Math.Abs() / Math.Round() / Math.Ceiling() / Math.Floor()

using System;

// Returns the absolute value (converts negative numbers to positive).
Math.Abs(double value)

// Rounds to the nearest integer (banker's rounding by default).
Math.Round(double value)
Math.Round(double value, int digits)
Math.Round(double value, MidpointRounding mode)

// Rounds up and returns the smallest integer greater than or equal to the value.
Math.Ceiling(double value)

// Rounds down and returns the largest integer less than or equal to the value.
Math.Floor(double value)

// Truncates toward zero (removes the fractional part).
Math.Truncate(double value)

using System;

// Math.Abs() — Gets the absolute value.
Console.WriteLine(Math.Abs(-42));    // 42
Console.WriteLine(Math.Abs(3.14));   // 3.14
Console.WriteLine(Math.Abs(-7.5));   // 7.5

// Math.Round() — Rounds a number.
// Uses banker's rounding by default: 0.5 rounds to the nearest even number.
Console.WriteLine(Math.Round(2.5));  // 2 (banker's rounding)
Console.WriteLine(Math.Round(3.5));  // 4 (banker's rounding)

// Use MidpointRounding.AwayFromZero for conventional rounding.
Console.WriteLine(Math.Round(2.5, MidpointRounding.AwayFromZero)); // 3
Console.WriteLine(Math.Round(3.14159, 2)); // 3.14 (2 decimal places)

// Math.Ceiling() — Rounds up (always toward positive infinity).
Console.WriteLine(Math.Ceiling(4.1));  // 5
Console.WriteLine(Math.Ceiling(4.9));  // 5
Console.WriteLine(Math.Ceiling(-4.1)); // -4 (rounds toward zero)

// Math.Floor() — Rounds down (always toward negative infinity).
Console.WriteLine(Math.Floor(4.9));    // 4
Console.WriteLine(Math.Floor(-4.1));   // -5 (rounds away from zero)

// Math.Truncate() — Returns the integer part, removing the fractional part.
Console.WriteLine(Math.Truncate(4.9));  // 4
Console.WriteLine(Math.Truncate(-4.9)); // -4 (rounds toward zero, unlike Floor)

The default behavior of Math.Round() is banker's rounding (round-half-to-even). Note that rounding 2.5 gives 2, not 3. To use conventional rounding where 0.5 always rounds up, specify MidpointRounding.AwayFromZero.

Math.Ceiling() and Math.Floor() can produce counterintuitive results with negative numbers. Ceiling(-4.1) returns -4, and Floor(-4.1) returns -5. Use Math.Truncate() to truncate toward zero. For other math methods, see Math.Max() / Min() / Pow() / Sqrt().