🐛 ReadonlyCollectionPropertiesBehavior does not account for collection property being null

[ArturDorochowicz]

Describe the Bug

ReadonlyCollectionPropertiesBehavior does not account for the collection property being null.

The code in ReadonlyCollectionPropertiesCommand retrieves the value of the collection property and immediately passes it to the InstanceMethodQuery constructor which throws ArgumentNullException when the value is null.

AutoFixture/Src/AutoFixture/Kernel/ReadonlyCollectionPropertiesCommand.cs

Lines 57 to 63 in 730b478

	foreach (var pi in this.PropertyQuery.SelectProperties(specimenType))
	{
	var addMethod = new InstanceMethodQuery(pi.GetValue(specimen), nameof(ICollection<object>.Add))
	.SelectMethods()
	.SingleOrDefault();
	if (addMethod == null) continue;

Scenario

using System.Collections.Generic;
using System.Linq;
using AutoFixture;
using Xunit;

#nullable enable

public class Bugs
{
    class NullCollectionModel
    {
        public Dictionary<int, int> Items { get; } = default!;
    }

    [Fact]
    public void NullCollection()
    {
        var fixture = new Fixture();
        fixture.Behaviors.Add(new ReadonlyCollectionPropertiesBehavior());

        // Throws
        fixture.Create<NullCollectionModel>();
    }
}
/*
System.ArgumentNullException
Value cannot be null. (Parameter 'owner')
   at AutoFixture.Kernel.ReadonlyCollectionPropertiesCommand.Execute(Object specimen, ISpecimenContext context)
   at AutoFixture.Kernel.Postprocessor`1.Create(Object request, ISpecimenContext context)
   at AutoFixture.Fixture.Create(Object request, ISpecimenContext context)
   at AutoFixture.Kernel.SpecimenContext.Resolve(Object request)
   at AutoFixture.Kernel.SeedIgnoringRelay.Create(Object request, ISpecimenContext context)
   at AutoFixture.Kernel.CompositeSpecimenBuilder.Create(Object request, ISpecimenContext context)
   at AutoFixture.Kernel.CompositeSpecimenBuilder.Create(Object request, ISpecimenContext context)
   at AutoFixture.Kernel.Postprocessor`1.Create(Object request, ISpecimenContext context)
   at AutoFixture.AutoPropertiesTarget.Create(Object request, ISpecimenContext context)
   at AutoFixture.Kernel.CompositeSpecimenBuilder.Create(Object request, ISpecimenContext context)
   at AutoFixture.Kernel.TerminatingWithPathSpecimenBuilder.Create(Object request, ISpecimenContext context)
*/

Expected Behavior

The code above should not throw.

More Information

Using AutoFixture 4.17