Adjusting decimal precision, .net

Question

These lines in C#

decimal a = 2m;
decimal b = 2.0m;
decimal c = 2.00000000m;
decimal d = 2.000000000000000000000000000m;

Console.WriteLine(a);
Console.WriteLine(b);
Console.WriteLine(c);
Console.WriteLine(d);

Generates this output:

2
2.0
2.00000000
2.000000000000000000000000000

So I can see that creating a decimal variable from a literal allows me to control the precision.

• Can I adjust the precision of decimal variables without using literals?
• How can I create b from a? How can I create b from c?

Answer 1

You are just seeing different representations of the exact same data. The precision of a decimal will be scaled to be as big as it needs to be (within reason).

From System.Decimal:

A decimal number is a floating-point value that consists of a sign, a numeric value where each digit in the value ranges from 0 to 9, and a scaling factor that indicates the position of a floating decimal point that separates the integral and fractional parts of the numeric value.

The binary representation of a Decimal value consists of a 1-bit sign, a 96-bit integer number, and a scaling factor used to divide the 96-bit integer and specify what portion of it is a decimal fraction. The scaling factor is implicitly the number 10, raised to an exponent ranging from 0 to 28. Therefore, the binary representation of a Decimal value is of the form, ((-296 to 296) / 10(0 to 28)), where -296-1 is equal to MinValue, and 296-1 is equal to MaxValue.

The scaling factor also preserves any trailing zeroes in a Decimal number. Trailing zeroes do not affect the value of a Decimal number in arithmetic or comparison operations. However, trailing zeroes can be revealed by the ToString method if an appropriate format string is applied.

Answer 2

What about Math.Round(decimal d, int decimals)?

Answer 3

This behavior seems strange, but maybe it suits the applications of the decimal data type. Have you tried casting the value to another type before displaying the value? (This will result in a loss of precision in the output.)

Answer 4

The question is - do you really need the precision stored in the decimal, rather than just displaying the decimal to the required precision. Most applications know internally how precise they want to be and display to that level of precision. For example, even if a user enters an invoice for 100 in an accounts package, it still prints out as 100.00 using something like val.ToString("n2").

How can I create b from a? How can I create b from c?

c to b is possible.

Console.WriteLine(Math.Round(2.00000000m, 1))

produces 2.0

a to b is tricky as the concept of introducing precision is a little alien to mathematics.

I guess a horrible hack could be a round trip.

decimal b = Decimal.Parse(a.ToString("#.0"));
Console.WriteLine(b);

produces 2.0

Answer 5

I found that I could "tamper" with the scale by multiplying or dividing by a fancy 1.

decimal a = 2m;
decimal c = 2.00000000m;
decimal PreciseOne = 1.000000000000000000000000000000m;
  //add maximum trailing zeros to a
decimal x = a * PreciseOne;
  //remove all trailing zeros from c
decimal y = c / PreciseOne;

I can fabricate a sufficiently precise 1 to change scale factors by known sizes.

decimal scaleFactorBase = 1.0m;
decimal scaleFactor = 1m;
int scaleFactorSize = 3;

for (int i = 0; i < scaleFactorSize; i++)
{
  scaleFactor *= scaleFactorBase;
}

decimal z = a * scaleFactor;

Answer 6

Preserving trailing zeroes like this was introduced in .NET 1.1 for more strict conformance with the ECMA CLI specification.

There is some info on this on MSDN, e.g. here.

You can adjust the precision as follows:

• Math.Round (or Ceiling, Floor etc) to decrease precision (b from c)

• Multiply by 1.000... (with the number of decimals you want) to increase precision - e.g. multiply by 1.0M to get b from a.

Answer 7

It's tempting to confuse decimal in SQL Server with decimal in .NET; they are quite different.

A SQL Server decimal is a fixed-point number whose precision and scale are fixed when the column or variable is defined.

A .NET decimal is a floating-point number like float and double (the difference being that decimal accurately preserves decimal digits whereas float and double accurately preserve binary digits). Attempting to control the precision of a .NET decimal is pointless, since all calculations will yield the same results regardless of the presence or absence of padding zeros.