System.Text.Json contract customization ignoring custom CreateObject delegates in certain collection types

[eiriktsarpalis]

Consider the following code sample:

var options = new JsonSerializerOptions
{
    TypeInfoResolver = new DefaultJsonTypeInfoResolver
    {
        Modifiers =
        {
            static typeInfo =>
            {
                if (typeInfo.Type == typeof(IEnumerable<int>))
                {
                    typeInfo.CreateObject = () => new int[50];
                }
            }
        }
    }
};

var result = JsonSerializer.Deserialize<IEnumerable<int>>("[]");
Console.WriteLine(result is int[]); // Prints "false"

This is because many collection converters are hardcoding their CreateObject implementation which ignores any user-defined delegates:

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Collection/IEnumerableOfTConverter.cs

Lines 23 to 31 in 02910b4

	protected override void CreateCollection(ref Utf8JsonReader reader, ref ReadStack state, JsonSerializerOptions options)
	{
	if (!_isDeserializable)
	{
	ThrowHelper.ThrowNotSupportedException_CannotPopulateCollection(TypeToConvert, ref reader, ref state);
	}
	state.Current.ReturnValue = new List<TElement>();
	}

Note that this is currently not possible to fix unless we expose an API equivalent to the JsonCollectionConverter.Add method. For .NET 7 we should add detection for converters hardcoding CreateObject/Add implementations to specific types and have the JsonTypeInfo.CreateObject setter throw if not supported by the converter.

Related to #71944.

[ghost]

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Issue Details

---

Consider the following code sample:

var options = new JsonSerializerOptions
{
    TypeInfoResolver = new DefaultJsonTypeInfoResolver
    {
        Modifiers =
        {
            static typeInfo =>
            {
                if (typeInfo.Type == typeof(IEnumerable<int>))
                {
                    typeInfo.CreateObject = () => new int[50];
                }
            }
        }
    }
};

var result = JsonSerializer.Deserialize<IEnumerable<int>>("[]");
Console.WriteLine(result is int[]); // Prints "false"

This is because many collection converters are hardcoding their CreateObject implementation which ignores any user-defined delegates:

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Collection/IEnumerableOfTConverter.cs

Lines 23 to 31 in 02910b4

	protected override void CreateCollection(ref Utf8JsonReader reader, ref ReadStack state, JsonSerializerOptions options)
	{
	if (!_isDeserializable)
	{
	ThrowHelper.ThrowNotSupportedException_CannotPopulateCollection(TypeToConvert, ref reader, ref state);
	}
	state.Current.ReturnValue = new List<TElement>();
	}

Note that this is currently not possible to fix unless we expose an API equivalent to the JsonCollectionConverter.Add method. For .NET 7 we should add detection for converters hardcoding CreateObject/Add implementations to specific types and have the JsonTypeInfo.CreateObject setter throw if not supported by the converter.

Related to #71944.

Author:	eiriktsarpalis
Assignees:	-
Labels:	bug, area-System.Text.Json
Milestone:	8.0.0