To convert decimal number to binary format use base 2

Int32 Number = 15;
Console.WriteLine(Convert.ToString(Number, 2));  //OUTPUT : 1111

To convert decimal number to octal format use base 8

int Number = 15;
Console.WriteLine(Convert.ToString(Number, 8));  //OUTPUT : 17

To convert decimal number to hexadecimal format use base 16

var Number = 15;
Console.WriteLine(Convert.ToString(Number, 16));  //OUTPUT : f