Math.Round Method (Decimal, Int32, MidpointRounding)

Updated: March 2012

Rounds a decimal value to a specified number of fractional digits. A parameter specifies how to round the value if it is midway between two other numbers.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)

public static decimal Round(
	decimal d,
	int decimals,
	MidpointRounding mode
)

Parameters

d
Type: System.Decimal

A decimal number to be rounded.

decimals
Type: System.Int32

The number of decimal places in the return value.

mode
Type: System.MidpointRounding

Specification for how to round d if it is midway between two other numbers.

Return Value

Type: System.Decimal
The number nearest to d that contains a number of fractional digits equal to decimals. If d has fewer fractional digits than decimals, d is returned unchanged.

ExceptionCondition
ArgumentOutOfRangeException

decimals is less than 0 or greater than 28.

ArgumentException

mode is not a valid value of System.MidpointRounding.

OverflowException

The result is outside the range of a Decimal.

The decimals parameter specifies the number of fractional digits in the return value and ranges from 0 to 28. If decimals is zero, an integer is returned.

The mode parameter controls how d is rounded if there is a single non-zero digit in d to the right of the decimals decimal position and its value is 5 — that is, if it is halfway between the digit in the decimals position and the next highest digit. The mode parameter can have one of the following two values:

  • MidpointRounding.ToEven. If the digit in the decimals position is odd, it is changed to an even digit. Otherwise, it is left unchanged. This behavior follows IEEE Standard 754, section 4. It is sometimes called rounding to nearest, or banker's rounding. It minimizes rounding errors that result from consistently rounding a midpoint value in a single direction.

  • MidpointRounding.AwayFromZero. The digit in the decimals position is always rounded up to the next digit. This is the most commonly known rounding method. It is known as symmetric arithmetic rounding.

The following example demonstrates how to use the Round method with the MidpointRounding enumeration.

// This example demonstrates the Math.Round() method in conjunction  
// with the MidpointRounding enumeration. 
using System;

class Sample 
{
    public static void Main() 
    {
    decimal result = 0.0m;
    decimal posValue =  3.45m;
    decimal negValue = -3.45m;

// By default, round a positive and a negative value to the nearest even number.  
// The precision of the result is 1 decimal place.

    result = Math.Round(posValue, 1);
    Console.WriteLine("{0,4} = Math.Round({1,5}, 1)", result, posValue);
    result = Math.Round(negValue, 1);
    Console.WriteLine("{0,4} = Math.Round({1,5}, 1)", result, negValue);
    Console.WriteLine();

// Round a positive value to the nearest even number, then to the nearest number away from zero.  
// The precision of the result is 1 decimal place.

    result = Math.Round(posValue, 1, MidpointRounding.ToEven);
    Console.WriteLine("{0,4} = Math.Round({1,5}, 1, MidpointRounding.ToEven)", result, posValue);
    result = Math.Round(posValue, 1, MidpointRounding.AwayFromZero);
    Console.WriteLine("{0,4} = Math.Round({1,5}, 1, MidpointRounding.AwayFromZero)", result, posValue);
    Console.WriteLine();

// Round a negative value to the nearest even number, then to the nearest number away from zero.  
// The precision of the result is 1 decimal place.

    result = Math.Round(negValue, 1, MidpointRounding.ToEven);
    Console.WriteLine("{0,4} = Math.Round({1,5}, 1, MidpointRounding.ToEven)", result, negValue);
    result = Math.Round(negValue, 1, MidpointRounding.AwayFromZero);
    Console.WriteLine("{0,4} = Math.Round({1,5}, 1, MidpointRounding.AwayFromZero)", result, negValue);
    Console.WriteLine();
    }
}
/*
This code example produces the following results:

 3.4 = Math.Round( 3.45, 1)
-3.4 = Math.Round(-3.45, 1)

 3.4 = Math.Round( 3.45, 1, MidpointRounding.ToEven)
 3.5 = Math.Round( 3.45, 1, MidpointRounding.AwayFromZero)

-3.4 = Math.Round(-3.45, 1, MidpointRounding.ToEven)
-3.5 = Math.Round(-3.45, 1, MidpointRounding.AwayFromZero)

*/

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0

Date

History

Reason

March 2012

Clarified the midpoint rounding operation.

Customer feedback.

August 2008

Added detail on midpoint rounding to the Remarks section.

Customer feedback.

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