Program.cs

using System;

while (true)
{
    Console.Write("Enter up to 8 binary digits, enter [e]xit to close the program: ");
    string input = Console.ReadLine()!;

    if (input == "e" || input == "exit")
    {
        break;
    }

    if (input.Length <= 8 && input.Length > 0)
    {
        int convert = ConvertBin2Dec(input);
        Console.WriteLine($"Decimal: {convert} ");
    }
    else
    {
        Console.WriteLine("Not a binary number");
    }

}
int ConvertBin2Dec(string input)
{
    int convert = 0;
    foreach (char digit in input)
    {
        if (digit == '0' || digit == '1')
        {
            convert <<= 1;
            convert += digit - '0';
        }
        else
        {
            Console.WriteLine($"{digit} is not 0 or 1");
            convert = 0;
        }
    }
    return convert;
}