I think Naresh's answers gave you what you need, but I'll add a few comments:
1. a number is independent of its representation in .NET. if you write:
int Y = 0xAA + 0xBB;
2. you can easily get the hex representation of a number as a string:
string hexOfY = 357.ToString("X4");
3. going from a TextBox string to a hex value is easy:
string HexFromTextBox = Convert.ToInt32(textBox1.Text).ToString("X4");
Int16 tbValue;
bool okay16BitValue = Int16.TryParse(textBox1.Text, out tbValue);
if (! okay16BitValue) throw new ArgumentException("TextBox string not a 16-bit integer");
4. to get the high and low bytes of a 16-bit number > 255 and < 65536:
byte highByte = (byte)(357 >> 8);
byte lowByte = (byte)(357 & 0xff);