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Adding a New Unit

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Andreas Gullberg Larsen edited this page Dec 28, 2020 · 88 revisions

So you want to add a quantity or unit that is not yet part of Units.NET?

Great, but before you start!

Sometimes we just have to say no, sorry! 😅 We simply want to avoid bloating the library.

A quantity is a good fit to add, if it

  • Is widely used.
  • Has multiple units to convert between (e.g. Length has kilometer, feet, nanometer etc.)
  • Can be represented by a double numeric value, integer values are not well supported and may suffer from precision errors.
  • Is not dimensionless/unitless (consider using Ratio)

A unit is a good fit to add to a quantity, if it

  • Is widely used.
  • Can be converted to other units of the same quantity.
  • The conversion function is well established without ambiguous competing standards.

We have made some exceptions to these "rules" so start a discussion with us if you still think it belongs in the library.

Ok, enough of that. Let's move on! 😃

Requirements

In order to build and run tests locally you need to have some tools installed.

Quick Summary of Steps

Units.NET uses CodeGen, a C# command line app that reads JSON files with quantity and unit definitions and generates C# code.

To add a quantity or a unit:

  • Add or change a quantity JSON file.
  • Run generate-code.bat file.
  • Specify test values for the new units in the generated test code.

Easy peasy. Below are the detailed steps.

Detailed steps

1. Add or modify JSON file for a quantity class

Conventions

  • Multiply for FromBaseToUnit and divide for FromUnitToBase, so that Length.Centimeter is defined as "FromBaseToUnit": "x*100" and "FromUnitToBase": "x/100" where base unit is Meter
  • Prefer 1e3 and 1e-5 notation instead of 1000 and 0.00001
  • Prefer a constant if the conversion factor is finite and well known (Inch FromUnitToBase: x*2.54e-2)
  • Prefer a calculation if the conversion factor is infinite (PrinterPoint FromUnitToBase: (x/72.27)*2.54e-2). If the calculation is not available, specify the most precise constant you can. double numeric type can represent 15-17 significant decimal digits

BaseDimensions

The base unit dimensions of the quantity, such as "L": 1 for Length and "L": 2 for Area (Length*Length).

The 7 SI base units are:

  • L - Length
  • M - Mass
  • T - Time
  • I - ElectricCurrent
  • Θ - Temperature
  • N - AmountOfSubstance
  • J - LuminousIntensity

BaseUnit - the intermediate unit of a quantity

when converting from one unit to another with FromUnitToBaseFunc and FromBaseToUnitFunc conversion functions. It is typically chosen as an SI derived unit (Meter, Newtonmeter, Squaremeter etc). This choice affects the precision of conversions for much bigger/smaller units than BaseUnit.

BaseUnits (optional) - the SI base units of a unit

Don't confuse this with the quantity's BaseUnit, which is discussed to be renamed.

If specified, you can create quantities with consistent units for a given unit system:

new Length(1, UnitSystem.SI).ToString() // "1 m"
new Length(1, myBritishEngineeringUnitSystem).ToString() // "1 ft"
Examples on BaseUnits values:
  • LengthUnit.Inch has { "L": "Inch" } (L=1)
  • AreaUnit.SquareCentimeter has { "L": "Centimeter" }, because we ignore dimensions (L=2)
  • VolumeUnit.Cubicfeet has { "L": "Foot" }, because we ignore dimensions (L=3)
  • ForceUnit.Newton has { "L": "Meter", "M": "Kilogram", "T": "Second" }, because N = 1 kg * 1 m / s² = Kilogram * Meter / Second² and we ignore the dimensions
  • ForceUnit.PoundForce has { "L": "Foot", "M": "Pound", "T": "Second" }, because N = 1 lbm * 1 ft / s² = Pound * Foot / Second² and we ignore the dimensions
  • MassConcentrationUnit.GramPerDeciliter has { "L": "Centimeter", "M": "Gram" }, because Deciliter = 1 cm * 1cm * 1cm = Centimeter³ and we ignore the dimensions
Examples of units with no meaningful mapping to SI base units ⚠️

The only consequence of not specifying BaseUnits is that you cannot construct these units by passing a UnitSystem to the quantity constructor as in the example above.

  • VolumeUnit.ImperialGallon has no BaseUnits, because Volume = Length³ and there is no length unit that when multiplied three times would result in imperial gallon.
  • RatioUnit.DecimalFraction has no BaseUnits, because dimensionless units are not made up by any SI base units.

2. Run generate-code.bat

to generate unit classes, unit enumerations and base class for tests.

3. Reopen solution to load all new files

This step might no longer be necessary, I think Visual Studio 2017 and the new .csproj format automatically loads new files automatically.

4. Fix generated test stubs to resolve compile errors tests

  • Override the missing abstract properties in the unit test class (ex: LengthTests.cs)
  • Specify value as a constant, not a calculation, with preferably at least 7 significant figures where possible.
  • Triple-check the number you write here, this is the most important piece as it verifies your conversion function in the .JSON file
  • Example: InchesInOneMeter in LengthTests.cs
    • I find the conversion factor to be 39.37007874 from an online unit conversion calculator, it has 10 significant figures so that is plenty
    • I add the code: protected override double InchesInOneMeter => 39.37007874;
    • I Google to double-check: Inches In 1 Meter and it tells me 1 Meter = 39.3701 Inches (Google typically has fewer significant figures)
    • If Google can't help me, I find a second source to confirm the conversion factor (another conversion website, wikipedia, Wolfram Alpha etc)
    • I check again by intuition, is there really around 40 inches in a meter? Yes, sounds about right.

5. Run tests

Make sure all the tests pass. Either run build.bat or run the tests from within Visual Studio with ReSharper or the built-in test runner.

6. Create pull request

Please see GitHub: Creating a pull request. If you still have any questions, please ask on our Gitter chat or create an issue.

Logarithmic Units

Units.NET supports logarithmic units by adding Logarithmic and LogarithmicScalingFactor (optional) properties.

  • LogarithmicScalingFactor is used to provide a scaling factor in the logarithmic conversion. For example, a scaling factor of 2 is required when implementing the ratio of the squares of two field amplitude quantities such as voltage. In most cases LogarithmicScalingFactor will be 1.

To create a logarithmic unit, follow the same steps from the previous section making the following adjustments:

Step 1. Add property "Logarithmic": "True" to the JSON file, just after BaseUnit. LogarithmicScalingFactor defaults to 1 if not defined.

Step 4. Provide custom implementations for logarithmic addition and subtraction unit tests. See LevelTests.cs for an example.

Refer to Level.json as an example implementation of logarithmic units.

Code Style

  • If you have the ReSharper plugin installed, there are code formatting settings checked into the repository that will take effect automatically.
  • If you don't use ReSharper, at least follow the same conventions as in the existing code.
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