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Math.Round Method (Decimal, MidpointRounding)

Updated: March 2012

Rounds a decimal value to the nearest integer. 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,
	MidpointRounding mode
)

Parameters

d
Type: System.Decimal

A decimal number to be rounded.

mode
Type: System.MidpointRounding

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

Return Value

Type: System.Decimal
The integer nearest d. If d is halfway between two numbers, one of which is even and the other odd, then mode determines which of the two is returned.
ExceptionCondition
ArgumentException

mode is not a valid value of System.MidpointRounding.

OverflowException

The result is outside the range of a Decimal.

The mode parameter controls how d is rounded if the fractional component of d is halfway between the one's digit and a value that is one greater than the one's digit. The mode can have one of the following two values:

  • MidpointRounding.ToEven. If the one's digit 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 one's digit 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 code example demonstrates the Round method in conjunction 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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