C# has allowed numeric constants 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 anywhere you like within your numeric literals to aid in readability. They are ignored entirely at compile time.

Code

Now

const utf16littleEndian = 0xFE_FF;
const utf16bigEndian    = 0xFF_FE;

Before

const utf16littleEndian = 0xFEFF;
const utf16bigEndian    = 0xFFFE;

Notes

• Extended to leading position by Numeric literal leading underscores ^{C# 7.2}

More information

• Specification
• C# Literals and Digit Separators @ Christian Nagel