Why can Collections be nullable, but exceptions is thrown on null?

[sveinhelge]

I have this mapping (I'm using List in the example, but it could have been any collection type)

[Mapper(PropertyNameMappingStrategy = PropertyNameMappingStrategy.CaseInsensitive)]
public static partial class ResponseMapper
{
   private static partial List<Target> ToList(List<Source>? model);
}

The generated code(without my namespaces)

[global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "4.0.0.0")]
private static partial global::System.Collections.Generic.List<global::...Target> ToList(global::System.Collections.Generic.List<global::...Source>? model)
{
    if (model == null)
       throw new System.ArgumentNullException(nameof(model));
    var target = new global::System.Collections.Generic.List<global::...Target>(model.Count);
    foreach (var item in model)
    {
       target.Add(MapToTarget(item));
    }
    return target;
}

I would expect it to not throw null exceptions, since the source is nullable.

More like

if (model == null)
return new List<Target>();  

Temporarly I have solved this by using a private static method pointing to a generated mapping

private static List<Target> NullToList(List<Source>? list) => ToList(list ?? []);

I could also have solved this with JsonConstructor, but then I have to manually do much plumbing code. That is not what we want.

[JsonConstructor]
public SomeClass(List<Source>? sourceList)
{
   SourceList = sourceList ?? [];
} 

Am I missing something here?

[sveinhelge]

If I added ThrowOnMappingNullMismatch = false to the attribute it works as expected

See docs

[Mapper(PropertyNameMappingStrategy = PropertyNameMappingStrategy.CaseInsensitive, ThrowOnMappingNullMismatch  = false)]

Then we will get this is the generated code

if (model == null)
   return new global::System.Collections.Generic.List<global::...Target>();

instead of

if (model == null)
   throw new System.ArgumentNullException(nameof(model));