Converting a byte to a binary string in c#

Just change your code to:

string yourByteString = Convert.ToString(byteArray[20], 2).PadLeft(8, '0');
// produces "00111111"

If I understand correctly, you have 20 values that you want to convert, so it's just a simple change of what you wrote.

To change single byte to 8 char string: Convert.ToString( x, 2 ).PadLeft( 8, '0' )

To change full array:

byte[] a = new byte[] { 1, 10, 100, 255, 200, 20, 2 };
string[] b = a.Select( x => Convert.ToString( x, 2 ).PadLeft( 8, '0' ) ).ToArray();
// Returns array:
// 00000010
// 00010100
// 11001000
// 11111111
// 01100100
// 00001010
// 00000001

To change your byte array to single string, with bytes separated with space:

byte[] a = new byte[] { 1, 10, 100, 255, 200, 20, 2 };
string s = string.Join( " ",
    a.Select( x => Convert.ToString( x, 2 ).PadLeft( 8, '0' ) ) );
// Returns: 00000001 00001010 01100100 11111111 11001000 00010100 00000010

if ordering of bytes is incorrect use IEnumerable.Reverse():

byte[] a = new byte[] { 1, 10, 100, 255, 200, 20, 2 };
string s = string.Join( " ",
    a.Reverse().Select( x => Convert.ToString( x, 2 ).PadLeft( 8, '0' ) ) );
// Returns: 00000010 00010100 11001000 11111111 01100100 00001010 00000001

public static string ByteArrayToString(byte[] ba)
{
    StringBuilder hex = new StringBuilder(ba.Length * 2);
    foreach (byte b in ba)
        hex.AppendFormat("{0:x2}", b);
    return hex.ToString();
}