Mapping a nested list onto a readonly list can lead to nullreference exception when the source list is null.

[wieke-osw]

Please do the checklist before filing an issue:

• I have read the documentation, including the FAQ
• I can reproduce the bug using the latest prerelease version
• I have searched existing discussion and issue to avoid duplicates

Describe the bug
When mapping a nested list onto a readonly list the null check on the source list is done improperly and can lead to a nullreference exception and possibly a CS8602 warning.

It is similar to already reported issues (e.g. #1650) but it concerns the source object not the target being null.

Repro
https://github.com/wieke-osw/FailingMap

I was unable to reproduce the CS8602 warning. However the problematic part of the code is basically the same.

Declaration code

using Riok.Mapperly.Abstractions;
using System.Collections.Generic;

namespace FailingMapTest;

[Mapper]
public static partial class TestMap
{
    [MapProperty(nameof(@Source.Nested.Items), nameof(Target.Items))]
    public static partial Target Map(Source source);
}

public class Source
{
    public Nested Nested { get; set; }
}

public class Nested
{
    public IEnumerable<int> Items { get; set; }
}

public class Target()
{
    public readonly List<int> Items = [];
}

public class Test
{
    [Fact]
    public void Fails()
    {
        var target = TestMap.Map(new Source());
        Assert.Empty(target.Items);
    }
}

Actual relevant generated code

public static partial global::ConsoleApp.Target? Map(global::ConsoleApp.Source? source)
{
    if (source == null)
        return default;
    var target = new global::ConsoleApp.Target();
    if (source.Nested.Items != null && target.Items != null) // This check will fail if source.Nested is null
    {
        if (global::System.Linq.Enumerable.TryGetNonEnumeratedCount(source.Nested.Items, out var sourceCount))
        {
            target.Items.EnsureCapacity(sourceCount + target.Items.Count);
        }
        foreach (var item in source.Nested.Items)
        {
            target.Items.Add(item);
        }
    }
    return target;
}

Expected relevant generated code

public static partial global::ConsoleApp.Target? Map(global::ConsoleApp.Source? source)
{
    if (source == null)
        return default;
    var target = new global::ConsoleApp.Target();
    if (source.Nested?.Items != null && target.Items != null) // Added a null-conditional operator ?
    {
        if (global::System.Linq.Enumerable.TryGetNonEnumeratedCount(source.Nested.Items, out var sourceCount))
        {
            target.Items.EnsureCapacity(sourceCount + target.Items.Count);
        }
        foreach (var item in source.Nested.Items)
        {
            target.Items.Add(item);
        }
    }
    return target;
}

Reported relevant diagnostics
I am getting this warning in my non-reproduction code, not in my reproduction project.

• CS8602 Dereference of a possibly null reference.

Environment (please complete the following information):

• Mapperly Version: 5.0.0-next.8
• Nullable reference types: disabled
• .NET Version: 10.0.300
• Target Framework: .net10.0
• Compiler Version: 5.6.0-2.26230.15 (a0ccfc09f8af4a6d4756397fa89c65efd95f70a6)
• C# Language Version: 14.0
• IDE: Visual Studio 2026 18.6.2
• OS: Windows 11

Additional context
This only happens if the target list is readonly, otherwise it will generate a null coalesing nullcheck.

public static partial global::FailingMapTest.Target? Map(global::FailingMapTest.Source? source)
{
    if (source == null)
        return default;
    var target = new global::FailingMapTest.Target();
    if (source.Nested?.Items != null)
    {
        target.Items = global::System.Linq.Enumerable.ToList(source.Nested.Items);
    }
    else
    {
        target.Items = null;
    }
    return target;
}

(But I cannot change the target property in the non-reproduction project.)