Factorial C#

  public class FactorialTest
  {   
      [Fact]
      public void Factorial0()
      {
          Factorial(0).ShouldEqual(1);
      }

      [Fact]
      public void Factorial1()
      {
          Factorial(1).ShouldEqual(1);
      }

      [Fact]
      public void Factorial2()
      {
          Factorial(2).ShouldEqual(2);
      }

      [Fact]
      public void Factorial3()
      {
          Factorial(3).ShouldEqual(6);
      }

      [Fact]
      public void Factorial4()
      {
          Factorial(4).ShouldEqual(24);
      }

      [Fact]
      public void Factorial5()
      {
          Factorial(5).ShouldEqual(120);
      }

      [Fact]
      public void Factorial100()
      {
          Factorial(100)
              .ShouldEqual(
                  BigInteger.Parse(
                      "93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000"));
      }

      public BigInteger Factorial(BigInteger argument)
      {
          return argument > 1 ? argument*Factorial(--argument) : 1;
      }
  }