integer conversion problem

The input string has to represent the number in decimal for Convert.ToInt16(String)[^] to work.
Its output is an integer. That has neither decimal nor hexadecimal format. It's just the correct value.
You can convert it into a string again to display it to the user. Use the ToString() method[^] for that.
If you want the output string to be in hexadecimal format, use

Int16 number = 255;
string output = number.ToString("X");

for that.
 
If you want to convert a string that represents a number in hexadecimal form to an integer, use

Int16 number;
bool success = Int16.TryParse(
    "0xFF",
    NumberStyles.HexNumber,
    CultureInfo.InvariantCulture,
    out value
);