🐛 ReadonlyCollectionPropertiesBehavior does not select collection Add method correctly

[ArturDorochowicz]

Describe the Bug

ReadonlyCollectionPropertiesBehavior does not select collection Add method correctly.

The ReadonlyCollectionPropertiesCommand class tries to select any method called Add:

AutoFixture/Src/AutoFixture/Kernel/ReadonlyCollectionPropertiesCommand.cs

Lines 57 to 62 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;

This is incorrect in two ways:

1. There may be more than one Add method which then causes an exception from SingleOrDefault
2. It finds any method called Add. It should likely specifically select ICollection<>.Add (though that's probably still not fully specific as the type may implement multiple ICollection<>)

Scenario

In this scenario the readonly collection property is a Dictionary<,> which has multiple Add methods.

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

public class Bugs
{
    class MultipleAddMethodsModel
    {
        public Dictionary<int, int> Items { get; } = new();
    }

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

        // Throws
        fixture.Create<MultipleAddMethodsModel>();
    }
}
/*
System.InvalidOperationException
Sequence contains more than one element
   at System.Linq.ThrowHelper.ThrowMoreThanOneElementException()
   at System.Linq.Enumerable.TryGetSingle[TSource](IEnumerable`1 source, Boolean& found)
   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

Code above should not throw. Items should be added to the dictionary.

Tasks

Using AutoFixture 4.17

[Ergamon]

You already figured out the problem.

The current source code expects the following:
The ReadOnlyCollection implements ICollection and has public Add(T item) method.

As you are using a Dictionary<TKey, TValue> that is not the case. As this class implements ICollection<KeyValuePair<TKey, TValue>> and explicitly implements the interface.

Simplified AutoFixture uses the following code:

MethodInfo GetAddMethod(object obj)
{
  var type = obj.GetType();

  return type.GetMethod(nameof(ICollection<object>.Add));
}

while it should use this

static MethodInfo GetAddMethod(object obj)
{
  var type = obj.GetType();

  var collectionType = type.GetInterfaces()
    .Where(i => i.IsGenericType)
    .Single(i => i.GetGenericTypeDefinition() == typeof(ICollection<>));

  return collectionType.GetMethod(nameof(ICollection<object>.Add));
}