Create QuantityType -> UnitInfo associations (DefaultUnits) in UnitSystem

- UnitSystem holds the list of default units for each (compatible?) quantity type and a method (GetDefaultUnitInfo) that provides the corresponding UnitInfo given a QuantityType
- All methods in Quantity that use a UnitSystem now use the new GetDefaultUnitInfo
- Added a method (in UnitSystem) for creating derived unit systems(WithDefaultUnit), providing a new QuantityType->UnitInfo association and optionally a new BaseUnits definition (as currently used by Equals and visible in public ctor- otherwise obsolete?)

Author: lipchev

CodeGen/Generators/UnitsNetGen/QuantityGenerator.cs

@@ -183,23 +183,25 @@ private void GenerateInstanceConstructors()
         /// <param name=""value"">The numeric value to construct this quantity with.</param>
         /// <param name=""unitSystem"">The unit system to create the quantity with.</param>
         /// <exception cref=""ArgumentNullException"">The given <see cref=""UnitSystem""/> is null.</exception>
-        /// <exception cref=""ArgumentException"">No unit was found for the given <see cref=""UnitSystem""/>.</exception>
+        /// <exception cref=""ArgumentException"">No default unit was found for the given <see cref=""UnitSystem""/>.</exception>
         public {_quantity.Name}({_valueType} value, UnitSystem unitSystem)
         {{
-            if(unitSystem == null) throw new ArgumentNullException(nameof(unitSystem));
-
-            var unitInfos = Info.GetUnitInfosFor(unitSystem.BaseUnits);
-            var firstUnitInfo = unitInfos.FirstOrDefault();
+            if(unitSystem == null) throw new ArgumentNullException(nameof(unitSystem)); 
 ");
 
             Writer.WL(_quantity.BaseType == "double"
                 ? @"
-            _value = Guard.EnsureValidNumber(value, nameof(value));"
+            _value = Guard.EnsureValidNumber(value, nameof(value));
+"
                 : @"
-            _value = value;");
-            Writer.WL(@"
-            _unit = firstUnitInfo?.Value ?? throw new ArgumentException(""No units were found for the given UnitSystem."", nameof(unitSystem));
-        }
+            _value = value;
+");
+
+            Writer.WL($@"
+            var defaultUnitInfo = unitSystem.GetDefaultUnitInfo(QuantityType) as UnitInfo<{_unitEnumName}>;
+
+            _unit = defaultUnitInfo?.Value ?? throw new ArgumentException(""No default unit was defined for the given UnitSystem."", nameof(unitSystem));
+        }}
 ");
         }
 
@@ -821,13 +823,12 @@ public double As(UnitSystem unitSystem)
             if(unitSystem == null)
                 throw new ArgumentNullException(nameof(unitSystem));
 
-            var unitInfos = Info.GetUnitInfosFor(unitSystem.BaseUnits);
+            var defaultUnitInfo = unitSystem.GetDefaultUnitInfo(QuantityType) as UnitInfo<{_unitEnumName}>;
 
-            var firstUnitInfo = unitInfos.FirstOrDefault();
-            if(firstUnitInfo == null)
-                throw new ArgumentException(""No units were found for the given UnitSystem."", nameof(unitSystem));
+            if(defaultUnitInfo == null)
+                throw new ArgumentException(""No default unit was found for the given UnitSystem."", nameof(unitSystem));
 
-            return As(firstUnitInfo.Value);
+            return As(defaultUnitInfo.Value);
         }}
 
         /// <inheritdoc />
@@ -864,13 +865,12 @@ IQuantity IQuantity.ToUnit(Enum unit)
             if(unitSystem == null)
                 throw new ArgumentNullException(nameof(unitSystem));
 
-            var unitInfos = Info.GetUnitInfosFor(unitSystem.BaseUnits);
+            var defaultUnitInfo = unitSystem.GetDefaultUnitInfo(QuantityType) as UnitInfo<{_unitEnumName}>;
 
-            var firstUnitInfo = unitInfos.FirstOrDefault();
-            if(firstUnitInfo == null)
-                throw new ArgumentException(""No units were found for the given UnitSystem."", nameof(unitSystem));
+            if(defaultUnitInfo == null)
+                throw new ArgumentException(""No default unit was found for the given UnitSystem."", nameof(unitSystem));
 
-            return ToUnit(firstUnitInfo.Value);
+            return ToUnit(defaultUnitInfo.Value);
         }}
 
         /// <inheritdoc />

UnitsNet.Tests/CustomCode/LengthTests.cs

@@ -4,6 +4,7 @@
 using Xunit;
 using UnitsNet.Units;
 using System;
+using System.Linq;
 
 namespace UnitsNet.Tests.CustomCode
 {
@@ -186,6 +187,16 @@ public void Constructor_UnitSystemSI_AssignsSIUnit()
             Assert.Equal(LengthUnit.Meter, length.Unit);
         }
 
+        [Fact]
+        public void Constructor_UnitSystemMySmallSI_AssignsMillimiters()
+        {
+            var myDefaultLengthUnit = Length.Info.UnitInfos.First(x => x.Value == LengthUnit.Millimeter);
+            var myUnitSystem = UnitSystem.SI.WithDefaultUnit(QuantityType.Length, myDefaultLengthUnit);
+
+            var length = new Length(1.0, myUnitSystem);
+            Assert.Equal(LengthUnit.Millimeter, length.Unit);
+        }
+        
         [Fact]
         public void Constructor_UnitSystemWithNoMatchingBaseUnits_ThrowsArgumentException()
         {