C#で階乗の計算（再帰処理） ref: https://qiita.com/gushwell/items/51abfaa8bd05a38a5dd0

Factorial.cs

public static long Factorial(int n) {
    if (n == 0)
        return 1L;
    return n * Factorial(n - 1);
}

Factorial60.cs

static long Factorial(int n) =>
    n == 0 ? 1L : n * Factorial(n - 1);

file0.txt

n! = n * (n-1) * (n-2) * ... * 3 * 2 * 1    (n > 0 のとき)
   = 1                                      (n = 0 のとき)

file1.txt

(n-1) * (n-2) * ... *  3 * 2 * 1

file2.txt

n! = n * (n-1)!

file3.txt

n! = n * (n-1)!    (n > 0 のとき)
   = 1             (n = 0 のとき)

file6.cs

public static long Factorial(int n) {
    checked {
        if (n == 0)
            return 1L;
        return n * Factorial(n - 1);
    }
}

file7.cs

static long Factorial(int n) =>
    checked (n == 0 ? 1L : n * Factorial(n - 1));