Add HowMuch custom quantity

UnitsNet.Tests/CustomQuantities/HowMuch.cs

@@ -0,0 +1,53 @@
+using System;
+
+namespace UnitsNet.Tests.CustomQuantities
+{
+    /// <inheritdoc cref="IQuantity"/>
+    /// <summary>
+    /// Example of a custom/third-party quantity implementation, for plugging in quantities and units at runtime.
+    /// </summary>
+    public struct HowMuch : IQuantity
+    {
+        public HowMuch(double value, Enum unit) : this()
+        {
+            Unit = unit;
+            Value = value;
+        }
+
+        public Enum Unit { get; }
+        public double Value { get; }
+
+        #region Crud to satisfy IQuantity, but not really used for anything
+
+        private static readonly HowMuch Zero = new HowMuch(0, HowMuchUnit.Some);
+
+        public QuantityType Type => QuantityType.Undefined;
+        public BaseDimensions Dimensions => BaseDimensions.Dimensionless;
+
+        public QuantityInfo QuantityInfo => new QuantityInfo(Type,
+            new UnitInfo[]
+            {
+                new UnitInfo<HowMuchUnit>(HowMuchUnit.Some, BaseUnits.Undefined),
+                new UnitInfo<HowMuchUnit>(HowMuchUnit.ATon, BaseUnits.Undefined),
+                new UnitInfo<HowMuchUnit>(HowMuchUnit.AShitTon, BaseUnits.Undefined),
+            },
+            HowMuchUnit.Some,
+            Zero,
+            BaseDimensions.Dimensionless);
+
+        public double As(Enum unit) => Convert.ToDouble(unit);
+
+        public double As(UnitSystem unitSystem) => throw new NotImplementedException();
+
+        public IQuantity ToUnit(Enum unit) => new HowMuch(As(unit), unit);
+
+        public IQuantity ToUnit(UnitSystem unitSystem) => throw new NotImplementedException();
+
+        public string ToString(string format, IFormatProvider formatProvider) => $"HowMuch ({format}, {formatProvider})";
+        public string ToString(IFormatProvider provider) => $"HowMuch ({provider})";
+        public string ToString(IFormatProvider provider, int significantDigitsAfterRadix) => $"HowMuch ({provider}, {significantDigitsAfterRadix})";
+        public string ToString(IFormatProvider provider, string format, params object[] args) => $"HowMuch ({provider}, {string.Join(", ", args)})";
+
+        #endregion
+    }
+}

UnitsNet.Tests/CustomQuantities/HowMuchUnit.cs

@@ -0,0 +1,12 @@
+namespace UnitsNet.Tests.CustomQuantities
+{
+    /// <summary>
+    /// Example of a custom/third-party quantity implementation, for plugging in quantities and units at runtime.
+    /// </summary>
+    public enum HowMuchUnit
+    {
+        Some,
+        ATon,
+        AShitTon
+    }
+}