dotnet 7 System.Text.Json JsonSerializer.Serialize exception: is marked required but does not specify a setter.

[kzhui125]

Description

using JsonSerializer.Serialize to Serialize object.

Reproduction Steps

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

TestDto t = new() { Items = new() };

string jsonString = JsonSerializer.Serialize(t);
Console.WriteLine(jsonString);

public class TestDto
{
    public string? Field1 { get; set; }
    [JsonIgnore]
    public required List<string> Items { get; set; }
}

Expected behavior

no exception

Actual behavior

exception throws

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

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Issue Details

---

Description

using JsonSerializer.Serialize to Serialize object.

Reproduction Steps

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

TestDto t = new() { Items = new() };

string jsonString = JsonSerializer.Serialize(t);
Console.WriteLine(jsonString);

public class TestDto
{
    public string? Field1 { get; set; }
    [JsonIgnore]
    public required List<string> Items { get; set; }
}

Expected behavior

no exception

Actual behavior

exception throws

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

Author:	kzhui125
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[eiriktsarpalis]

I believe this is by design. The required keyword is picked up by the serializer and has special meaning (equivalent to JsonRequiredAttribute). However "required" is the exact opposite to "ignore" so the two configurations are mutually exclusive. If the property needs to be ignored you should remove the required keyword. If the property is required for initialization then you should remove the JsonIgnore annotation.

[kzhui125]

using Newtonsoft.Json;

TestDto t = new() { Field1 = "Value1", Items = new() };
List<TestDto> ts = new() { t };

string json = JsonConvert.SerializeObject(t);
Console.WriteLine();

public class TestDto
{
    public required string Field1 { get; set; }

    [JsonIgnore]
    public required List<string> Items { get; set; }
}

when using Newtonsoft.Json it works

[kzhui125]

The property is required for object initialization (what the required keyword mean), but not required for json serialization.

They are two different things.

[eiriktsarpalis]

I would guess that's because Json.NET doesn't support required properties yet.

[eiriktsarpalis]

The property is required for object initialization (what the required keyword mean), but not required for json serialization.

They are two different things.

They are not. Deserialization if anything does perform object initialization, albeit indirectly. You can always bypass required/init properties when using reflection (which is why you see this working in the case of Json.NET), however you can't do it in the case of source generators. STJ honors required properties both because it preserves invariants of the deserialized class and because it has to in the case of source generators.

[kzhui125]

Deserialization throws exception make sense. since deserialization will create object, and the required modifier should be respected.

But serialization is object to json. The object is already created(required modifier was also respected). Serialization throw exception make people confuse, since it doesn't create object, it only create string from concat object properties.

I meet this issue when I want to return dictionary as json to frontend, but I don't want to return the original list (duplicate).

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

SomeVm vm = new() { SomeDtos = new() };
foreach (var dto in vm.SomeDtos)
{
    dto.ItemsDict = dto.Items
        .ToDictionary(i => i.Key, i => i);
}

string jsonString = JsonSerializer.Serialize(vm);
Console.WriteLine(jsonString);


public class SomeVm
{
    public required List<SomeDto> SomeDtos { get; set; }
    public string? Field1 { get; set; }
}

public class SomeDto
{
    public string? Field1 { get; set; }

    [JsonIgnore]
    public required List<SomeItemDto> Items { get; set; }
    public Dictionary<string, SomeItemDto>? ItemsDict { get; set; }
}

public class SomeItemDto
{
    public required string Key { get; set; }
    public string? Value { get; set; }
}

Thanks for your help.

[eiriktsarpalis]

JsonIgnoreAttribute is not specific to serialization -- it also controls deserialization. It's precisely this mode (disabling deserialization for a property marked required) that triggers the error you are seeing.