Conversion from binary to octal in c#

Csharp program for Conversion from binary to octal. Here mentioned other language solution.

// Include namespace system
using System; 
//  C# program for
//  Convert Binary number into Octal number
public class Convert
{
    // Convert decimal number into octal
    public int octal(int number)
    {
        var result = 0;
        var multiplier = 1;
        var remainder = 0;
        while (number != 0)
        {
            remainder = number % 8;
            result = (remainder * multiplier) + result;
            multiplier *= 10;
            number = (int)(number / 8);
        }
        return result;
    }
    public void binaryToOctal(String num)
    {
        if (num.Length == 0)
        {
            // When empty binary number
            return;
        }
        // Some useful variable
        var flag = false;
        var decimalNo = 0;
        var counter = 0;
        var index = num.Length - 1;
        // We Assume that given binary number is valid
        // Here - indicates negative binary number
        // First convert binary to decimal
        // Example = 10111 => 23
        while (index >= 0)
        {
            if (num[index] == '1')
            {
                decimalNo += (1 << counter);
            }
            else
                if (num[index] != '0')
                {
                    if (index == 0 && num[index] == '-')
                    {
                        // When get negative number
                        flag = true;
                    }
                    else
                    {
                        // Not a valid binary number
                        return;
                    }
                }
            counter += 1;
            index -= 1;
        }
        // When given number is
        var output = this.octal(decimalNo);
        if (flag == true)
        {
            output = -output;
        }
        // Display given number
        Console.Write("Binary : " + num);
        // Display result
        Console.WriteLine(" Octal : " + output);
    }
    public static void Main(String[] args)
    {
        var task = new Convert();
        // Test Case
        task.binaryToOctal("1111");
        task.binaryToOctal("10111");
        task.binaryToOctal("101011");
        task.binaryToOctal("11011");
        task.binaryToOctal("-1000110");
    }
}

Output

Binary : 1111 Octal : 17
Binary : 10111 Octal : 27
Binary : 101011 Octal : 53
Binary : 11011 Octal : 33
Binary : -1000110 Octal : -106