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)

Math.Ceiling(double value)

Math.Floor(double value)

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

Program.cs

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.
Console.WriteLine(Math.Round(2.5)); // 2 (banker's rounding: rounds to even)
Console.WriteLine(Math.Round(3.5)); // 4 (banker's rounding: rounds to even)

// 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)

This produces the following output:

dotnet run
42
3.14
7.5
2
4
3
3.14
5
5
-4
4
-5
4
-4

The following example shows a practical use of Ceiling for calculating the total number of pages needed to display a dataset.

PagingHelper.cs

using System;

// Calculate the total number of pages for n items at k items per page.
int itemCount = 25;
int pageSize = 7;
int pageTotal = (int)Math.Ceiling((double)itemCount / pageSize);
Console.WriteLine($"Total pages: {pageTotal}"); // Total pages: 4

// Calculate the price after 10% tax, truncating the fractional part.
double price = 980;
double taxRate = 0.10;
double total = Math.Floor(price * (1 + taxRate));
Console.WriteLine($"Total: {total}"); // Total: 1078

This produces the following output:

dotnet run
Total pages: 4
Total: 1078

Common Mistake: Math.Round() Uses Banker's Rounding by Default

Math.Round() uses banker's rounding (round-half-to-even) by default. When the value is exactly 0.5, it rounds to the nearest even number — so 2.5 rounds to 2 and 3.5 rounds to 4. Using it with the expectation of conventional rounding (always round 0.5 up) leads to bugs.

using System;

// NG: default Round uses banker's rounding — 0.5 rounds to even
Console.WriteLine(Math.Round(0.5)); // 0 (expected 1)
Console.WriteLine(Math.Round(1.5)); // 2
Console.WriteLine(Math.Round(2.5)); // 2 (expected 3)

The corrected version looks like this:

using System;

// OK: use MidpointRounding.AwayFromZero for conventional rounding
Console.WriteLine(Math.Round(0.5, MidpointRounding.AwayFromZero)); // 1
Console.WriteLine(Math.Round(1.5, MidpointRounding.AwayFromZero)); // 2
Console.WriteLine(Math.Round(2.5, MidpointRounding.AwayFromZero)); // 3

This produces the following output:

dotnet run
0
2
2
1
2
3

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().