Add quantity constructor overload for UnitSystem

[angularsen]

Per discussion in #519, we want to construct quantities using a unit system such as SI or a custom unit system. The advantage is consistent base units and not having to specify the unit when constructing quantities.

So you can do

new Length(1); // 1 meter, SI unit system
new Length(1, UnitSystem.SI); // same
new Length(1, myUnitSystem); // whatever length unit you configured in your system

Implementation

• Add constructor MyQuantity(double numericValue, UnitSystem unitSystem) {}
• Add constructor MyQuantity(double numericValue) : MyQuantity(numericValue, UnitSystem.SI) {}

Quantities with dimensions matching base units is easy

public Length(double numericValue, UnitSystem unitSystem) 
    : this(numericValue, unitSystem.BaseUnits.Length) { }

Dimensionless quantities?

Information, Ratio, Angle, etc...

public Information(decimal numericValue, UnitSystem unitSystem) 
    : this(numericValue, unitSystem.BaseUnits./*???*/) { }

These are not coupled to any unit system, so we can keep the existing choice of base unit?

Logarithmic quantities?

Level (dB), PoweRatio, AmplitudeRatio

public PowerRatio(decimal numericValue, UnitSystem unitSystem) 
    : this(numericValue, unitSystem.BaseUnits./*???*/) { }

AmplitudeRatio dBV

Unit system: (used in any unit system)

PoweRatio dBW

Unit system: (not in a named unit system)

They seem uncoupled from unit systems, so we can keep the existing choices of base unit?

Non-trivial quantities?

Most quantities fall in this category. How do we construct them given a non-SI instance of UnitSystem?

ElectricChargeDensity C/m³

Unit system: Système International d'Unités (SI)

public ElectricChargeDensity(decimal numericValue, UnitSystem unitSystem) 
    : this(numericValue, unitSystem.BaseUnits./*???*/) { }

[angularsen]

@tmilnthorp I started on this and quickly realized I didn't fully understand how we should implement it. Hoping you have some ideas :-)

I hoped to bring it into v4, but it can wait to v5 if it's not straight forward.

[tmilnthorp]

• For base quantities it is indeed as easy as picking up the unit directly from the UnitSystem's BaseUnits
• Dimensionless don't need to do anything. Always Dimensionless.
• I am not 100% certain what to do for log quantities. They're really just a ratio of something (power, sound pressure, etc.). I believe a 3dB change in W is the same as a 3dB change in Btu/hr. Not sure if it's as easy as converting the ratio unit?
• For derived quantities, we'll need to specify the base units for each. For example Area in SquareMile will need BaseUnit.Length = Length.Mile. If you pass a UnitSystem where BaseUnit.Length = Length.Mile, you match the unit.

[tmilnthorp]

I wonder how we'll resolve things like this:

      "SingularName": "SquareFoot",
      "PluralName": "SquareFeet",
      "BaseUnits": {
        "Length": "LengthUnit.Foot"
      },
      "FromUnitToBaseFunc": "x*0.092903",
      "FromBaseToUnitFunc": "x/0.092903",
      "Localization": [
        {
          "Culture": "en-US",
          "Abbreviations": [ "ft²" ]
        },
        {
          "Culture": "ru-RU",
          "Abbreviations": [ "фут²" ]
        }
      ]
    },
    {
      "SingularName": "UsSurveySquareFoot",
      "PluralName": "UsSurveySquareFeet",
      "BaseUnits": {
        "Length": "LengthUnit.Foot"
      },
      "FromUnitToBaseFunc": "x*0.09290341161",
      "FromBaseToUnitFunc": "x/0.09290341161",
      "Localization": [
        {
          "Culture": "en-US",
          "Abbreviations": [ "ft² (US)" ]
        }
      ]
    },

Both have a base unit of feet. Similar to ambiguous parsing we'll have ambiguous unit system values potentially.

[angularsen]

Well, in this particular scenario, UsSurveySquareFoot should have BaseUnits.Length = LengthUnit.UsSurveyFoot. I can't immediately think of a scenario where units have overlapping base units like you describe here, but I'm also not certain there aren't any.

I think maybe we'll just have to try mapping up all the units and see if we find any ambiguity?

[angularsen]

One thing we can run into is unmapped units. You've defined a UnitSystem with base length unit UsSurveyFoot. Then you try to create an Area with this unit system and maybe we didn't get around to adding UsSurveySquareFoot yet. That would throw an exception. Not sure how big of a problem this would be, but I kind of don't like running into runtime exceptions like this. At least something to be aware of when using these constructors.

Another thing is, we are adding significant complexity here and I sincerely hope it adds enough value that people will make use of it.

[angularsen]

I'm not sure how to move this discussion along. There seems to be some uncertainty how this should all work, I think I need to see a PR with some actual code and tests to consider it further. I'm closing this issue due to inactivity, please create a PR or reopen this issue to discuss further.

[tmilnthorp]

In my example PR you can see I added a constructor for it. Just need to finalize that and add base units for everything