System.Text.Json constructor deserializing with covariant return type read-only properties sets values to null

[krispenner]

Description

I would expect the below to work, however it does not. Maybe because covariant return types are new and not yet handled in System.Text.Json yet. I think the example speaks for itself but let me know if more information is required.

Configuration

• Which version of .NET is the code running on? .NET 5.0
• What is the architecture (x64, x86, ARM, ARM64)? x64
• Do you know whether it is specific to that configuration? It is not.

Non-working Example

Using covariant return type read-only properties passed through derived constructor does not work.

https://dotnetfiddle.net/IsNX3q

using System;
using System.Text.Json;
using System.Text.Json.Serialization;
					
public class Program
{
	public static void Main()
	{
		var json = "{ \"prop\": { \"id\": \"abc\", \"num\": 2 } }";
		var obj = JsonSerializer.Deserialize<DerivedClass>(json);
		Console.WriteLine(obj?.Property?.Id ?? "null");
		Console.WriteLine(obj?.Property?.Number?.ToString() ?? "null");
	}
	
	public class BaseClass
	{
		public BaseClass(BaseProperty property)
		{
			Property = property;
		}

		[JsonPropertyName("prop")]
		public virtual BaseProperty Property { get; }
	}
	
	public class DerivedClass : BaseClass
	{
		public DerivedClass(DerivedProperty property)
			: base(property)
		{
		}

		public override DerivedProperty Property { get; }
	}
	
	public class BaseProperty
	{
		[JsonPropertyName("id")]
		public string Id { get; set; }
	}
	
	public class DerivedProperty : BaseProperty
	{
		[JsonPropertyName("num")]
		public int? Number { get; set; }
	}
}

Output:

null
null

Working Example

Using non-covariant return type read-only properties passed through derived constructor does work:

using System;
using System.Text.Json;
using System.Text.Json.Serialization;
					
public class Program
{
	public static void Main()
	{
		var json = "{ \"prop\": { \"id\": \"abc\", \"num\": 2 } }";
		var obj = JsonSerializer.Deserialize<DerivedClass>(json);
		Console.WriteLine(obj?.Property?.Id ?? "null");
	}
	
	public class BaseClass
	{
		public BaseClass(BaseProperty property)
		{
			Property = property;
		}

		[JsonPropertyName("prop")]
		public virtual BaseProperty Property { get; }
	}
	
	public class DerivedClass : BaseClass
	{
		public DerivedClass(BaseProperty property)
			: base(property)
		{
		}
	}
	
	public class BaseProperty
	{
		[JsonPropertyName("id")]
		public string Id { get; set; }
	}
}

Output:

abc

[ghost]

Tagging subscribers to this area: @eiriktsarpalis, @layomia
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

I would expect the below to work, however it does not. Maybe because covariant return types are new and not yet handled in System.Text.Json yet. I think the example speaks for itself but let me know if more information is required.

Configuration

• Which version of .NET is the code running on? .NET 5.0
• What is the architecture (x64, x86, ARM, ARM64)? x64
• Do you know whether it is specific to that configuration? It is not.

Non-working Example

Using covariant return type read-only properties passed through derived constructor does not work.

using System;
using System.Text.Json;
using System.Text.Json.Serialization;
					
public class Program
{
	public static void Main()
	{
		var json = "{ \"prop\": { \"id\": \"abc\", \"num\": 2 } }";
		var obj = JsonSerializer.Deserialize<DerivedClass>(json);
		Console.WriteLine(obj?.Property?.Id ?? "null");
		Console.WriteLine(obj?.Property?.Number?.ToString() ?? "null");
	}
	
	public class BaseClass
	{
		public BaseClass(BaseProperty property)
		{
			Property = property;
		}

		[JsonPropertyName("prop")]
		public virtual BaseProperty Property { get; }
	}
	
	public class DerivedClass : BaseClass
	{
		public DerivedClass(DerivedProperty property)
			: base(property)
		{
		}

		public override DerivedProperty Property { get; }
	}
	
	public class BaseProperty
	{
		[JsonPropertyName("id")]
		public string Id { get; set; }
	}
	
	public class DerivedProperty : BaseProperty
	{
		[JsonPropertyName("num")]
		public int? Number { get; set; }
	}
}

Output:

null
null

Working Example

Using non-covariant return type read-only properties passed through derived constructor does work:

using System;
using System.Text.Json;
using System.Text.Json.Serialization;
					
public class Program
{
	public static void Main()
	{
		var json = "{ \"prop\": { \"id\": \"abc\", \"num\": 2 } }";
		var obj = JsonSerializer.Deserialize<DerivedClass>(json);
		Console.WriteLine(obj?.Property?.Id ?? "null");
	}
	
	public class BaseClass
	{
		public BaseClass(BaseProperty property)
		{
			Property = property;
		}

		[JsonPropertyName("prop")]
		public virtual BaseProperty Property { get; }
	}
	
	public class DerivedClass : BaseClass
	{
		public DerivedClass(BasePropertyproperty)
			: base(property)
		{
		}
	}
	
	public class BaseProperty
	{
		[JsonPropertyName("id")]
		public string Id { get; set; }
	}
}

Output:

"abc"

Author:	krispenner
Assignees:	-
Labels:	area-System.Text.Json, untriaged
Milestone:	-

[eiriktsarpalis]

I can reproduce; when I change the code to specify a non-covariant override then the serializer would throw the exception:

Unhandled exception. System.InvalidOperationException: Members 'Property' and 'Property' on type 'Program+NonCovariantReturnDerivedClass' cannot both bind with parameter 'property' in constructor 'Void .ctor(System.Object)' on deserialization.

I was not able to produce a workaround that didn't involve renaming the base property. cc @layomia

Minimal reproduction:

using System;
using System.Text.Json;
using System.Text.Json.Serialization;

public class Program
{
    public static void Main()
    {
        var json = "{ \"prop\": \"value\" }";

        var obj = JsonSerializer.Deserialize<BaseClass>(json);
        Console.WriteLine(obj.Property is not null); // True

        obj = JsonSerializer.Deserialize<CovariantReturnDerivedClass>(json);
        Console.WriteLine(obj.Property is not null); // False

        // Unhandled exception. System.InvalidOperationException: Members 'Property' and 'Property' on type 'Program+NonCovariantReturnDerivedClass'
        // cannot both bind with parameter 'property' in constructor 'Void .ctor(System.Object)' on deserialization.
        obj = JsonSerializer.Deserialize<DerivedClass>(json);
    }

    public class BaseClass
    {
        public BaseClass(object property)
        {
            Property = property;
        }

        [JsonPropertyName("prop")]
        public virtual object Property { get; }
    }

    public class CovariantReturnDerivedClass : BaseClass
    {
        public CovariantReturnDerivedClass(string property)
            : base(property)
        {
        }

        public override string Property { get; }
    }

    public class DerivedClass : BaseClass
    {
        public DerivedClass(object property)
            : base(property)
        {
        }

        public override object Property { get; }
    }
}

[eiriktsarpalis]

Related to #46522.

[layomia]

We should address as part of #44428.