int to hex string

Try C# string interpolation introduced in C# 6:

var id = 100;
var hexid = $"0x{id:X}";

hexid value:

"0x64"

Use ToString("X4").

The 4 means that the string will be 4 digits long.

Reference: The Hexadecimal ("X") Format Specifier on MSDN.

Try the following:

ToString("X4")

See The X format specifier on MSDN.