Equals to this in C# String s = String.join(String.Empty,

 public static void main(String[] args) {
        System.out.println(doIt(new byte[] {1} ));
        System.out.println(doIt(new byte[] {2} ));
        System.out.println(doIt(new byte[] {1,1} ));
        System.out.println(doIt(new byte[] {1,2} ));
    }

    public static String paddLeft(String source, char padding, int length) {
        StringBuilder sb = new StringBuilder();
        sb.append(source);
        while(sb.length() < length) {
            sb.insert(0, padding);
        }
        return sb.toString();
    }

    public static String convertToString(int number) {
        return Integer.toBinaryString(number);
    }


    public static String doIt(byte[] bytes) {
        StringBuilder sb = new StringBuilder();
        for(byte b : bytes) {
            sb.append(paddLeft(convertToString(b), '0', 8));
        }
        return sb.toString();
    }