if ( isDivisibleBy5 )
        return "Buzz"
    return n.ToString();
}

public String FizzBuzz(int n) {

    if (n < 1)
        throw new InvalidArgException("n must be >= 1);

    var isDivisibleBy3 = isDivisibleBy(3, n);
    var isDivisibleBy5 = isDivisibleBy(5, n);

    if ( isDivisibleBy3 && isDivisibleBy5 )
        return "FizzBuzz";
    if ( isDivisibleBy3 )
        return "Fizz";
    if ( isDivisibleBy5 )
        return "Buzz"
    return n.ToString();
}