Throw if JsonNet custom quantity instantiation fails, not null

UnitsNet.Serialization.JsonNet.Tests/UnitsNetBaseJsonConverterTest.cs

@@ -7,6 +7,7 @@
 using Newtonsoft.Json.Converters;
 using Newtonsoft.Json.Linq;
 using Newtonsoft.Json.Serialization;
+using UnitsNet.Tests.CustomQuantities;
 using Xunit;
 
 namespace UnitsNet.Serialization.JsonNet.Tests
@@ -67,6 +68,39 @@ public void UnitsNetBaseJsonConverter_ConvertValueUnit_throws_UnitsNetException_
             Assert.Equal("PowerUnit Watt", result.Data["type"]);
         }
 
+        [Fact]
+        public void UnitsNetBaseJsonConverter_ConvertValueUnit_throws_UnitsNetException_when_registered_quantity_cannot_be_instantiated()
+        {
+            var sut = new TestQuantityConverter();
+            sut.RegisterCustomType(typeof(IQuantity), typeof(HowMuchUnit));
+
+            var result = Assert.Throws<UnitsNetException>(() => sut.Test_ConvertValueUnit("HowMuchUnit.Some", 10.2365));
+
+            Assert.Contains("Unable to instantiate registered quantity type", result.Message);
+            Assert.Equal("JsonNetRegisteredQuantityInstantiationFailed", result.Data[UnitsNetException.ErrorCodeDataKey]);
+            Assert.Equal(typeof(IQuantity), result.Data["quantityType"]);
+            Assert.Equal(typeof(HowMuchUnit), result.Data["unitType"]);
+            Assert.Equal(HowMuchUnit.Some, result.Data["unit"]);
+            Assert.NotNull(result.InnerException);
+        }
+
+        [Fact]
+        public void UnitsNetBaseJsonConverter_ConvertValueUnit_wraps_exception_when_registered_quantity_constructor_throws()
+        {
+            var sut = new TestQuantityConverter();
+            sut.RegisterCustomType(typeof(ThrowingQuantity), typeof(HowMuchUnit));
+
+            var result = Assert.Throws<UnitsNetException>(() => sut.Test_ConvertValueUnit("HowMuchUnit.Some", 10.2365));
+
+            Assert.Contains("Unable to instantiate registered quantity type", result.Message);
+            Assert.Equal("JsonNetRegisteredQuantityInstantiationFailed", result.Data[UnitsNetException.ErrorCodeDataKey]);
+            Assert.Equal(typeof(ThrowingQuantity), result.Data["quantityType"]);
+            Assert.Equal(typeof(HowMuchUnit), result.Data["unitType"]);
+            Assert.Equal(HowMuchUnit.Some, result.Data["unit"]);
+            Assert.NotNull(result.InnerException);
+            Assert.Contains("Thrown from registered quantity constructor.", result.InnerException.ToString());
+        }
+
         [Fact]
         public void UnitsNetBaseJsonConverter_CreateLocalSerializer_works_as_expected()
         {
@@ -226,5 +260,32 @@ private class TestConverter : UnitsNetBaseJsonConverter<string>
                 return (result.Unit, result.Value);
             }
         }
+
+        private class TestQuantityConverter : UnitsNetBaseJsonConverter<IQuantity>
+        {
+            public override bool CanRead => false;
+            public override bool CanWrite => false;
+            public override void WriteJson(JsonWriter writer, IQuantity value, JsonSerializer serializer) => throw new NotImplementedException();
+            public override IQuantity ReadJson(JsonReader reader, Type objectType, IQuantity existingValue, bool hasExistingValue, JsonSerializer serializer) => throw new NotImplementedException();
+
+            public IQuantity Test_ConvertValueUnit(string unit, double value) => ConvertValueUnit(new ValueUnit {Unit = unit, Value = value});
+        }
+
+        private class ThrowingQuantity : IQuantity
+        {
+            public ThrowingQuantity(double value, HowMuchUnit unit)
+            {
+                throw new UnitsNetException("Thrown from registered quantity constructor.");
+            }
+
+            public QuantityInfo QuantityInfo => throw new NotImplementedException();
+            public Enum Unit => throw new NotImplementedException();
+            public double Value => throw new NotImplementedException();
+            public UnitKey UnitKey => throw new NotImplementedException();
+            public double As(Enum unit) => throw new NotImplementedException();
+            public double As(UnitKey unitKey) => throw new NotImplementedException();
+            public IQuantity ToUnit(Enum unit) => throw new NotImplementedException();
+            public string ToString(string format, IFormatProvider formatProvider) => throw new NotImplementedException();
+        }
     }
 }

UnitsNet.Serialization.JsonNet/UnitsNetBaseJsonConverter.cs

@@ -21,12 +21,14 @@ namespace UnitsNet.Serialization.JsonNet
 #endif
     public abstract class UnitsNetBaseJsonConverter<T> : NullableQuantityConverter<T>
     {
+        private const string RegisteredQuantityInstantiationFailedErrorCode = "JsonNetRegisteredQuantityInstantiationFailed";
+
         private readonly ConcurrentDictionary<string, (Type Quantity, Type Unit)> _registeredTypes = new();
 
         /// <summary>
         /// Register custom types so that the converter can instantiate these quantities.
-        /// Instead of calling <see cref="Quantity.From(double,UnitKey)"/>, the <see cref="Activator"/> will be used to instantiate the object.
-        /// It is therefore assumed that the constructor of <paramref name="quantity"/> is specified with <c>new T(double value, typeof(<paramref name="unit"/>) unit)</c>.
+        /// Instead of calling <see cref="Quantity.From(double,UnitKey)"/>, <see cref="Activator.CreateInstance(Type, object?[])"/> will be used to instantiate the object.
+        /// It is therefore assumed that <paramref name="quantity"/> has a public constructor that accepts <c>(double value, <paramref name="unit"/> unit)</c>.
         /// Registering the same <paramref name="unit"/> multiple times, it will overwrite the one registered.
         /// </summary>
         public void RegisterCustomType(Type quantity, Type unit)
@@ -92,16 +94,39 @@ protected IQuantity ConvertValueUnit(ValueUnit valueUnit)
             }
 
             var unit = GetUnit(valueUnit.Unit);
-            var registeredQuantity = GetRegisteredType(valueUnit.Unit).Quantity;
+            var registeredType = GetRegisteredType(valueUnit.Unit);
+            var registeredQuantity = registeredType.Quantity;
 
             if (registeredQuantity is not null)
             {
-                return (IQuantity)Activator.CreateInstance(registeredQuantity, valueUnit.Value, unit)!;
+                try
+                {
+                    IQuantity? instance = (IQuantity?)Activator.CreateInstance(registeredQuantity, valueUnit.Value, unit);
+                    return instance ?? throw CreateRegisteredQuantityInstantiationException(registeredQuantity, registeredType.Unit ?? unit.GetType(), unit);
+                }
+                catch (Exception ex) when (!IsRegisteredQuantityInstantiationException(ex))
+                {
+                    throw CreateRegisteredQuantityInstantiationException(registeredQuantity, registeredType.Unit ?? unit.GetType(), unit, ex);
+                }
             }
 
             return Quantity.From(valueUnit.Value, unit);
         }
 
+        private static bool IsRegisteredQuantityInstantiationException(Exception ex) =>
+            ex is UnitsNetException && Equals(ex.Data[UnitsNetException.ErrorCodeDataKey], RegisteredQuantityInstantiationFailedErrorCode);
+
+        private static UnitsNetException CreateRegisteredQuantityInstantiationException(Type quantityType, Type unitType, Enum unit, Exception? innerException = null)
+        {
+            string message = $"Unable to instantiate registered quantity type \"{quantityType.FullName}\" for unit \"{unitType.FullName}.{unit}\". The converter expected a non-null quantity instance from a public constructor accepting (double value, {unitType.Name} unit).";
+            var ex = innerException is null ? new UnitsNetException(message) : new UnitsNetException(message, innerException);
+            ex.Data[UnitsNetException.ErrorCodeDataKey] = RegisteredQuantityInstantiationFailedErrorCode;
+            ex.Data["quantityType"] = quantityType;
+            ex.Data["unitType"] = unitType;
+            ex.Data["unit"] = unit;
+            return ex;
+        }
+
         private (Type? Quantity, Type? Unit) GetRegisteredType(string unit)
         {
             var (unitEnumTypeName, _) = SplitUnitString(unit);

UnitsNet/Exceptions/UnitsNetException.cs

@@ -10,6 +10,11 @@ namespace UnitsNet
     /// </summary>
     public class UnitsNetException : Exception
     {
+        /// <summary>
+        ///     Key used in <see cref="Exception.Data" /> for stable UnitsNet error codes.
+        /// </summary>
+        public const string ErrorCodeDataKey = "errorCode";
+
         /// <inheritdoc />
         public UnitsNetException()
         {