Finished first C# code

Factorial/Factorial.cs

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+using System;
+
+public class Functions
+{
+    public static long Factorial(int n)
+    {
+        // Test for invalid input.
+        if ((n < 0) || (n > 20))
+        {
+            return -1;
+        }
+
+        // Calculate the factorial iteratively rather than recursively.
+        long tempResult = 1;
+        for (int i = 1; i <= n; i++)
+        {
+            tempResult *= i;
+        }
+        return tempResult;
+    }
+}
+
+class MainClass
+{
+    static int Main(string[] args)
+    {
+        // Test if input arguments were supplied.
+        if (args.Length == 0)
+        {
+            Console.WriteLine("Please enter a numeric argument.");
+            Console.WriteLine("Usage: Factorial <num>");
+            return 1;
+        }
+
+        // Try to convert the input arguments to numbers. This will throw
+        // an exception if the argument is not a number.
+        // num = int.Parse(args[0]);
+        int num;
+        bool test = int.TryParse(args[0], out num);
+        if (!test)
+        {
+            Console.WriteLine("Please enter a numeric argument.");
+            Console.WriteLine("Usage: Factorial <num>");
+            return 1;
+        }
+
+        // Calculate factorial.
+        long result = Functions.Factorial(num);
+
+        // Print result.
+        if (result == -1)
+            Console.WriteLine("Input must be >= 0 and <= 20.");
+        else
+            Console.WriteLine($"The Factorial of {num} is {result}.");
+
+        return 0;
+    }
+}
+// If 3 is entered on command line, the
+// output reads: The factorial of 3 is 6.

Factorial/compile.txt

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+To compile Factorial.cs.
+
+In Windows, open Developer PowerShell for VS 2019, and enter command `csc Factorial.cs`