How to convert a byte array with hexadecimal contents to string with decimal in c#

It's not entirely clear what you mean.

Byte arrays just contain byte values - they're just numbers. In other words:

byte x = 0x20;
byte y = 32;

are exactly the same - they both just set the value to be 32.

Now, if you want to convert a byte array into a number, look at BitConverter and its methods like BitConverter.ToInt32. That will convert the byte array into a number (an int in that particular case) - you can then just call ToString() on the number to get a decimal representation as a string.

How many bytes is your original data? That will be a key factor in determining which BitConverter method to call. You will also need to know the endianness of the data - if BitConverter in "normal" .NET is little endian; you might be interested in the EndianBitConverter class in my MiscUtil library if your data is really big-endian.

If your array really contains the character codes of the hexadecimal representation of numbers, then you don't have to bother with decoding. As the character codes for those characters are the same in all regular encodings, you can just cast the bytes to characters.

I wrote an extension that parses a stream of characters into a stream of bytes:

static class Hex {

    public static IEnumerable<byte> ParseHex(this IEnumerable<char> chars) {
        int buffer = 0;
        bool first = true;
        foreach (char c in chars) {
            int b = (c - '0') % 32;
            if (b > 9) b -= 7;
            if (first) {
                buffer |= b << 4;
            } else {
                yield return (byte)(buffer | b);
                buffer = 0;
            }
            first = !first;
        }
        if (!first) {
                yield return (byte)buffer;
        }
    }

}

(If someone recognises part of the code, it's based on my code in this answer.)

Usage:

byte[] data = { 48, 70, 51, 67, 70, 56, 48, 55, 57, 49 };

string result = string.Join(",",
    data.Cast<char>().ParseHex().Select(b => b.ToString()).ToArray()
);

I'm a little unsure what you're trying to do. Are you trying to create a comma separated string of the decimal numbers - e.g:

byte[] hexValues = { 0x9, 0xA, 0xB, 0xC, 0xD, 0xE, 0xF };
string decimalValuesAsStringList = string.Join(",", hexValues.Select(i => i.ToString()).ToArray());

...or like this:

char[] hexCharacters = { '9', 'A', 'B', 'C', 'D', 'E', 'F' };
byte[] hexValues = hexCharacters.Select(c => Convert.ToByte(c)).ToArray();
string decimalValuesAsStringList = string.Join(",", hexValues.Select(i => Convert.ToInt32(((char)i).ToString(), 16).ToString()).ToArray());