byte b = 255;
 string s = b.ToString("X");

------解决方案--------------------
C# code

byte b = 0;
string s = b.ToString("X2");//这样更好

------解决方案--------------------
string str="abcdefg12345678";
byte[] b = System.Text.Encoding.Default.GetBytes(str);       
System.Text.StringBuilder sb = new System.Text.StringBuilder();
foreach (byte item in b)
{
sb.Append(item.ToString("x2"));
}