How to make AutoMapper throw when mapping null into non-nullable value type? - automapper

Question

I have two classes:
public class ClassA
{
public ClassA(int? value)
{
Value = value;
}
public int? Value { get; }
}
public class ClassB
{
public ClassB(int value)
{
Value = value;
}
public int Value { get; }
}
When mapping an instance of ClassA with Value == null into ClassB, AutoMapper seems to ignore the property and provide the default value instead:
var config = new MapperConfiguration(cfg =>
{
cfg.CreateMap<ClassA, ClassB>();
});
var mapper = new Mapper(config);
var classA = new ClassA(null);
var classB = mapper.Map<ClassB>(classA);
Debug.Assert(classB.Value == 0);
How do I make AutoMapper throw an exception when mapping an instance of ClassA with Value == null into ClassB instead? Optimally, can I enable this behavior for all mappings from nullable value types to non-nullable value types?

Answer 1

I can add a custom BeforeMap function that tries to identify these kind of situations via reflection, but I'd prefer a simpler solution and/or one that also handled custom mappings of differently named properties, etc.
cfg.ForAllMaps((_, e) => e.BeforeMap(CheckForDisallowedNulls));
void CheckForDisallowedNulls(object source, object destination)
{
foreach (var sourceProperty in source.GetType().GetProperties())
{
var underlyingType = Nullable.GetUnderlyingType(sourceProperty.PropertyType);
if (underlyingType is { })
{
var destinationProperty = destination.GetType().GetProperty(sourceProperty.Name);
if (destinationProperty is { } && destinationProperty.PropertyType == underlyingType && sourceProperty.GetValue(source) == null)
{
throw new ArgumentNullException(sourceProperty.Name);
}
}
}
}

Answer 2

I can create a map from int? to int that throws an exception when the source value is null:
cfg.CreateMap<int?, int>().ConvertUsing((s, _) => s.Value);
However, this requires adding such a map for each nullable value type I want the behavior for.