StringBuilder方式

var str = DateTime.Now.ToString();
var encode = Encoding.UTF8;
var bytes = encode.GetBytes(str);
StringBuilder ret = new StringBuilder();
foreach (byte b in bytes)
{
//{0:X2} 大写
ret.AppendFormat("{0:x2}", b);
}
var hex = ret.ToString();
Console.WriteLine(hex);

16进制字符串转换 byte[]

var inputByteArray = new byte[hex.Length / 2];
for (var x = 0; x < inputByteArray.Length; x++)
{
    var i = Convert.ToInt32(hex.Substring(x * 2, 2), 16);
    inputByteArray[x] = (byte)i;
}
str = encode.GetString(inputByteArray);
Console.WriteLine(str);

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C# byte[] 转换16进制字符串