C# 7.0 gave us digit separators C# 7.0 but did not permit them to be used as the first digit in a literal.
C# 7.2 permits the _ character to also be used before the first numeric digit.
Code
C#
const int utf16bigEndian = 0x_FE_FF;
const int utf16littleEndian = 0x_FF_FE;C#
const int utf16bigEndian = 0xFE_FF;
const int utf16littleEndian = 0xFF_FE;