C# has allowed numeric literals since the start but previously there was no valid syntax for separating long or confusing sequences of numeric digits.

C# 7.0 allows _ to be placed between the digits of your numeric literals to aid in readability. They are ignored entirely at compile time.

Code

Now

const int maxUploadBytes    = 10_000_000;
const int utf16bigEndian    = 0xFE_FF;
const int utf16littleEndian = 0xFF_FE;

Before

const int maxUploadBytes    = 10000000;
const int utf16bigEndian    = 0xFEFF;
const int utf16littleEndian = 0xFFFE;

Notes

• Separators can be repeated (1__000) but cannot appear at the end of a literal or next to the decimal point, exponent or type suffix
• Works with decimal, hex and Binary literals ^{C# 7.0}
• Extended to directly after the 0x or 0b prefix by Numeric literal leading underscores ^{C# 7.2}

More information

• Integer literals (C# reference)
• C# Literals and Digit Separators @ Christian Nagel