Unable to deserialize JSON into a collection property without a getter (with a misleading error message)

[ezhevita]

Description

When trying to deserialize a JSON string into a class containing a property with a collection type (Dictionary or IEnumerable) serializer throws an exception which says about a property missing a setter despite it being present.

Reproduction Steps

JsonSerializer.Deserialize<Test>("{\"Data\":[1]}");

public class Test
{
    [JsonRequired]
    public List<int> Data
    {
        set => FirstValue = value[0];
    }

    [JsonIgnore]
    public int FirstValue { get; private set; }
}

Expected behavior

JSON is deserialized correctly.

Actual behavior

The following exception is thrown:

System.InvalidOperationException: JsonPropertyInfo 'Data' defined in type 'Test' is marked required but does not specify a setter.
   at System.Text.Json.ThrowHelper.ThrowInvalidOperationException_JsonPropertyRequiredAndNotDeserializable(JsonPropertyInfo jsonPropertyInfo)
   at System.Text.Json.Serialization.Metadata.JsonPropertyInfo.Configure()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.ConfigureProperties()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.Configure()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.<EnsureConfigured>g__ConfigureSynchronized|172_0()
   at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType)
   at Program.<Main>$(String[] args)

Regression?

No, this behavior is present at least since .NET 7 when required properties were introduced.

Known Workarounds

Adding a getter to the property, it is allowed for it to return null or just outright to throw an exception.

Configuration

.NET 8.0.6 running on macOS 14.5 ARM64; issue is not specific to the configuration.

Other information

Similar to (although different scenario): https://github.com/dotnet/runtime/issues/92330

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[eiriktsarpalis]

I can reproduce, thanks. One workaround is to specify a getter it seems.

[julien98Qc]

Indeed, properties of collection types that only have setters are not supported.

In DetermineSerializationCapabilities(), CanDeserialize is set to false when an Enumerable or Dictionary has no getter. In Configure(), we trigger the exception based on IsRequired and CanDeserialize. The exception message assumes it is a setter problem, but that is not always the case.

The resolution might be one of the following:

• Add another verification that throws a separate error for this specific case.
• Change the exception message to a more generic one.

I would go for the other verification in my humble opinion. I could work on this, just tell me which direction to go.

[julien98Qc]

@eiriktsarpalis Just wanted to let you know that I've made a PR for this issue.

[julien98Qc]

The issue is not about a misleading error message anymore but to make it work when required and no getter specified. Since then, I'm really confused as why it was purposefuly dermined not deserializable in DetermineSerializationCapabilities(). I don't have enough time to investigate further.

See this comment below for more context : https://github.com/dotnet/runtime/pull/104861#pullrequestreview-2204684810

[ezhevita]

I've updated the issue title to reflect that the expected solution is making this scenario work.

[devsko]

I don't think this is the correct way to address the problem. A fix should instead try to make the scenario work as expected. If you remove JsonRequired from the repro in #104700 then deserialization works as expected and this should too.

@eiriktsarpalis are you aware that the setter is not called even when [JsonRequired] is removed. No exception but still unexpected.

The following

• Throws InvalidOperationException with [JsonRequired] and without getter
• Prints '0' without [JsonRequired] and without getter
• Prints '1' with getter (even though it is never called)

Console.WriteLine(JsonSerializer.Deserialize<Test>("{\"Data\":[1]}")?.FirstValue);

public class Test
{
    //[JsonRequired]
    public List<int> Data
    {
        //get => throw new Exception();
        set => FirstValue = value[0]; // Is NOT called as long as no getter is declared
    }

    [JsonIgnore]
    public int FirstValue { get; private set; }
}

[ShawnWu33]

Hi I am trying to looking into this issue as got some time in upcoming few days.

Upon a shallow investigation, the following logic in DetermineSerializationCapabilities() is where the CanDeserilize being set false.

if ((EffectiveConverter.ConverterStrategy & (ConverterStrategy.Enumerable | ConverterStrategy.Dictionary)) != 0)
{
    // Properties of collections types that only have setters are not supported.
    if (Get == null && Set != null && !_isUserSpecifiedSetter)
    {
        CanDeserialize = false;
    }
}

The comment // Properties of collections types that only have setters are not supported. is introduced in #72186. @eiriktsarpalis Could you shed a light on which direction you consider the proper fix should be? Do we want to fix what's underlying to let collection type that only have setters be supported?

Btw Should _isUserSpecifiedSetter being set as true in this case? Its false when running above case.