Convert hexadecimal to binary (directly)

using System;

class HexToBinary
{
    static void Main()
    {
        string hexadec = Console.ReadLine();
        ConvertHexToBinary(hexadec);
    }
    static void ConvertHexToBinary(string input)
    {
        int len = input.Length;
        int binary = 0;
        int count = 0;
        for (int i = len-1; i >= 0; i--, count++)
        {
            switch (input[i])
            {
                case 'A':   binary = binary | (10 << 4*count); break;
                case 'B':   binary = binary | (11 << 4*count); break;
                case 'C':   binary = binary | (12 << 4*count); break;
                case 'D':   binary = binary | (13 << 4*count); break;
                case 'E':   binary = binary | (14 << 4*count); break;
                case 'F':   binary = binary | (15 << 4*count); break;
                default: binary = binary | ((input[i] - 48) << 4*count); break;
            }
        }
        Console.WriteLine(Convert.ToString(binary, 2).PadLeft(32, '0'));
    }
}