a very annoing exception

Hi , I have the following code: -> in class Calculator: ============== private void radionBtnHexa_CheckedChanged(object sender, System.EventArgs e) { if (radioBtnDecimal.Checked == true) { String str = textBoxResult.Text.Substring(0,textBoxResult.Text.Length-1); textBoxResult.Text = myConvert.ToHex(str); } else { if (radioBtnOctal.Checked == true) textBoxResult.Text = myConvert.ToDecimal(textBoxResult.Text,8); else if (radioBtnBinary.Checked == true) textBoxResult.Text = myConvert.ToDecimal(textBoxResult.Text,2); textBoxResult.Text = myConvert.ToHex(textBoxResult.Text); } // ... } -> in class Converting: ================ public String ToHex(String str) { String hexStr; int counter,reminder; int hexInt; hexStr = str; //hexStr = ToDecimal(str,16); hexInt = Int32.Parse(hexStr); counter=1; hexStr=""; while (hexInt+15>Math.Pow(16,counter-1)) { reminder=(int)(hexInt%Math.Pow(16,counter)); reminder=(int)(reminder/Math.Pow(16,counter-1)); if (reminder<=9) { hexStr=hexStr+(char)(reminder+48); } else { hexStr=hexStr+(char)(reminder+55); } hexInt-=reminder; counter++; } return ReverseString(hexStr); } and I get the following exception: "Input string was not in a correct format" ! :( Any ideas ?!? Please help me because I must get to the bottom of this ! It's bugging me for days ! :( Any thing that would help appreciated ! Thankx, Adisor